Stress and Sociality in a Patrilocal Primate: Do Female Spider-Monkeys Tend-and-
Befriend?
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University
By
Michelle Amanda Rodrigues
Graduate Program in Anthropology
The Ohio State University
2013
Dissertation Committee:
Dawn M. Kitchen, Advisor
W. Scott McGraw
Douglas E. Crews
Randy Nelson
ii
Copyrighted by
Michelle Amanda Rodrigues
2013
iii
Abstract
Stress provokes an adaptive strategy that mobilizes the body for acute physical challenges. However, chronic stress has detrimental effects that can reduce health and reproductive fitness. Thus, coping mechanisms are valuable in reducing chronic stress.
One such mechanism, the “tend-and-befriend” strategy, refers to affiliation between females as an adaptive strategy to deal with stress. This mechanism is proposed to be a widespread strategy throughout the primate order, and one that underlies patterns of female bonding in humans. Although this strategy has been documented in matrilineal primates characterized by female kinship bonds, there has not been documentation of this strategy among unrelated females. Such documentation is necessary to demonstrate that this strategy is unrelated to female philopatry. Since our hominid ancestors are presumed to be male-philopatric, examining if this strategy applies to unrelated females is essential to understanding the evolutionary context of this mechanism. Here, I examine the tend- and-befriend strategy in a species characterized by fission-fusion social organization and female dispersal. I examine the patterns of female-female social relationships, male aggression, and ecological variables on glucocorticoid concentrations, a measure of physiological stress, among female black-handed spider monkeys. Behavioral, hormonal and ecological data were collected in a wild, habituated community. I validated a cortisol
ii assay for black-handed spider monkeys, and determined that cortisol concentrations do not significantly vary between reproductive states. The only activity variable or ecological variable that was significantly associated was time engaged in rest, with rest and cortisol concentrations inversely related. I found that affiliative behavior was significantly correlated with cortisol concentrations. I further found that females engaged in higher rates of affiliative behavior on days when fecal samples with high cortisol concentrations were collected. Most females exhibited a pattern of low cortisol concentrations punctuated by spikes in cortisol that returned to baseline, rather than chronically elevated cortisol concentrations. I found that patterns of association and affiliation were highly variable, and that rates of affiliative behavior were significantly correlated with association indices. I conclude that these patterns indicate that female spider monkeys are engaging in high rates of affiliative behavior when they are experiencing stress, and that engaging in affiliation brings cortisol concentrations back to baseline values. This research has direct implications for understanding the evolution of the stress response, and whether bonding among unrelated females is a result of ancestral tendencies within the primate order or a more derived feature limited to certain taxa.
iii
For Roger Rodrigues
iv
Acknowledgments
There are many people, organizations, and animals that have helped make this project possible. First, I have to thank Dawn Kitchen for both encouraging and challenging me. Dawn has been a great friend and mentor throughout my PhD program, and continually challenges me to improve my work, even if it means endless cycle revisions and learning new statistical tests. I also am very grateful to Scott McGraw,
Douglas Crews, and Randy Nelson for serving on my committee, and teaching me about primate ecology and behavioral endocrinology, and providing feedback to strengthen this project at various stages.
I am very grateful to Jill Pruetz for giving me the opportunity to first study at El
Zota and supporting my research. Jill is a mentor, a colleague, and a friend, and she is responsible for the early years of research on the study of the El Zota spiders. I also owe a great deal of gratitude to Stacy Lindshield. Stacy helped me so much in establishing planning my research, and she first introduced me to the beautiful forest and the spider monkeys. Stacy has also provided feedback on this project at various stages, particularly on the ecological component. I also appreciate all of the assistance that Matt Lattanzio has given me at every stage of my research. He taught me how to identify snakes
v venomous snakes, provided feedback and suggestions at the early stages of this project and has provided statistical advice whenever I am stuck on analysis. Matt has been a great colleague and friend, and I would not have made it through graduate school without his support.
Many people at the El Zota Biological Field Station helped me tremendously in this research. I am eternal gratitude to Israel Mesen for facilitating so many logistical aspects of my dissertation research, especially translating everything from financial transactions to rather embarrassing medical problems. Marbeli, Tomas, Hinder, Daisy, and Abigail, Victor and Noely, Wilmara Maria and Carlos Luis, Dona Anna and Douglas,
Marjorie and Joel all went out of their way to make me feel comfortable at El Zota.
Marbeli, Maria, Dona Rosa, and Marjorie made an extra effort to make my favorite foods and surprise us with desserts when torrential rain got our spirits down. Maria and Luis were wonderful friends and companions, and made El Zota truly feel like home. I am also very grateful to Hiner Ramirez, Enid Ramirez, Maribel Ramirez, and their entire family for allowing me permission to work at El Zota, supporting conservation of the forest, and being wonderful hosts.
I appreciate the assistance of Jessica Walz in pilot research field, and her feedback and support throughout this project. She was a fantastic assistant and companion in the field, and a great colleague and collaborator. Agnieska Sukiennik and
Katy Gilbert assisted me in identifying and naming individuals. I am also grateful to
Jason Ferrell and Katy for establishing the ecological transects.
vi
I am incredibly indebted to three wonderful assistants that went above and beyond in their dedication to finding and keeping the monkeys, even when it meant getting perpetually getting stuck in the swamp and having to crawl out. They did their best to collect fecal samples no matter what: whether they were splattered on tall leaves or the assistant’s eyelids, or if they needed to sniff it to distinguish it from rotting fruit. They then patiently sorted through fecals to remove debris and seeds, and Emily went even further with her own side project that involved sorting and washing the remaining fecals to recover and identify seeds. Each field assistant helped train the next, and they monitored transects, recorded party encounters, identified animals and collected fecals any time I needed to travel away from the field site or was not feeling well. They got very dirty They got eaten alive by mosquitoes. They motivated me when my own motivation was flagging. They walked through more spider webs than we can ever count and stayed calm even if when there were spiders in their hair. They helped me to see the forest through new eyes and appreciate so many aspects of the forest ecosystem . This project owes so much to Emily “Little Chair” Stulik, Anna “Fondles Poo” Kordek, and Lindsay
“One Feather” Mahovetz. “Sings with Monkeys” is incredibly appreciative. Both Emily and Anna also worked simultaneously on side projects that I hope will further our knowledge of the Pilon community’s feeding ecology.
I am also grateful to the friends and colleagues who have provided advice, feedback, and moral support throughout graduate school. Cathy Cooke, Elizabeth
Hellmer, Stacy Lindshield, Matt Lattanzio, Kristina Walkup, Tracie McKinney, Erin
Ehmke, Laurie Kauffman, Liz Beggrow, Ellen Furlong, Anna Yocom Laurie Reitsema,
vii
Jennifer Spence, Hedy Justus, Erin Kane, Dara Ulmer, Sarah Martin, Amy Eakins, Lori
Critcher, Jessica Walz, Jocelyn Bryant, Darcy Hannibal, Raymond Vagell, and many others have provided advice, support, or feedback at various stages of my research. I also want to acknowledge the great female friends I rely on when I need to tend-and-befriend:
Amy Laude, Angie Sansguiri, Divya Kansagra, Phillipa Soskin, Cathy Cooke, Elizabeth
Hellmer, Karen Bullock, Melissa Tanouye Conger, Paulomi Shah, Danielle Van Kampen,
Anna Geletka, Sabrina Ahmad McCollough, Claire Speirs, Shuba Bindra, Meredith
Palmer, Agnieska Sukiennik, Tejal Chande, and Anjal Chande have all been wonderful and supportive friends. I am also grateful to Caleb Bullock, Gracie Bullock, and Nathan
Cooke Moussopo for providing stress relief, and Sirius, Maise, Stitchy, and Cleo for being my furry dissertation assistants. I am also appreciative of the support of my family, including my sister, Belinda Rodrigues, and my father Roger Rodrigues. My father has been supportive of my decision to study monkeys, even though he still wishes I had gone to medical school instead. And finally, I want to thank all the spider monkeys who helped contribute to this project. Goldie, Clydette, Travis and Udi inspired me to study spider monkeys, and Rita, Marielita, Margarita, Evita, and Chita provided behavioral data and fecal samples. Most of all, I appreciate the cooperation of each of the wild spider monkeys that provided me with behavioral data and fecal samples. It is nearly impossible to study spider monkeys unless they are willing to put up with human stalkers, and I am grateful that they gave me enough data to complete my project.
I am also grateful to Dan Wittwer and Toni Ziegler at the Wisconsin National
Primate Center for their advice and assistance in hormone analysis, and Jay Peterson,
viii
Nicole Howlett, Vince Sodaro, and Sheila Wojciechowski of Brookfield Zoo for their assistance in collection of fecal samples and providing background information on the captive spider monkeys.
Finally, I appreciate all the agencies that provided funding for this project. I am grateful to the Wenner-Gren Foundation, The American Philosophical Society, The Ohio
State University Alumni Fund, The Ohio State University Chapter of Sigma Xi, and the
Department of Anthropology for funding this research.
ix
Vita
2003……………………….B.S. Ecology, Ethology, Evolution, University of Illinois
2003………………………...B.S. Psychology, University of Illinois
2005………………………...M.A. Anthropology, Iowa State University
2007 to present ……………Department of Anthropology, The Ohio State University
Publications
Lindshield, SM, Rodrigues, MA. 2009. Tool use in wild spider monkeys (Ateles geoffroyi). Primates 50(3): 269-272.
Rodrigues, MA. 2007. Age and Sex-Based Differences in Social Interactions and Social Spacing in Mantled Howler Monkeys (Alouatta palliata): Implications for Juvenile Social Development. J Dev Proc 2(2): 103-114.
Beck, B, Walkup, K, Rodrigues, M, Unwin, S, Travis, D, Stoinski, T. 2007. Guidelines for Great Ape Re-introduction. Gland, Switzerland: IUCN SSC Primate Specialist Group (PSG). 48 pp.
Fields of Study
Major Field: Anthropology
x
Table of Contents
Abstract……………………………………………………………………………………ii
Acknowledgments...... v
Vita………………………………………………………………………………………...x
List of Tables…………………………………………………………………………....xvi
List of Figures………………………………………………………………………….xviii
Chapter 1: Introduction…………………………………………………………………....1
Stress and Sociality………………………………………………………………..3
The Importance of Social Organization…………………………………………...5
Dissertation Overview…………………………………………………………….9
Chapter 2: Measuring Stress in Female Black-handed Spider Monkeys: Validation and the Influence of Reproductive State……………………………………………………...12
Introduction………………………………………………………………………12
Metabolite and Assay Selection………………………………………….15
Validation………………………………………………………………...16
Effects of Reproductive Condition………………………………………17
Research Objective and Hypotheses……………………………………..18
xi
Methods…………………………………………………………………………..19
Study Site and Animals………………………………………………….19
Captive Animals…………………………………………………19
Wild Animals…………………………………………………….20
Sample Collection and Preparation………………………………………23
Hormone Analysis……………………………………………………….24
Cortisol Assay……………………………………………………24
Estradiol Assay…………………………………………………..25
Data Analysis…………………………………………………………….25
Results……………………………………………………………………………27
Captive Validation……………………………………………………….27
Wild Validation…………………………………………………………..28
Wild Longitudinal Data………………………………………………….38
Discussion………………………………………………………………………..30
Validation………………………………………………………………..31
Pregnancy………………………………………………………………...33
Lactation…………………………….…………………………………...36
Reproductive Cycling……………………………………………………39
Conclusions………………………………………………………………41
Tables…………………………………………………………………………….43
Figures……………………………………………………………………………45
xii
Chapter 3: Seasonality, Activity Patterns, and Cortisol in Female Spider Monkeys in a
Wet Forest Environment…………………………………………………………………55
Introduction………………………………………………………………………55
Allostasis…………………………………………………………………56
Seasonality in the Tropics………………………………………………..57
Glucocorticoids and Activity Patterns…………………………………...58
Cortisol in Spider Monkeys……………………………………………...59
Research Objectives and Hypothesis…………………………………….61
Methods………………………………………………………………………….62
Study Site………………………………………………………………...62
Behavioral Data Collection………………………………………………63
Ecological Data Collection………………………………………………65
Hormonal Data Analysis and Collection…………………..…………….66
Statistical Analysis……………………………………………………….66
Results……………………………………………………………………………67
Discussion………………………………………………………………………..69
Climactic Variables affecting Activity Budgets…………………………70
Diet and Fruit Abundance………………………………………………..72
Rest and Cortisol…………………………………………………………75
Conclusions………………………………………………………………76
Tables…………………………………………………………………………….78
Figures……………………………………………………………………………81
xiii
Chapter 4: Friendship among Female Spider Monkeys………………………………….90
Introduction……………………………………………………………………....90
Stress and Coping Mechanisms………………………………………….93
Limitations of the Tend-and-befriend Hypothesis……………………….94
Fission-fusion social dynamics…………………………………………..96
Methods…………………………………………………………………………..98
Focal Subjects……………………………………………………………98
Behavioral Data Collection………………………………………………98
Statistical Analysis…………………………………………………….....99
Results……………………………………………………………………………99
Discussion………………………………………………………………………104
Affiliation in spider Monkeys...………………………………………...105
Personality…………………...………………………………………….106
Patterns of Grooming, Embraces, and Vocalizations………..…………107
Social Bonds in Species with Female Dispersal………………………..109
Factors affecting Inter-site Variation…………………………………...112
Potential Stressors………………………………………………………113
Implications for Primate and Human Evolution………………………..115
Tables…………………………………………………………………………...118
Figures…………………………………………………………………………..124
Chapter 5: Conclusions…………………………………………………………………131
Research Objective……………………………………………………………..131
xiv
Major Findings………………………………………………………………….132
Limitations of this Study………………………………………………………..132
Theoretical Significance………………………………………………………..134
Applied Significance……………………………………………………………140
Future Directions……………………………………………………………….141
Conclusions……………………………………………………………………..143
References………………………………………………………………………………144
Appendix A: Focal subjects…………………………………………………………….158
Appendix B: Individual Hormone profiles……………………………………………..160
xv
List of Tables
2.1. Fecal samples collected from captive female black-handed spider monkeys at Brookfield Zoo before and after the physical exam (PE)……………………..…………43 2.2. Individually recognized focal subjects in the Pilón community at El Zota Biological Field Station, their offspring, and number of fecal samples collected…………………..43 2.3. Categories of reproductive states for adult and subadult female spider monkeys at El Zota Biological Field Station, Costa Rica……………………………………………….44 2.4. Results of the GLMM assessing the effects of reproductive state, age, parity, time of fecal collection, and lactation stage on cortisol concentrations for wild female spider monkeys at El Zota Biological Field Station, Costa Rica……………………………….44 2.5. Results of the GLMM assessing effects of parity, time of fecal collection, reproductive state, age, and lactation stage on estradiol concentrations of female black- handed spider monkeys at El Zota Biological Field Station, Costa Rica………………..44 3.1. Distribution of focal samples, hours of observation, and fecal samples per individuals for black-handed spider monkeys at El Zota Biological Field Station, Costa Rica……...78 3.2. Behavioral catalogue for focal follows of female black-handed spider monkeys at El Zota Biological Field Station. …………………………………………………………...78 3.3. Fruit consumption of female black-handed spider monkeys and temporal availability from Sept 2010-Aug-2011 in the secondary forest of El Zota Biological Field Station, Costa Rica………………………………………………………………………………..79 3.4. Individual activity budgets for female spider monkeys at El Zota Biological Field Station, Costa Rica……………………………………………………………………...80 3.5. Results of GLMM assessing effects of activity variables, fruit abundance, party sizes, and season on cortisol concentrations of female black-handed spider monkeys at El Zota Biological Field Station………………………………………………………..80 4.1. Distribution of Focal samples, hours, and fecal samples per individuals female black- handed spider monkey at El Zota Biological Field Station…………………………….118 4.2. Behavioral catalogue used for focal sampling of female spider monkeys…………118 4.3. Affiliative behaviors recorded for female spider monkeys………………………...119
xvi
4.4. Agonistic behaviors recorded in female spider monkeys………………………….119 4.5. Association and interaction with males for female black-handed spider monkeys at El Zota Biological Field Station…………………………………………………………...120 4.6. Post-conflict affiliation for female black-handed spider monkeys at El Zota Biological Field Station………………………………………………………………...120 4.7. Agonism, cortisol, and rank for female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica……………………………………………………121 4.8. Association indices for female spider monkey dyads that did not engage in affiliative interactions……………………………………………………………………………...122 4.9. Affiliation indices and rates of affiliative behaviors for female spider monkey dyads that engaged in affiliation………………………………………………………………123 A.1. Focal subjects, reproductive stage, and offspring for female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica………………………………158 A.2 Number of focals, hours, fecals, and mean cortisol and estradiol concentrations for all female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica……………………………………………………………………………………. 159
xvii
List of Figures.
2.1. Mean cortisol concentrations before and after veterinary exam for black-handed female spider monkey Evita at Brookfield Zoo, Illinois…………………………………45 2.2. Mean cortisol concentrations before and after a veterinary exam for adult female spider monkey Rita at Brookfield Zoo, Illinois………………………………………….45 2.3. Mean cortisol concentrations before and after a veterinary exam for adult female spider monkey Margarita at Brookfield Zoo, Illinois……………………………………46 2.4. Figure 2.4. Mean cortisol concentrations for adult female spider monkey Marielita at Brookfield Zoo, Illinois, before and after a veterinary exam……………………………46 2.5. Cortisol profile for adult female spider monkey Evita at Brookfield Zoo, Illinois, before and after veterinary exam………………………………………………………...47 2.6. Cortisol profile for adult female spider monkey Margarita at Brookfield Zoo, Illinois, before and after veterinary exam………………………………………………………...47 2.7. Cortisol profile for adult female spider monkey Marielita at Brookfield Zoo, Illinois before and after veterinary exam………………………………………………………...48 2.8. Cortisol profile for adult female spider monkey Rita at Brookfield Zoo, Illinois, before and after veterinary exam………………………………………………………...48 2.9. Scatterplot of estradiol and cortisol concentrations of adult female spider monkeys at Brookfield Zoo, Illinois…………………………...……………………………………..49 2.10. Scatterplot of cortisol and estradiol concentrations in pilot study of wild female spider monkeys at El Zota Biological Field Station……………………………………..49 2.11. Scatterplot of the estradiol and cortisol concentrations from long-term data set of adult female spider monkeys at El Zota Biological Field Station, Costa Rica………….50 2.12. Mean +SE of cortisol concentrations of female black-handed spider monkeys in different reproductive states at El Zota Biological Field Station………………………..50 2.13. Mean +SE of estradiol concentrations of female black-handed spider monkeys in different reproductive states at El Zota Biological Field Station, Costa Rica…………...51 2.14. Cortisol and estradiol profiles of pregnant female black-handed spider monkey Jill at El Zota Biological Field Station, Costa Rica………………………………………….51 2.15. Mean +SE of cortisol concentrations from female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica…………………………………………….52 xviii
2.16. Mean+SE of estradiol concentrations from female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica…………………………………………….52 2.17. Mean +SE of cortisol concentrations in adult and subadult cycling female black- handed spider monkeys at El Zota Biological Field Station, Costa Rica………………..53 2.18. Mean + SE of estradiol concentrations in adult and subadult cycling female black- handed spider monkeys at El Zota Biological Field Station, Costa Rica………………..53 2.19. Mean +SE of cortisol concentrations in nulliparous versus parous female black- handed spider monkeys at El Zota Biological Field Station, Costa Rica………………. 54 2.20. Mean+SE of estradiol concentrations in nulliparous versus parous female black- handed spider monkeys at El Zota Biological Field Station, Costa Rica………………..54 3.1. Map of the secondary forest of El Zota Biological Field Station and transect locations for assessing bimonthly fruit abundance…………………………………………………81 3.2. Monthly consumption of Spondias by female black-handed spider monkeys at el Zota Biological Field Station, Costa Rica, from Sept 2010-Aug 2011…………………….82 3.3. Consumption of Hyeronima fruit by female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica, from Sept 2010-Aug 2011………………………..82 3.4. Consumption of Ficus fruits by female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica……………………………………………………..83 3.5. Consumption of palm fruits (Socretea and Iriartea) by female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica, from Sept 2010-Aug 2011…..83 3.6. Consuption of Dipteryx fruit by female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica……………………………………………………..84 3.7. Monthly fruit abundance in the secondary forest of El Zota Biological Field Station, Costa Rica, from Sept 2010-Aug 2011…………………………………………………..84 3.8. Mean + SE of fruit abundance in the wet versus dry seasons at El Zota Biological Field Station, Costa Rica, from Sept 2010-Aug 2011…………………………………...85 3.9. Monthly mean sizes for female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica from Sept 2010-Aug 2011……………………………………85 3.10. Party sizes between the wet and dry seasons for female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica……………………………….86 3.11. Monthly mean cortisol concentrations for female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica…………………………………………….86 3.12. Mean+ SE of cortisol concentrations of female black-handed spider monkeys in the wet and dry seasons……………………………………………………………………...87 3.13. Mean+ SE of cortisol concentrations from female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica, in the morning versus afternoon………... 87
xix
3.14. Mean activity budgets for female black-handed spider monkeys in the secondary forest of El Zota Biological Field Station, Costa Rica…………………………………..88 3.15. Individual activity budgets for female black-handed spider monkeys in the secondary forest of El Zota Biological Field Station, Costa Rica……………………….88 3.16. Scatterplot of mean cortisol values and time engaged in rest over the entire field season (Jun 2010-Aug 2011) for individual female black-handed spider monkeys at El Zota Biological Field Station…………………………………………………………….89 3.17. Monthly cortisol concentrations and resting rates for individual female black- handed spider monkeys at El Zota Biological Field Station, Costa Rica, from Sept 2010- Aug 2011…………………………………………………………………………………89 4.1. Equation to calculate twice-weight association index……………………………..124 4.2. Affiliation and cortisol concentrations in individual female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica………………………………124 4.3. Mean + SE for rates of affiliation on days with cortisol spikes versus baseline values for female black-handed spider monkeys at El Zota Biological Field Station…………125 4.4. Scatterplot of association with males and mean cortisol concentration for female black-handed spider monkeys at El Zota Biological Field Station……………………..125 4.5. Scatterplot of agonism received from males and mean cortisol concentrations for black-handed spider monkeys at El Zota Biological Field Station, Costa Rica………..126 4.6. Scatterplot of rates of agonism received and embraces for female spider monkeys at El Zota Biological Field Station, Costa Rica…………………………………. ……….126 4.7. Mean + SE for mean cortisol concentrations of female spider monkeys of different rank at El Zota Biological Field Station………………………………………………..127 4.8. Scatterplot of mean cortisol concentrations versus number of associates for female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica………..127 4.9. Association indices for females that engaged in no affiliative behavior versus females who did among female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica………………………………………………………………………………128 4.10 Scatterplot of association index and affiliation in dyads of female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica……………………….128 4.11. Scatterplot of mean cortisol concentrations versus number of friends for female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica………..129 4.12. Scatterplot of whinny rates versus estradiol concentrations in female black-handed spider monkeys at El Zota Biological Field Station Costa Rica………………………..129 4.13. Scatterplot of whinny rates versus grooming rates in individual female black- handed spider monkeys at El Zota Biological Field Station, Costa Rica……………………………………………………………………………………..130 xx
4.14. Scatterplot of estradiol concentrations versus grooming rates in individual female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica………..130 B.1. Cortisol and estradiol profiles for Agata (AG), an adult female black-handed spider monkey at El Zota Biological Field Station, Costa Rica……………………………….160 B.2. Cortisol and estradiol profile for Anne Shirley (AS), a subadult female black-handed spider monkey at El Zota Biological Field Station……………………………………..161 B.3. Cortisol and estradiol profile for Ariadne (AR), an adult female black-handed spider monkey at El Zota Biological Field Station…………………………………………….161 B.4. Cortisol and estradiol profile for Buttercup (BU), a subadult female spider monkey at El Zota Biological Field Station, Costa Rica………………………………………...162 B.5. Cortisol and estradiol profile for Houdini (HO), a subadult female black-handed spider monkey at El Zota Biological Field Station, Costa Rica………………………..162 B.6. Cortisol and estradiol profile for Isela (IS), an adult female black-handed spider monkey at El Zota Biological Field Station, Costa Rica……………………………….163 B.7. Cortisol and Estradiol profile for Jill (JI), an adult female black-handed spider monkey at El Zota Biological Field Station…………………………………………….163 B.8. Cortisol and estradiol profile for Jlo (JL), an adult female spider monkey at El Zota Biological Field Station, Costa Rica……………………………………………………164 B.9. Cortisol and estradiol profile for Leila (LE), an adult female spider monkey at El Zota Biological Field Station, Costa Rica………………………………………………164 B.10. Cortisol and estradiol profile for Muttonchop Mindy (MC), an adult female black- handed spider monkey at El Zota Biological Field Station, Costa Rica………………..165 B.11. Cortisol and estradiol profile for Rudy (RU), an adult female black-handed spider monkey at El Zota Biological Field Station, Costa Rica……………………………….165
xxi
CHAPTER 1: INTRODUCTION
The effects of stressors and body’s response to these stressors are key factors affecting health. While the stress response is an adaptive strategy that aids individuals in dealing with life’s challenges, a sustained stress response can result in a number of physical and mental ailments, including immune suppression, cardiovascular disease, depression, and anxiety (Sapolsky 1992, 2009; McEwen and Wingfield, 2003). For example, chronic stressors increases risk of heart disease, which is the most predominant cause of death in humans in the United States, accounting for approximately 27% of deaths in a given year (Kung et al., 2008). Conversely, social factors can alleviate stress responses; social integration in humans is associated with decreased mortality due to lowered risk for stress-related disease (Cohen et al., 1992; Cohen, 2004). By understanding the evolutionary context that has shaped the stress response, crucial connections between biology and sociality can be made to understand how psychosocial factors can both promote and alleviate the effects of chronic stress. Here, I investigate the evolutionary context of the stress response by examining coping strategies utilized by female primates to prevent chronic stress, and evaluate whether such strategies are part of 1 a widespread, ancestral pattern. Using spider monkeys as a model, I examine how a fission-fusion social structure and female dispersal influence stress levels among females, in order to identify how such selective pressures may have shaped sex-specific coping mechanisms throughout the primate order.
In this study, I examine the relationship between female sociality and glucocorticoid (GC) concentrations in a species with patrilocal social organization. I test the predictions of the tend-and-befriend hypothesis and compare patterns of female affiliation, male aggression, and GC concentrations in female spider monkeys to patterns reported for other primate species. Following the predictions of the tend-and-befriend hypothesis, I predict that females with strong social bonds will exhibit lower GC levels, and their offspring will have greater access to social support, both of which would be expected to promote reproductive success.
Selye (1973: 61) defines stress as “the nonspecific response of the body to any demand made on it.” A stressor is any factor that disturbs the body’s internal balances and causes an organism to mount a stress response (Selye, 1973). The stress response is mediated through the action of glucocorticoids (GCs), a class of hormones that increase the availability of glucose circulating in the bloodstream. Traditionally, organisms were thought to maintain physiological stability through changing environmental conditions, and this stability was defined as homeostasis (Selye, 1973; McEwen and Wingfield,
2003). However, because definitions of “stress,” “stressors,” the “stress response,” and
“homeostasis” have often been vague, McEwen and Wingfield 2003) proposed the concepts of allostasis and allostatic load as an alternate way to understand organisms’
2 internal balance and reaction to stressors. Allostasis is stability through change, in which the body alters physiological setpoints in relation to different challenges and life stages
(McEwen and Wingfield, 2003; Landys et al., 2006). Allostatic load refers to the cumulative results of sustained responses to predictable or unpredictable stressors, while allostatic overload refers the costs that accumulate when continued stressors exceed the body’s capacity to meet the demands of its environment (McEwen and Wingfield, 2003;
Landys et al., 2006). Wingfield (1990) posed the “challenge hypothesis,” which predicts that testosterone in males is maintained at different baselines during the breeding and non-breeding season. This hypothesis predicts that testosterone concentrations are maintained at higher baseline values during the breeding seasons, and when social instability increases the frequency of male-male aggression. Landys and colleagues
(2006) proposed a similar pattern in GCs for both males and females, in which individuals maintain different baselines in preparation for predictable life history challenges or seasonally-mediated stressors.
Stress and sociality
The physiological response to stress can be measured using glucocorticoids, the
“stress hormones.” The secretion of GCs is an adaptive response to immediate stressors that mobilizes glucose in the bloodstream. However, in situations of chronic stress, a positive feedback system becomes established and further increases GC concentrations, rather than bringing them down to baseline (Sapolsky 1992). As a consequence, the immune system and other aspects of physiological functioning become compromised.
However, other hormones can mediate this process by increasing or decreasing GC
3 concentrations (Sapolsky, 1992; Taylor et al., 2000; von der Ohe and Servheen, 2002;
Taylor, 2006). For example, during pregnancy, high levels of estradiol promote a marked increase in GCs and associated binding factors. Conversely, oxytocin serves to inhibit the release of GCs. This hormone is associated with birth and lactation, and aspects of social bonding (Uvnäs-Moberg, 1998). Thus, engaging in social affiliation or offspring care, particularly nursing, should increase oxytocin levels and reduce GC concentrations.
Research on humans has demonstrated several social factors that affect the stress response. Whereas loneliness and bereavement are associated with increased GC levels, social support and physical contact can reduce the stress response (Taylor, et al. 2000;
Heinrichs et al. 2003; Segerstrom and Miller 2004). Humans involved in personal conflict are twice as susceptible to the common cold as counterparts without such social conflict.
Similarly, greater social integration is associated with reduced susceptibly to colds
(Cohen, 2004).
Such findings have been mirrored by studies on non-human primates, both in terms of the causes of psychosocial stress and the coping mechanisms that relieve it. For example, baboons that lose close kin to predators are more likely than others in the group to have increased glucocorticoid concentrations. Of females that lose a kin member, those that increase their grooming networks return more quickly to baseline levels (Engh et al.,
2006 a; b). The potential for infanticide is another of the biggest stressors for female primates, and studies have documented that lactating females exhibit elevated GCs relative to other females in response to takeovers by immigrant males (baboons: Beehner et al. 2005; Engh et al. 2006b; howler monkeys: Cristóbal-Azkarate et al. 2007). Thus,
4 although direct aggression does raise GC concentrations (Wallner et al., 1999) for cognitively sophisticated animals, the threat of aggression alone is enough to elevate physiological stress levels.
The social mechanisms of coping are labeled “tend-and-befriend” by Taylor and colleagues (2000). This phrase refers both to tending offspring as well as seeking out affiliative support, particularly from other females. Taylor and colleagues (2000) hypothesize that this strategy is an adaptive trait that is a widespread, ancestral trait among female primates, and is related to increased activation of oxytocin in female physiology. In this present study, I apply this hypothesis to examine the relationship between female-female social relationships and GC concentrations in black-handed spider monkeys (Ateles geoffroyi). This hypothesis provides the framework to examine whether the “tend-and-befriend” strategy is a derived trait within specific lineages, or if it may be part of a broader evolutionary heritage.
The Importance of Social Organization
Taylor and colleagues (2000) draw on evidence from Western psychological studies. However, this sociological model may be too simplistic. Much of their human research is limited to situations artificially created in laboratory settings and may not be representative of how people behave in normal social settings. Furthermore, it is also possible that the “befriending” that they document is simply due to sociocultural pressures favoring politeness. While Taylor and colleagues (2000) assert that cross- cultural evidence supports this hypothesis, they base this on studies that demonstrate that females in various cultures are more likely than men to seek and provide help (Edwards,
5
1993). However, this evidence does not directly refer to seeking social affiliation.
Further, their evidence for an evolutionary basis for the tend-and-befriend strategy of stress reduction is based predominantly on matrilineal monkeys, such as gelada baboons and rhesus macaques. These species may not be representative of over-arching primate patterns, especially since matrilineal social organization may be a derived trait that is characteristic only of Old World monkeys (Strier, 1994). In female-philopatric species, related females are characterized by strong bonds and stable dominance hierarchies, whereas in species characterized by female dispersal, females are not as strongly bonded
(Wrangham 1980; Van Schaik 1989; Sterck et al. 1997). Nonetheless, these broad categorizations may be too simplistic (Isbell & Young 2002). While the strongest bonds among female primates are determined largely by kin relationships, strong “friendship” bonds have also been documented among unrelated females (Silk, 2002).
The tend-and-befriend hypothesis is intended to demonstrate an evolutionary basis for human female bonding and coping strategies. However, in contrast to many other primate species that form cohesive social groups, humans have a highly flexible, dispersed social organization (Manson and Wrangham, 1991; Rodseth et al., 1991; Geary and Flinn, 2002; Rodseth and Novak, 2006; Aureli et al., 2008). Furthermore, Manson and Wrangham (1991) argue that humans have a tendency toward patrilocal societies with weak social networks among females, and that this may be indicative of the ancestral hominid condition. Under these conditions, human females would be predominantly unrelated to other females in their adult lives.
6
While dispersed, patrilocal societies are rare among the primate order, they are found among our closest relatives, the chimpanzees and bonobos (Goodall, 1986;
Ghiglieri, 1987; Parish, 1994, 1996; Parish and De Waal, 2000; Stevens et al., 2006;
Stumpf, 2007; Aureli et al., 2008). In both of these species, males reside in the natal community, while most females disperse. Bonobo females, despite being immigrants to their adult communities, form strong social bonds with other females (Parish, 1996;
Parish and De Waal, 2000; Stevens et al., 2006; Stumpf, 2007). While female chimpanzees are generally considered weakly bonded, recent evidence has indicated that there is a great deal of variation. At some sites, such as Täi National Park, female chimpanzees do exhibit strong bonds despite their lack of kinship ties ( Lehmann &
Boesch 2008; Lehmann & Boesch 2009). Furthermore, female chimpanzees are often found in “neighborhoods,” which are regions of the total community where female’s core areas overlap (Williams et al., 2002; Kahlenberg et al., 2008 a; b).
Studies on chimpanzees and bonobos are valuable for providing an evolutionary context to examine human patterns, but they can only document patterns characteristic of the African apes. In order to evaluate whether the tend-and-befriend pattern is part of a wider ancestral pattern, these questions need to be studied in species that are phylogenetically distant to humans. Spider monkeys are a New World species that exhibit fission-fusion social organization in conjunction with male philopatry and female dispersal. By examining this question in spider monkeys, I focus on the selective pressures that shaped social relationships and coping responses in a convergent social organization.
7
Fission-fusion communities provide a unique and flexible solution to dealing with the costs and benefits of sociality (Chapman, 1990; Symington, 1990; Chapman et al.,
1995; Lehmann and Boesch, 2004; Otali and Gilchrist, 2005; Aureli et al., 2008). Spider monkeys, like chimpanzees, form small parties when food resources are scarce, but aggregate in larger parties when resources are abundant (Carpenter 1935; Janson &
Robinson 1987; Chapman 1990; Symington 1990; Chapman et al. 1995). Females without offspring travel in larger parties than females with immatures, suggesting that the latter are maximizing foraging benefits while minimizing potential social stress
(Chapman 1990; Rodrigues 2008). However, data indicate that lactating females travel in larger parties than other females (Rodrigues et al. 2009) and that individuals actively choose which parties to join (Ramos-Fernández, 2005). Presumably, greater choice of party size and social partners should allow individuals to avoid stressful conspecifics and range with close associates.
Spider monkeys exhibit a sex-segregated pattern of social organization (Fedigan
& Baxter 1984) and females are often the target of male-initiated aggression and forced copulation (Chapman et al. 1989; Campbell 2003; Gibson et al. 2008; Slater et al. 2008).
Furthermore, while rare, infanticide has been reported among spider monkeys (Gibson et al., 2008). Previous data from my study community suggest that females with juvenile offspring spend little time in association with males, although there are differences based on sex of offspring. Mothers with male offspring associated with males for 18% of their time, while mothers with female offspring are only associated with males approximately
1% of their time (Rodrigues 2007). Thus, even though they are likely unrelated, other
8 females are each other’s most likely social partners. Therefore, grouping with males, who may be aggressive to females, should increase GC concentrations of female spider monkeys relative to baseline levels. This should be particularly true for females with dependent offspring. Likewise, I would expect a relationship between GC concentrations and the frequency of female-directed male aggression.
While female spider monkeys are reported to have weak social bonds, a large degree of variability has been demonstrated. Some females, but not all, show close affiliative bonds with other females (Symington, 1990). Furthermore, mother-juvenile dyads are often consistently found ranging with a particular adult female (Rodrigues
2007). Finally, Webster and Suarez (2008) report that immigrant females “shadow” certain resident females, and associate almost exclusively with those females. These relationships should help them to learn locations of feeding trees and ease their transition into the community. Thus, female-female bonds may develop early after immigration into the new community. Based on this evidence, I predict social bonds, as a coping mechanism for stress, should reduce elevated GC concentrations in female spider monkeys back to baseline values.
Dissertation Overview
In Chapter Two, “Measuring stress responses in female black-handed spider monkeys: Validation and the influence of reproductive state,” I lay the groundwork for examining GC concentrations in female spider monkeys. This chapter includes the results of the pilot project, in which I validate a fecal cortisol EIA for female black-handed spider monkeys, and examine the relationship between cortisol and estradiol in both
9 captive and wild spider monkeys. Additionally, in this chapter, I further examine relationships among cortisol, estradiol, and reproductive state based on long-term fieldwork conducted from June 2010-August 2011 in individually recognized wild females. I then discuss how these findings inform our understanding of stress across reproduction states in the primates, especially the relationship between stress and lactation.
In Chapter Three, “Seasonality, activity patterns, and cortisol in a wet forest environment” I examine the relationship between cortisol and temporal and ecological variables. Specifically, I examine effects of time of collection, activity variables, and ecological variables on GC concentrations over the entire period of data collection.
Additionally, I examine how monthly patterns of fruit abundance, activity levels, and party sizes influence monthly cortisol concentrations. I then evaluate how these findings inform our understanding of the relationship between stress and ecology in a wet forest environment.
In Chapter Four, “Friendship among female spider monkeys,” I examine main predictions of this project. Specifically, I examine the relationship between social behaviors and GC concentrations, as well as effects of male aggression, female affiliation, offspring care, and subgrouping patterns on GC concentrations. Additionally, I investigate rates of post-conflict offspring care and female affiliation to test the predictions of the tend-and-befriend hypothesis. Finally, I examine rates of female affiliation after spikes in cortisol relative to baseline values, in order to determine if female spider monkeys actively seek out and increase affiliation with female friends to
10 reduce stress levels. I then discuss how these findings fit into our understanding of social bonding among unrelated females in the primate order, and how these findings inform theoretical models of human evolution and primate sociality
In Chapter Five, “Conclusion: Do female spider monkeys tend-and befriend?” I synthesize results from all three data chapters, draw conclusions regarding my central hypotheses, and evaluate how these findings fit into current theoretical frameworks regarding the evolution of social relationships and primate socioecology. I then address practical applications of the methods and results of this study, and suggest avenues for further research.
11
CHAPTER 2: MEASURING STRESS RESPONSES IN FEMALE BLACK- HANDED SPIDER MONKEYS: VALIDATION AND THE INFLUENCE OF REPRODUCTIVE STATE
Introduction
Stress mobilizes the body to prepare for acute challenges, but chronic stress can be detrimental to health and reproductive fitness. The so-called stress hormones, or glucocorticoids (GCs) mobilize glucose in the bloodstream so that animals can quickly respond to severe threats (Sapolsky 1992). However, when animals are chronically stressed, a positive feedback loop in the hypothalamic-pituitary-adrenal (HPA) axis increases GC concentrations above baseline values, and over time this can result in compromised immune and reproductive functions. While research on GCs has proliferated, there remain a number of obstacles to accurately measuring them. In particular, for females mammals, GCs can vary based on social, environmental, and physiological factors associated with reproductive state (Sapolsky, 1992; Landys et al.
2006; Ehmke, 2010; Carnegie et al. 2011). In particular, data on GC concentrations during lactation are conflicting. Some mammals, particularly rodents, suppress GC reactivity during lactation (Windle et al. 1997; Shanks et al. 1999; Brunton and Russell,
2003; Brunton et al. 2008). While some lactating primates exhibit normal GC 12 concentrations, others exhibit elevated GC activity as a result of the risks of predation or threat of infanticide (Weingrill et al. 2004; Engh et al. 2006 a; Maestripieri et al. 2008;
Carnegie et al. 2011). Furthermore, some research on humans suggests that breastfeeding suppresses GC reactivity, supporting the patterns found in rodents (Altemus, 1995). Here,
I validate the GC assay for female spider monkeys (Ateles geoffroyi), and examine how
GC concentrations vary with reproductive state, in order to elucidate how lactation affects GC reactivity in primates.
Hormone metabolites extracted from fecal samples provide a non-invasive method of investigating physiological stress in birds and mammals (Whitten et al. 1998;
Beehner and Whitten 2004; Touma and Palme 2005; Heistermann et al. 2006). Though great strides have been made in methodology to collect and store samples in the field
(Beehner and Whitten 2004; Ziegler and Wittwer 2005), there are a number of obstacles to accurately using metabolites from fecal samples to examine the relationship between glucocorticoids (GCs) and behavioral variables. Cortisol or corticosterone metabolite often are assumed to reflect physiological stress. Any assay GC metabolite assay must be validated to ensure measurement of metabolites genuinely reflect physiological stress
(Touma and Palme, 2005; Heistermann et al. 2006; Martínez-Mota et al. 2008). Whereas some studies rely on validations performed on related species (Martínez-Mota et al. 2007;
Rangel-Negrín et al. 2009; Ange-van Heugten et al. 2009), the assay ideally should be validated for both the species and sex examined (Beehner and Whitten 2004; Touma and
Palme 2005; Heistermann et al. 2006). Second, when studying GCs in females, the
13 relationship between GCs and reproductive state must be determined (Sapolsky, 1992;
Landys et al. 2006; Ehmke, 2010; Carnegie et al. 2011).
Despite these obstacles, fecal sampling is less invasive than sampling fblood
(Touma and Palme 2005; Ehmke 2010) and often feces are easier to collect than urine.
Furthermore, steroid hormone metabolism in the liver pools hormone concentrations in fecal samples. This should minimize effects of minor fluctuations, pulsatile secretion patterns and circadian rhythms (Goymann et al. 1999; Touma and Palme 2005). Time lag between the experience of stress and its reflection in the feces depends on both steroid metabolism and gut passage rate (Touma and Palme 2005; Ziegler and Wittwer 2005).
Milton (1981) determined gut passage rate for spider monkeys is generally 4-8 hours; however, some markers were excreted over 24 hours after initial ingestion.
Extraction of hormones from spider monkey fecal samples was successfully demonstrated by Campbell’s (2000; Campbell et al. 2001) study of reproductive hormones. This study validated assays for estrone (EIC) and progesterone (PDG), and found that fecal steroid metabolites lag urinary steroid metabolites by one to two days.
Campbell’s findings (2000; Campbell et al. 2001) suggest steroid metabolism in the liver may slow excretion of fecal metabolites of these hormones, as this lag exceeds typical gut passage rates by 16-44 hours. Pregnant females exhibited significant increases in both estrone and progesterone, and in cycling females estrone and progesterone peaked within
1-2 days of each other. Since Campbell’s (2000; Campbell et al. 2001) work, Strier and
Ziegler (2005) examined fecal estradiol in muriquis, Hernández-López and colleagues
(2010) examined estradiol and progesterone among adult and elderly captive female
14 spider monkeys, and Rangel-Negrín and colleagues (2009) examined cortisol in wild and captive Yucatan spider monkeys. Additionally, Ange-van Heugten (2009) examined the effects of diet on cortisol concentrations in captive woolly and spider monkeys. However, with the exception of a stress challenge administered to two individuals by Rangel-
Negrín and colleagues (2009), no study has validated fecal cortisol in spider monkeys.
Furthermore, no studies have investigated how female reproductive state and associated hormones (estrogens and progesterone) affect cortisol concentrations.
Metabolite and Assay Selection
Choice of GC metabolite varies based on study species and assay method.
Cortisol is the primary GC in most primates, as well as most carnivores and ungulates, whereas corticosterone is the primary metabolite in rodents, birds, and reptiles (Touma and Palme, 2005). Assays methods are more responsive to one GC metabolite over another. Beehner and Whitten (2004) used an RIA kit for corticosterone due to high cross-reactivity with other GC metabolites and its previous validation in baboons.
Additionally, different primate taxa vary in both their circulating concentrations of cortisol as well as rates of metabolism and excretion of native cortisol and corticosterone in fecal metabolites. For example, New World monkeys have higher concentrations of circulating cortisol, up to 10 times that of Old World Monkeys (Chrousos et al. 1982;
Ange-van Heugten et al. 2009). Similar differences are reported for salivary cortisol as well (Ange-van Heugten et al. 2009) and would be expected to occur in fecal metabolites.
Heistermann and colleagues (Heistermann et al. 2006) tested assays for cortisol, corticosterone, and two group-specific antibodies among five primate species, and found
15 no consistent pattern due to interspecific differences. For example, while the cortisol enzymeimmunoassay (EIA) was best at documenting GC changes in common marmosets, the 11X-hydroxy- etiocholanolone EIA was the most responsive for common chimpanzees. Thus, choice of metabolite and assay differ according to taxonomic differences as well as other factors, including previous work on related species.
Validation
The adrenocorticotropic hormone (ACTH) challenge is considered the standard method to validate the efficacy of using fecal metabolites as an accurate gauge of physiological stress (Touma and Palme 2005; Wasser et al. 2000). This procedure involves artificially introducing the pituitary hormone ACTH via injection to stimulate the adrenal glands to increase GC secretion. The ACTH challenge has been used to validate fecal GC metabolite assays in baboons, long-tailed macaques, common marmosets, Barbary macaques, and chimpanzees (Wasser et al. 2000; Heistermann et al.
2006). Additionally, Heintz and colleagues (2011) have used the ACTH challenge to validate a salivary cortisol assay for chimpanzees. However, because there are practical and ethical obstacles to performing ACTH challenges in primates, many studies use a modified version using a stressful situation or anesthesia instead. For example, Beehner and Whitten (2004) created a stress challenge for wild hybrid baboons by baiting traps with corn to generate competition over the high-calorie resource. Several studies have used anesthesia (Whitten et al. 1998; Heistermann et al. 2006), which stimulates GC activity (Touma and Palme 2005; Wasser et al. 2000) and thus can be a reliable alternative to an ACTH challenge. Although Rangel-Negrín and colleagues (2009)
16 exposed captive spider monkeys to a stress challenge utilizing capture and isolation, no anesthesia was administered. Furthermore, only one female and one male were exposed to this challenge.
Effects of Reproductive Condition
Although female stress hormone concentrations are affected by reproductive condition, the relationship between GCs and reproductive state is not consistent across species (Sapolsky 1992; Ehmke 2010; Carnegie et al. 2011). Life history theories that incorporate the concept of allostasis predict that GC concentrations will rise in response to certain life stages, such as pregnancy and lactation (Landys et al. 2006) because these are predictable challenges that are crucial to reproductive success. Furthermore, research on some primates has indicated that parental GC concentrations are associated with responsiveness and increased vigilance in early months of an infant’s life (Almond et al.
2008; Nguyen et al. 2008). Additionally, estradiol concentrations spike toward the end of pregnancy, and higher estradiol concentrations promote an increase in GC binding factors that may result in higher concentrations of circulating GCs (Sapolsky 1992; von der Ohe and Servheen 2002). Similarly, the production of corticotropin-releasing hormone (CRH) by the placenta may also increase GC concentrations (Lockwood et al. 1996). Thus, we would expect GCs to increase during pregnancy. However, perhaps in compensation for these hormonal cascades, GC responsiveness is reported to decrease during late pregnancy and lactation in some mammals, particularly rodents (Windle et al. 1997;
Shanks et al. 1999; Brunton and Russell, 2003; Tu et al. 2005; Tilbrook et al. 2006;
Brunton et al. 2008). The majority of primate studies have reported higher levels of GCs
17 in the latter stages of pregnancy, including lemurs (Cavigelli, 1999) callitrichids ( Ziegler et al. 1995; Smith et al. 1997; Bales et al. 2005) capuchins (Ehmke, 2010; Carnegie et al.
2011) baboons (Weingrill et al. 2004; Gesquiere et al. 2008) and humans (Lockwood et al. 1996). However, some other female mammals show no change in GC concentrations across reproductive states ( dairy cows: Walker et al. 2008).
Research Objective and Hypotheses
Here, I examine patterns of fecal cortisol and estradiol metabolites in captive and wild female black-handed spider monkeys (Ateles geoffroyi). The goal of this study is two-fold: 1) to validate the fecal cortisol metabolite EIA and 2) determine the relationship between cortisol and estradiol metabolites across reproductive states in black-handed spider monkeys. Validation was performed using captive cycling females, while the effect of reproductive state was examined in wild female spider monkeys. I hypothesized that fecal cortisol metabolite concentrations would increase following anesthesia during a routinely-schedule veterinary exam, and that fecal cortisol metabolite concentrations would correlate with estradiol metabolite concentrations in both captive and wild females. I further hypothesized that pregnant females would have the highest cortisol concentrations, while cycling females were expected to have the lowest cortisol concentrations. Finally, I hypothesized that lactating females with young (ventral) infants would have higher cortisol concentrations compared to females with dorsal infants and young juveniles.
18
Methods
Study site and animals
Captive Animals
Fecal samples were collected from female black-handed spider monkeys (Ateles geoffroyi) at the Brookfield Zoo, in Brookfield, Illinois. This group consists of eight spider monkeys, including five female black-handed spider monkeys, two male black- handed spider monkeys, and one female black spider monkey (Ateles paniscus). The monkeys are housed in a mixed-species habitat measuring 30.48 x 30.48 m
(approximately 863,097.48 cubic meters of space) during the day, and four connected enclosures each 3.66 x 4.27 x 2.44 m at night. In their public enclosure, the spider monkeys share the enclosure with capuchin monkeys (Cebus apella), a great anteater, and a tapir, and have periodic interactions with the capuchins. Additionally, they have visual access to cotton-top tamarins (Saguinus oedipus), Goeldi’s monkeys (Callimico goeldii), golden lion tamarins (Leontopithecus rosalia) and a two-toed sloth.
The spider monkeys are fed canned Zupreem ® Primate Diet (435 g) and leafy greens (250 g), and vegetables (450 g) in the morning. Most of these items are fed in their night enclosure around 10 am, but leafy greens are scattered for foraging in their day enclosure. Upon their return to their night enclosure around 5 pm, they are fed Mazuri ®
New World monkey chow (500 g), fruit (765 g), sweet potatoes (123 g), leafy greens
(1500 g), and sometimes hard-boiled eggs (three times a week).
Twenty-two fecal samples were collected from four of the five female black- handed spider monkeys from June 7-14, 2008, before and after a veterinary exam (Table
19
2.1). Additional samples from the fifth female were excluded from analysis due lack of sampling after the exam. Furthermore, not all females were sampled equivalently after the veterinary exam. All females were cycling, and housed with vasectomized males.
The veterinary exams were performed on June 11, 2008, and each took between
45-75 minutes. Animals were anesthetized with a combination of metadomidine and ketamine, at varying doses determined by zoo veterinarians. Under anesthesia, each animal underwent a full physical exam, tuberculosis test, blood draws, and radiographs.
Additionally, some animals underwent rectal cultures, pinworm checks, and tongue scrapes.
Wild Animals
Data were collected at El Zota Biological Field Station in Costa Rica. El Zota is a
1000 ha private reserve situated in the northeastern region of the country at 10°57.6 N,
83°75.9’W approximately 20 km from Tortuguero National Park and Barro del Colorado
Reserve (Pruetz and LaDuke, 2001; Lindshield, 2006). This area receives approximately
4000-5000 mm of rainfall annually (Sanford et al., 1994; Wolfe and Ralph, 2009).
Seasonality is mild, with rainfall occurring year-round, but with slight peaks in early
November-December and June-July, and drier periods September-October and February-
March. The forest is characterized predominantly by wet and swamp forest (Lindshield,
2006). Low-lying areas in the secondary community tend to be seasonally inundated, while some areas are perpetually inundated. Spider monkeys share the forest with capuchin (Cebus capucinus) and howler monkeys (Aloutta palliata), and a variety of
20 other fauna, including potential predators jaguar (Panthera onca) and tayra (Eira barbara).
Two spider monkey communities are present at El Zota: one community ranges in the northern primary forest, and the other ranges in the southern secondary forest (Pruetz and LaDuke, 2001; Lindshield, 2006; Rodrigues, 2007). Research was conducted on the
Pilón community, which ranges throughout the secondary forest and on neighboring properties to northwest and southeast of the reserve. This community is well-habituated, with exposure to student groups and researchers since the establishement of the the field station in 2001. Approximately 35-40 individual live in the Pilon community, including
17 individually recognized subadult and adult females, 15 of those females’ infant and juvenile offspring, four individually recognized adult and subadult males, and 3-5 males that were not individually recognized. Individuals were identified based on pelage, facial, and genital characteristics.
Fecal samples of wild spider monkeys were collected during two phases at El
Zota Biological Field Station, Costa Rica. During the pilot phase, 24 fecal samples were collected from female spider monkeys at El Zota from July 14-27, 2008. During this study, not all females were individually recognized, and multiple samples may have been collected from the same females on different days. Females were in various, unknown reproductive conditions. They were grouped into the two general categories of “lactating” or “cycling/pregnant” based on the presence or absence of a nursing infant/juvenile.
During the second phase, 134 fecal samples were collected from 17 individually recognized females at El Zota from July 2010-August 2011 (Table 2.2). Reproductive
21 condition of these individually recognized monkeys was determined retroactively as
“cycling”, “lactating,” or “pregnant.” Females were considered pregnant by noting estimated date of birth and calculating gestation periods to determine date of conception.
Females were considered lactating if they had an infant (0-12 months) or juvenile-1 (12-
24 months), because offspring are not fully weaned until at least 24 months. Females without offspring, and females with offspring greater than 24 months, were considered cycling females. Cycling females were further divided into adult and subadult categories, and lactating females were subdivided into three stages of lactation (early, middle, and late) depending on the age of their offspring (Table 2.3). Two females, JI and ST, gave birth during the study period, although only JI was sampled during pregnancy. These classifications follow developmental stages outlined by van Roosmalen and Klein (1988), in which offspring that travel independently of their mothers but still regularly nurse
(approximately 12-24 months) are considered juveniles.
Samples were collected opportunistically, with a target goal of obtaining one sample per female every two weeks. However, due to the difficult nature of finding individuals in a fission-fusion social context, challenges in finding and collecting fecal samples from fast-moving, arboreal animals, and loss of fecal samples to out-of-reach foliage, swamps, and creeks, this goal was not achieved. Thus, while sample collection approximated this goal for a few females (individuals AS, LE, and JL), sample collection was less frequent for most females, and fewer samples were collected from difficult-to- sample individuals (DA, FA, EV, MI, ST, and ZE).
22
Sample collection and preparation
Samples from the captive females were collected by keepers and immediately frozen at -20 C. Samples were later transported to the laboratory of Douglas E. Crews at
Ohio State University and extracted following the same protocol used to extract samples in the field. Samples from the wild females were collected by myself or a field assistant using either a Ziploc bag, or plastic spatula and test tube. They were then immediately stored in a thermos with a cold pack, and then returned to the field station for processing within an hour.
Samples were processed and stored using Solid Phase Extraction (SPE) following the protocol outlined by Ziegler and Wittwer (2005) with modifications suggested by
Erin Ehmke (pers. comm.). 0.1 grams of fecal material were mixed with 2.5 ml distilled water and 2.5 ml ethanol. The mixture was then shaken by hand for five minutes and hand-centrifuged for 10 minutes. The mixture was then allowed to settle for 30 minutes or more. Approximately 3 ml of the supernatant was removed and transferred to a clean test tube. Two ml of the filtered supernatant was removed and passed through an Alltech
Prevail C18 Maxi-Clean SPE Cartridge (Alltech ©, Lexington, KY). This allows for storage at ambient temperatures for approximately a year. Samples were stored for 72-
460 days. Beehner and Whitten (2004) report that fecal GCs extracted in methanol and stored in C18 cartridges exhibited limited degradation when stored up to 40 days at ambient temperature. This is in contrast to Khan and colleagues (2002) findings that storage of fecal GCs in ethanol resulted in an increase in GC concentrations up to 120 days, followed by a decrease between 120-180 days. The initial increase in
23 concentrations was due to the continued extraction of hormone metabolites while stored in ethanol.
To prevent continued extraction, I washed my cartridges with 2ml of distilled water to stop extraction (Ziegler and Wittwer, 2005). To ensure preservation in the field, cartridges were stored in Ziploc bags with silica gel, and then placed in a cooler with additional silica gel. A total of 134 fecal samples were collected from the 17 focal individuals, although six individuals were not adequately sampled.
Hormone Analysis
Samples were analyzed using enzyme-immunoassay (EIA) techniques and radio- immunoassay techniques (RIA) at the Wisconsin National Primate Center Core Assay facility with the assistance of Dan Wittwer. SPE cartridges were washed with 1 ml of a
95:5 water: methanol solution. 1 ml of methanol was then added to the SPE cartridge and collected. Methanol was dried and re-suspended in 1 ml and stored in the refrigerator until assayed.
Cortisol Assay
Cortisol was analyzed using an in-house EIA assay at the Wisconsin National
Primate Center by Dan Wittwer. This assay cross-reacts with other steroids, particularly cortisone. Antibody (R4866) and conjugate were purchased from Coralie Munro, UC-
Davis. The polyclonal antibody was raised in rabbits. Standards, H4001, were from
Sigma. 200 µl of sample were evaporated and then re-suspended in 300 µl of F:HRP at a concentration of 1:150,000 and plated. Plates were incubated for 2 hours, unbound material was washed off the plate and substrate was added. Stop solution was added after
24 sufficient color developed. Assays were calculated using log-logit regression. The cortisol assay was validated for accuracy (-93.49±1.09, n=6) and parallelism (–t=1.14, df=22, p>0.05, n=7) using internal controls in order to determine precision. Inter- and intra-assay coefficients of variation of two pools were 15.8/9 and 13.1/6.
Estradiol Assay
Estradiol was analyzed using an in-house RIA assay at the Wisconsin National
Primate Center by Dan Wittwer. The antibody was from Holly Hill Biologicals Inc. The polyclonal antibody was raised in rabbits. Tritiated estradiol was purchased from Perkin
Elmer. Standards, E8875 were from Simga. 50 µl of sample was evaporated and then refrigerated overnight after antibody and trace were added. After overnight incubation,
1ml of charcoal solution was added. After 15 minutes incubation, the solution was centrifuged to remove charcoal. Supernatant was poured into a scintillation vial and the radioactivity was counted in a beta counter. Assays were calculated using log-logit regression. The estradiol assay was validated for accuracy (-98.84±-1.53, n=8) and parallelism (t=-1.83, df=22 p>0.05, n=5) using internal controls in order to determine precision. The inter-assay and intra-assay coefficients of variation of the two pools were
8.4/2 and 7.4/1.8.
Data Analysis
Each data set was tested for normality, and log transformations and non- parametric statistics were used where necessary. For correlations, Spearman’s rank was used if both hormones were not normally distributed. If samples size were not equivalent,
Mann-Whitney U or Kruskall-Wallis tests were used. In the sample from the Brookfield
25
Zoo, cortisol was not normally distributed (skewness=4.32±0.49, kurtosis=19.58±0.95) while estradiol was normally distributed (skewness=1.19±0.49). Log transformations were performed on both hormones. After transformation cortisol was still not normal
(skewness: 1.05±0.49, kurtosis=2.04±0.95) so non-parametric tests were performed. In the pilot phase from El Zota, both cortisol (skewness=1.54±0.47, kurtosis=3.06±0.92) and estradiol (skewness=4.87±0.47, kurtosis=23.79±0.82) were not normally distributed.
Log transformations were performed on both hormones. After transformation, cortisol was normally distributed (skewness=-0.64±0.47, kurtosis =-0.25±0.92) while estradiol was not (skewness=2.66±0.47, kurtosis=9.80±0.92). Thus, non-parametric tests were performed. The major dataset from El Zota was not normally distributed (cortisol: skewness=7.23±0.21, kurtosis=62.88±0.42; estradiol: skewness=9.69±0.42) so all values were long-transformed. After log transformation, cortisol was normally distributed
(skewness=0.37±0.21, kurtosis=1.8±0.21), but estradiol was not (skewness=1.80±0.21, kurtosis=5.39±0.42).
A general linear mixed model (GLMM) was then used to evaluate the effects of multiple variables (reproductive state, parity age, time collected, and stage of lactation) on the hormone of interest (cortisol or estradiol). This method incorporates multiple variables, and is particularly suited to accounting for repeated measurements from the same individuals (Beehner and McCann, 2008; Bolker et al., 2009). Because individual is included as a random factor, pseudoreplication is prevented. Multiple independent variables were inter-correlated and therefore I generated multiple models and used the
26 lowest Akaike’s Information Criteria (AIC) value to determine which model was the best fit (Beehner and McCann, 2008; Bolker et al., 2009). Significance was set at p > 0.05.
Results
Captive Validation Study
All captive females exhibited elevated cortisol concentrations after the exam
(Figure 2.1-2.8). This increase in cortisol was statistically significant when values from all females were pooled (Mann-Whitney: U=22.00, two-tailed p=0.032, N=22). However, when individual peaks post-exam were compared to pre-exam medians, only a non- significant trend was observed (Wilcoxon signed rank=1.826, two-tailed p=0.68, N=4).
In order to include individual in my model, I used a GLMM, which works well with small sample sizes and allowed me to include individual as a random factor, with estradiol concentrations and time of measurement (before or after the veterinary exam) included as fixed factors. In the full model, the AIC was 175.67, and cortisol was significantly higher in samples after the veterinary exam (F1,19=4.805, p=0.044, N = 22).
Because fecal samples were not recovered from two individuals (Evita and Margarita) on days one and two after the veterinary exam, their peak cortisol values were likely missed
(Figures 2.5-2.6). The more complete cortisol profiles of Marielita and Rita indicate that peak cortisol concentrations were reached one day after the exam, with cortisol concentrations returning to approximately baseline by day two (Figures 2.7-2.8).
However, the magnitude of Rita’s spike in cortisol was several times greater than
Marielita’s, indicating that individual variation may play an important role in cortisol responsiveness. Estradiol did not significantly affect cortisol concentrations (F1,19=1.867,
27 p=0.191; N=22 Figure 2.9). When estradiol was removed from the model, the AIC was slightly lower (173.147) and cortisol exhibited a non-significant trend toward higher values after the exam (F1,19=3.663, p=0.073, N=22).
Wild Validation Study
As predicted, estradiol and cortisol were significantly correlated in the wild pilot samples (rs=0.53, two-tailed p<0.001, N=24; Figure 2.10). However, no significant differences were found for either estradiol (Mann-Whitney U=30.00, p=0.94, N=24) or cortisol (U=35.00, p=0.81, N=24) between lactating and cycling/pregnant females.
However, the majority of samples (N=21) were from presumably lactating females.
When a general linear mixed model was run, both reproductive state (F2,19=3.966, p=0.036, N=24) and estradiol concentration (F1,19=6.815, p=0.017, N=24) significantly predicted cortisol concentrations, and the model had an AIC of 122.781.
Wild Long-Term Study
In the long-term data set from individually recognized wild females, cortisol and estradiol were once again significantly correlated (r=0.24, p<0.001, N=134; Figure 2.11).
However, cortisol did not vary significantly based on age (U=-0.16, p=0.87, N=134), reproductive state (Kruskal-Wallis H=0.119, df=2, p=0.94, N=134: figure 12) or parity
(U=0.53, p=0.60, N=134).
Although estradiol did not significantly vary based on age (U=-0.73, p=0.46,
N=134) or parity (U=-0.25, p=0.80, N=134), it did vary significantly based on reproductive state (H=6.53, two-tailed p=0.038, df=2, N=134; Figure 2.13). Elevated concentrations of estradiol in samples from a single pregnant female account for this
28 result (Figure 14). Pregnant female Jill (JI) gave birth on or slightly before 11/15/2010, and exhibited a rise in estradiol 8-9 weeks prior to birth.
Distribution of cortisol and estradiol were further compared within the lactating and cycling states. Neither cortisol (Figure 2.15) nor estradiol (Figure 2.16) significantly varied based on infant age among lactating females (cortisol: H=0.70, df=3, p=0.88,
N=74; estradiol: H=3.60, df=3, p=0.31, N=74), although females in late lactation had greater variation in estradiol concentrations than females in early or middle lactation.
Neither cortisol (Figure 2.17) nor estradiol significantly varied by age among cycling females (cortisol: U=-0.23, p=0.82, N=56; estradiol: U-1.94, p=0.53, N=56), but there is a non-significant trend toward higher estradiol concentrations in the cycling adults versus cycling subadults (Figure 2.18). Cortisol and estradiol are significantly correlated in lactating females (r=0.35, p<0.001, N=74) but not in cycling females (r=0.14, p=0.123,
N=56).
Because these multiple variables may each affect overall cortisol or estradiol concentrations, general linear mixed models were generated to control for the effect of individual and interactions between variables. The initial cortisol model included individual as a random factor, and reproductive state, estradiol concentration, parity, age, time collected (AM or PM) and stage of lactation (depending on the age of infants) for lactating females as fixed factors. This full model had an AIC of 1,193.292 and no variable significantly predicted cortisol concentrations. The model with the lowest AIC included reproductive state, age, parity, time, and stage of lactation, and had an AIC of
1,191.648. Only parity exhibited a non-significant trend (F1,105=2.774, p=0.099, N=134;
29
Figure 2.19). No other variable exhibited any significant effect or non-significant trend
(Table 2.3).
The initial estradiol model included individual as a random factor, and reproductive state, parity, age, time collected (AM or PM) and stage of lactation
(depending on the age of infants) for lactating females as fixed factors. The model that best fit included individuals as a random factor, incorporated all of these variables, and had an AIC of 1222.27. However, none of those variables significantly explained variation in estradiol (Table 2.4). When the model was run including individual as a random factor and age, parity and time as fixed factors, parity did have a significant effect (F1,130 =4.393, p=0.038. N=134; Figure 2.20), with nulliparous females having higher cortisol concentrations. However, this model had a higher AIC of 1249.600.
Discussion
Results indicate that fecal cortisol metabolites are associated with physiological stress in female black-handed spider monkeys (Ateles geoffroyi). Data from all four females indicate elevated cortisol concentrations after the veterinary exam. Though small sample sizes limit an conclusions, data from the two best-sampled individual females in my captive study indicate that fecal cortisol peaked approximately one day after anesthesia.
Data from wild females indicate no significant differences in cortisol concentrations across reproductive states. Although conclusions for pregnant females are limited by sample size and sample collection, the number of samples collected from cycling and lactating females establish a lack of significant differences between these two
30 reproductive states. Practically, this means that reproductive state does not confound measurement of cortisol concentrations in females. Next, I discuss more details about cortisol levels in lactating and pregnant females in primates and other mammals and how they compare to my study. I discuss how lack of significant differences between the three reproductive states, and within different stages of lactation, are notable compared to reports on cortisol during lactation in primates and other mammals.
However, differences in relationships between estradiol and cortisol (estradiol and cortisol were significantly correlated in lactating females, but not cycling females, in both the pilot and full datasets) is a notable difference between cycling and lactating females.
This difference should be considered when studying lactating females. In particular, this may affect measurement of cortisol in females that are in the latter stages of lactation, in which females may be beginning to resume cycling.
Validation
Results of this study are generally congruent with similar studies that examined or validated fecal steroid metabolites in Atelines. In this study, fecal cortisol concentrations achieved peaks one day after exposure to anesthesia and the veterinary exam. Similarly,
Campbell (2000) found that fecal steroid metabolites (estrone and progesterone) lag urinary steroids by 1-2 days. This indicates that cortisol metabolites are reflected in fecal samples over a similar time scale as estrone and progesterone. Similarly, Ziegler and colleagues (1997) found that fecal estradiol metabolites lagged urinary estradiol concentrations by one day in muriquis. While the time scale of fecal collection in the current study does not provide hourly detail to compare cortisol peaks, these results
31 support time scales reported by Rangel-Negrín and colleagues (Rangel-Negrín et al.,
2009) for females.
From the results of this study, however, it is difficult to gauge the time it takes for cortisol metabolites to return to baseline after the stressor. The slightly elevated values on day 2 for Margarita and Marielita, and day 3 for Evita suggest that it may take several days for fecal cortisol concentrations to return baseline. However, the results of Rita’s cortisol profile indicate that her cortisol concentrations had returned to baseline by day 2.
This may be explained by Milton’s (1981) finds on gut passage rates. Milton (1981) determined that although the gut passage rate for spider monkeys is relatively fast (4-8 hours), some food materials remain in the digestive tract for longer than 24 hours.
Differences in food selection, individual steroid metabolism, gut passage rates, age, and rank likely contribute to variable inter-individuals rates of fecal steroid metabolite excretion in females. Furthermore, differences in cortisol excretion rates found by
Rangel-Negrín and colleagues (2009) between a male and female suggest that males’ metabolism and excretion rates of cortisol are more in line with general gut passage rates.
Females’ rate of steroid metabolism and excretion are much slower, and may be related to slower steroid catabolism within the liver. This finding, in conjunction with Milton’s
(1981) findings that some food remains in the gut passage for over 24 hours, may partially account for any remaining elevations in cortisol two to three days after the veterinary exam, even as cortisol returned toward baseline.
Findings for muriquis and spider monkeys also contrast with time lags Martínez-
Mota and colleagues (2008) report for fecal GCs in howler monkeys. They found GCs
32 show elevation after 24 hours, and peaked between 72-96 hours after anesthesia. This is consistent with differences in gut passage rate reported by Milton (1981) for spider monkeys and howler monkeys. Like Martínez-Mota and colleagues (2008), I observed inter-individual differences in peak GC levels. For example, while Rita peaked at 76.92 ng/gram, Marielita peaked at only 9.13 ng/gram.
These conclusions are limited to female spider monkeys, as cortisol metabolite excretion may differ from males. Rangel-Negrín (2009) found that male fecal cortisol concentrations peaked at 7-8 hours, well before the female’s concentrations peaked at 20-
25 hours. Furthermore, Ange-van Heugten (2009) reports captive male spider monkeys had significantly higher fecal cortisol concentrations than captive females. Whether this was due to a sex difference in cortisol secretion, metabolism, or excretion or a reflection of species-atypical housing is unclear. However, given that male spider monkeys face higher levels of stress and aggression due to both housing constraints and sex-typical behaviors (Davis et al. 2009), I suggest that future studies examining cortisol in male spider monkeys include both captive and wild specimens.
Pregnancy
Many primate studies document elevated GCs in pregnant females. Elevated GC concentrations during pregnancy occur in baboons (Gesquiere et al. 2008; Weingrill et al.
2004; Engh et al. 2006), mandrills (Setchell et al., 2008) capuchins (Ehmke, 2010;
Carnegie et al. 2011) callitrichids (Ziegler et al. 1995; Smith et al. 1997; Albuquerque et al. 2001; Ziegler & Sousa 2002; Bales et al. 2005) lemurs (Cavigelli, 1999), and humans
(Lockwood et al., 1996). However, the timing of when rises in GCs occur may vary both
33 within and across different primate taxa. In humans both maternal cortisol and fetal CRH increase with gestation age (Lockwood et al., 1996). However, despite this increase,
Kammerer and colleagues (2002) found that women in late pregnancy had a blunted cortisol response to a stress test, in contrast to non-pregnant controls. Among non-human primates, rises in GCs are reported during latter stages of pregnancy. Carnegie and colleagues (2011) found that pregnant female capuchin monkeys had significantly higher
GC concentrations compared to other reproductive states. However, this rise occurred gradually over pregnancy, with higher GC concentrations observed during the second half of pregnancy (Carnegie et al. 2011). Both captive and wild female marmosets exhibit elevated GC concentrations during pregnancy, but this elevation only occurs during late gestation, around the third trimester (Smith et al. 1997; Albuquerque et al. 2001; Ziegler and Sousa 2002). In contrast, Weingrill and colleagues (2004) found that while female baboons had elevated GCs during pregnancy, there was no increase during late gestation.
Furthermore, in wild golden lion tamarins, Bales and colleagues (2005) found that GC concentrations during the first trimester were actually lower than those observed for nonpregnant females. It was not until the third trimester that GC concentrations rose
(Bales et al. 2005).
Experience with previous offspring and offspring size also influence maternal GC concentrations during pregnancy. Primiparous females experience a greater increase in
CG concentrations during the third trimester relative to multiparous females.
Additionally, golden lion tamarin females who had larger infants experience a greater increase in GC concentrations toward the end of gestation (Bales et al. 2005). Bales and
34 colleagues (2005) hypothesize that higher concentrations of cortisol and estrogens in late pregnancy help primiparous females respond appropriately to their infants. Their hypothesis is supported by data in male cotton top tamarins (Almond et al. 2008) and female baboons (Nguyen et al. 2008) that parental rises in GCs during the latter part of gestation and the early post-partum period are associated with increased vigilance and paternal responsiveness to infants’ distress vocalizations.
Female spider monkeys have a gestation length of seven and a half months
(Chapman and Chapman 1990). In my study, Jill (JI), a pregnant female, was observed and sampled during the second half of her pregnancy, but fecal samples were not obtained during her third trimester. Fecal samples could not be obtained due to her foraging in predominantly swampy terrain. Just prior to the third trimester, between eight and nine weeks before gestation, JI’s estradiol concentrations were elevated (Figure 14).
It is likely that if I was able to adequately sample JI during the third trimester, a rise in cortisol concentrations would have been observed. In this respect, spider monkeys may follow the pattern exhibited by tamarins and marmosets, in which elevation in GC concentrations only occurs during the third trimester (Smith et al. 1997; Albuquerque et al. 2001; Ziegler and Sousa, 2002; Bales et al. 2005). However, it should also be considered that JI was a multiparous female, with an older male juvenile (Juvenile-3). If spider monkeys do follow the same pattern as the callitrichids (Smith et al. 1997;
Albuquerque et al. 2001; Ziegler and Sousa 2002; Bales et al. 2005) it would be expected that primiparous females would experience higher elevations in cortisol than multiparous females. Thus, based on this case study, I suggest that female spider monkeys experience
35 a rise in estradiol concentrations just prior to the third trimester, but do not experience any rise in cortisol concentrations until the third trimester. These conclusions are preliminary and more data are needed to adequately investigate patterns. Captive females also may be a better choice for further study, as consistent fecal sampling is difficult in wild populations.
Lactation
In this study, lactating female spider monkeys showed neither increased nor decreased cortisol responsiveness. These findings are particularly interesting given contrasting results from other studies on female mammals regarding cortisol responsiveness, and my results may shed light on the importance of cortisol responsiveness within primates. The vast majority of mammalian studies of cortisol response during lactation are from rodents, which are reported to have a suppressed cortisol response during pregnancy and lactation (Windle et al. 1997; Shanks et al. 1999;
Brunton & Russell 2003; Tu et al. 2005; Brunton et al. 2008). Altemus (1995) explains this is due to oxytocin released during lactation, which reduces stress responses and fearful behavior.
However, as Maestripieri and colleagues ( 2008) point out, this mechanism is most often studied in rodents, who often are tested without their pups. Maestripieri and colleagues (Maestripieri et al. 2008) further stress a crucial difference between primates and rodents. Rodents exhibit a relatively short period of lactation and maternal care compared to primates, so the immediate post-partum period and the total period of lactation heavily overlap among rodents. However, in primates, lactation extends far
36 beyond this initial post-partum period. Because primates are characterized by particularly long periods of lactation and infant care, the physiological, energetic, and psychosocial challenges posed by infant care greatly differ. Furthermore, whereas rodents park their infants while foraging, the majority of primates carry their dependent infants and thus their infants may be subject to potential threats from conspecifics or other animals
(Maestripieri et al. 2008).
Maestripieri and colleagues (2008) examined stress responses of free-ranging female macaques by lactating and non-lactating conditions and found no evidence of hypo-responsiveness to stress. Rather, lactating females had higher plasma cortisol concentrations in response to the stressor of capture and restraint than did non-lactating females. They hypothesize that lactating females had a stronger stress response due to the additional stressor of fear for infants’ safety.
The intricacies of the stress response during lactation may also be illuminated by considering experimental work conducted with sheep, which have longer periods of lactation comparable to those experienced by primates. Cook (1997) found that lactating sheep initially had higher baseline concentrations of cortisol compared to non-lactating sheep. However, after exposure to a stressor, this pattern was reversed, with non-lactating sheep exhibiting higher post-stressor cortisol concentrations. Additionally, Tilbrook and colleagues (2006) found that while non-lactating ewes experienced a significant rise in cortisol concentrations in response to the stresses of isolation and restraint, lactating ewes did not exhibit a similar rise in cortisol. Non-lactating ewes, lactating ewes separated from their infants, and lactating ewes whose infants were prevented from suckling all
37 experience a significance rise in cortisol in response to the stressor. However, lactating ewes whose infants were allowed to suckle did not experience a significant rise in cortisol in response to the same stressor. Of all four groups, non-lactating ewes had significantly higher cortisol concentrations than lactating ewes in all three conditions. From this study,
Tilbrook and colleagues (2006) conclude that both the presence of the infant and suckling play a role in attenuating the stress response.
Similar results have been reported in human women. Lactating women have lower cortisol responsiveness to energetic stresses, which is attributed to high oxytocin concentrations (Altemus, 1995). Furthermore, breastfeeding is reported to be protective against post-partum stress. Several studies indicate that breastfeeding mothers report less anxiety, reduced perceived stress, reduced blood pressure, reduced cortisol concentrations and more positive moods compared to bottle-feeding mothers (Wiesenfeld et al. 1985;
Mezzacappa et al. 2000; Mezzacappa et al. 2001; Groer et al. 2002).
Altemus (1995) reports that lactating women also have lower plasma concentrations of estrogens. Rodent studies indicate that lower concentrations of estrogens are associated with suppressed HPA-responses to stressors (Altemus, 1995).
Kammerer and colleagues (Kammerer et al. 2002) found that women at 8 weeks post- partum varied in cortisol response to a stress challenge, and showed an intermediate response between cycling females (who experience a significant rise in cortisol) and pregnant females (who did not experience a significant rise in cortisol). They suggest that lactation may play a role in this variability, as most, but not all of the women were breastfeeding at the time of the study.
38
In my study, lactating females had similar GC concentrations to cycling females.
This phenomenon was also observed among female baboons (Gesquiere et al. 2008).
However, the cortisol concentrations of lactating female spider monkeys exhibit less variation than those of cycling females. Additionally, although estradiol concentrations do not significantly differ, lactating females do have slightly lower means and ranges
(Figure 2.13). Given the non-significant trend between estradiol concentrations in adult and subadult cycling females, it is possible that the inclusion of subadult females in the cycling category may obscure differences between lactating females and fully adult, regularly cycling females.
Given the lack of significant differences in cortisol concentrations between lactating and cycling females, lactating female spider monkeys do not appear hypo- or hyper-responsive to stress. Rather, they appear to retain normal cortisol responsiveness.
Variation in GC concentrations in lactating female primates likely changes with ecological and social context. Both Weingrill and colleagues (2004) and Maestripieri and colleagues (2008) hypothesize that cortisol responses in lactating female primates may be largely dependent on the threat of potential infanticide, predation, or any factor that endangers offspring.
Reproductive Cycling
Estradiol and cortisol concentrations were significantly correlated in the pilot sample of wild females, who were predominantly lactating, and lactating females during the main study period. This contrasts to captive-cycling females of the pilot project and wild-cycling females during the main study period. Neither exhibited significant
39 correlation between estradiol and cortisol concentrations. Reasons for this are not clear.
Correlation between two hormones in lactating females may be due to inclusion of those with juvenile-1 offspring. They may already be experiencing rises in estradiol and resuming cycling. However, if this is the case, it is not clear why the cycling females do not exhibit significant correlations between these hormones.
Strier and Ziegler (2005) found that female muriquis need to achieve a minimum estradiol threshold before achieving reproductive cycling. For both females in late lactation and subadult females, thresholds may not be achieved yet. Hernández-López and colleagues (2010) report that estradiol concentrations were higher during ovulation for female spiders monkeys, as predicted. Furthermore, adult females had significantly higher estradiol concentrations during their ovulation, as compared to elderly females.
Similar age-related declines in estradiol are reported for rhesus macaques (Gore et al.
2004; Downs and Urbanski, 2006). Given the non-significant trend toward lower estradiol concentrations in subadult females, I suggest subadult females may not have achieved regular reproductive cycling, and have yet to reach peak fertility.
Weingrill and colleagues (2004) report that cycling females have the lowest GC concentrations compared to pregnant and lactating female baboons. Similar results are found in cotton-top tamarins (Ziegler et al. 1995). Additionally, Strier and colleagues
(Strier et al. 2003) found that female muriquis have the lowest cortisol concentrations during the mating period. Ehmke (2010) also reports that lactating female brown capuchins (Cebus apella) have higher cortisol concentrations than cycling females.
However Carnegie and colleagues (2011) found no significant differences between
40 lactating and non-pregnant, non-lactating female white-faced capuchins (Cebus capucinus). For female spider monkeys, no significant differences between cortisol concentrations were found for lactating and cycling individuals. The differences found between taxa may reflect differences in the social context that differentially affects lactating and cycling females. For example, lactating baboons and capuchins face higher risks of infanticide or predation (Fedigan 2003; Manson et al. 1994; Engh et al. 2006a;
Engh et al. 2006b). However, female-directed aggression initiated by males is well- documented among spider monkeys (Fedigan & Baxter 1984; Campbell 2000; Campbell
2003; Slater et al. 2008). While female-directed aggression does occur across reproductive states (Campbell 2003; Slater et al. 2008), Slater and colleagues (2008) report that prolonged chases are significantly more frequent when cycling females are involved. Thus, I conclude that for both lactating and cycling females, the cortisol concentrations may be influenced predominantly by social or ecological stressors.
Conclusions
Results of this study indicate that fecal cortisol metabolites reliably indicate physiological stress, and may be used to investigate stress in female spider monkeys.
Furthermore, cortisol and estradiol concentrations across reproductive states indicate that cortisol does not significantly vary, although more data are needed from pregnant females. Practically, I suggest that reproductive state is not a major confound when examining cortisol concentrations in relation to environmental or social factors.
Furthermore, for lactating and cycling females, I suggest social or ecological stressors may influence females’ cortisol concentrations more than does reproductive state.
41
However, conclusions are limited by the opportunistic nature of fecal sampling and lack of hormonal data (progesterone) to precisely determine female reproductive cycles.
Future studies of relationships between cortisol and reproductive state should utilize either captive or radio-collared animals. They also should specifically focus on 1) cortisol and estradiol concentrations throughout pregnancy and early lactation, and 2) how cortisol concentrations are affected by resumption of cycling, utilizing more frequent fecal sampling and assays of progesterone in addition to estradiol and cortisol.
42
TABLES
Individual Before PE After PE Total Chita* 5 0 5 Evita 3 1 4 Margarita 4 1 5 Marielita 5 2 7 Rita 3 3 6 Total samples 20 7 27 Total analyzed 15 7 22 Table 2.1. Fecal samples collected from captive female black-handed spider monkeys at Brookfield Zoo before and after the physical exam (PE). *Chita’s samples were excluded from analysis because no samples were collected from after PE.
Code Class Offspring age Fecal Samples AG Lactating Dorsal infant 5 AS Subadult cycling 17 AR Lactating Dorsal infant 13 BU Subadult cycling 9 DA Cycling Juvenile-2 2 EV Lactating Juvenile-1 1 FA Lactating Dorsal infant 1 HO Subadult cycling 9 IS Cycling Juvenile-2 5 JI Pregnant/lactating Juvenile-3/Ventral infant 8 JL Lactating Dorsal infant 17 LE Lactating/cycling Juvenile-1 18 MC Cycling 11 MI Cycling Juvenile-2 2 RU Lactating/cycling Juvenile-1 12 ST Cycling/pregnant/lactating Ventral infant 2 ZE Lactating Juvenile-3/Juvenile-1 2 Total 134 Table 2.2. Individually recognized focal subjects in the Pilón community at El Zota Biological Field Station, their offspring, and number of fecal samples collected. Ventral infants are approximately 0-6 months old, dorsal infants are approximately 6-12 months old, Juvenile-1 are approximately 12-24 months old, Juvenile-2 are approximately 24-36 months old, and Juvenile-3 are approximately 36-50 months old, following van Roosmalen and Klein (1988).
43
Category Definition Pregnant Determined retro-actively from birth of infant Subadult cycling Subadult based on known age or size Adult cycling No offspring/offspring greater than 24 months Early lactation Presence of newborn or ventral infant (VI) Middle lactation Presence of dorsal infant (DI) Late lactation Presence of Juvenile-1 (J1) Table 2.3. Categories of reproductive states for adult and subadult female spider monkeys at El Zota Biological Field Station, Costa Rica. Ventral infants are approximately 0-6 months old, and dorsal infants are approximately 6-12 months old, and Juvenile-1 are approximately 12-24 months old, following van Roosmalen and Klein (1988).
Effect Numerator df Denominator df F ratio p value State 2 105 0.153 0.858 Age 1 105 2.239 0.138 Parity 1 105 2.774 0.099 Time 1 105 1.628 0.205 Lactation stage 3 105 0.042 0.989 Table 2.4. Results of the GLMM assessing the effects of reproductive state (cycling, pregnant, or lactating), age (adult vs. subadult), parity (nulliparous vs. parous), time of fecal collection (am vs. pm), and lactation stage (early, middle, or late) on cortisol concentrations for wild females spider monkeys at El Zota Biological Field Station, Costa Rica.
Effect Numerator df Denominator df F ratio p value Parity 1 105 0.097 0.756 Time 1 105 0.234 0.630 State 2 105 1.31 0.274 Age 1 105 0.063 0.802 Lactation Stage 3 105 0.409 0.747 Table 2.5. Results of the GLMM assessing effects of parity (nulliparious vs. parous), time of fecal collection (am vs. pm), reproductive state (cyling, pregnant, or lactating), age (adult vs. subadult), and lactation stage (early, middle, or late) on estradiol concentrations of female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
44
FIGURES
Evita 16 14
12
10 8 13.61 6 N=1 4 8.89 Cortisolng/gram N=3 2 0 Before After
Figure 2.1. Mean cortisol concentrations before and after veterinary exam for black-handed female spider monkey Evita at Brookfield Zoo, Illinois.
Rita 35
30
25
20
15 29.15 N=3
Cortisolng/gram 10 4.07 5 N=3
0 Before After
Figure 2.2. Mean cortisol concentrations before and after a veterinary exam for adult female spider monkey Rita at Brookfield Zoo, Illinois.
45
Margarita 8 7
6 5 4 7.44
3 N=1 Cortisolng/gram 2 3.64 1 N=4 0 Before After
Figure 2.3. Mean cortisol concentrations before and after a veterinary exam for adult female spider monkey Margarita at Brookfield Zoo, Illinois.
Marielita 7
6
5
4
3 6.14 N=2
Cortisolng/gram 2 2.22 1 N=5 0 Before After
Figure 2.4. Mean cortisol concentrations for adult female spider monkey Marielita at Brookfield Zoo, Illinois, before and after a veterinary exam.
46
Evita 16 14 12 10 8 6
Cortisolng/gram 4 2 0 -3 -2 -1 3 Days before and after exam
Figure 2.5. Cortisol profile for adult female spider monkey Evita at Brookfield Zoo, Illinois, before and after veterinary exam.
Margarita 8 7 6 5 4 3
Cortisolng/gram 2 1 0 -4 -3 -2 -1 2 Days before and after exam
Figure 2.6. Cortisol profile for adult female spider monkey Margarita at Brookfield Zoo, Illinois, before and after veterinary exam.
47
Marielita 10 9
8 7 6 5 4 3 Cortisolng/gram 2 1 0 -4 -3 -2 -1 0 1 2 Days before and after exam
Figure 2.7. Cortisol profile for adult female spider monkey Marielita at Brookfield Zoo, Illinois before and after veterinary exam.
Rita 90 80
70 60 50 40 30
Cortisolng/gram 20 10 0 -3 -2 -1 0 1 2 Days before and after exam
Figure 2.8. Cortisol profile for adult female spider monkey Rita at Brookfield Zoo, Illinois, before and after veterinary exam.
48
16 14
12 10 8 6 Cortisolng/gram 4 2 0 0 50 100 150 200 250 300 Estradiol ng/gram
Figure 2.9. Scatterplot of estradiol and cortisol concentrations of adult female spider monkeys at Brookfield Zoo, Illinois. One outlying value with extremely high cortisol concentration (estradiol=133.2, cortisol=76.93) is removed to allow closer scale.
25 2 rs =0.28, p<-0.001
20
15
10 Cortisolng/gram 5
0 0 5 10 15 20 25 Estradiol ng/gram
Figure 2.10. Scatterplot of cortisol and estradiol concentrations in pilot study of wild female spider monkeys at El Zota Biological Field Station. One outlier with extremely high estradiol concentration (estradiol=445.3, cortisol 28.36) is removed to allow closer scale.
49
70.00 r2=0.058, p<0.001 60.00
50.00
40.00
30.00
Cortisolng/gram 20.00
10.00
0.00 0.00 20.00 40.00 60.00 80.00 100.00 Estradiol ng/gram
Figure 2.11. Scatterplot of the estradiol and cortisol concentrations from long-term data set of adult female spider monkeys at El Zota Biological Field Station, Costa Rica. Three outliers are removed to allow closer scale.
25 H=0.119, p=0.04, N=134
20
15
10 Cortisolng/gram
5
17.97 12.4 7.67 0 Cycling Lactating Pregnant
Figure 2.12. Mean +SE of cortisol concentrations of female black-handed spider monkeys in different reproductive states at El Zota Biological Field Station.
50
35 H=6.53, p=0.94, N=134 30
25
20
15
Estradiol Estradiol ng/gram 10
5 14.61 4.33 18.13 0 Cycling Lactating Pregnant
Figure 2.13. Mean +SE of estradiol concentrations of female black-handed spider monkeys in different reproductive states at El Zota Biological Field Station, Costa Rica.
45 40 35 30 25 20 Cortisol 15 Estradiol 10 5 0
Figure 2.14. Cortisol and estradiol profiles of pregnant female black-handed spider monkey Jill at El Zota Biological Field Station, Costa Rica. Birth is estimated to be between 11/4/2010 and 11/15/2010.
51
18 H=0.70, p=0.88, N=74 16
14
12 10 8
6 Cortisolng/gram 4 2 9.71 13.12 11.15 0 VI DI J1 Offspring age
Figure 2.15. Mean +SE of cortisol concentrations from female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica. Early lactation is denoted by VI (ventral infant: 0-6 months), middle lactation is denoted by DI (dorsal infant:6-12 months), and late lactation is denoted by J1 (Juvenile-1:12-24 months).
25 H=3.60, p=0.31, N=74 20
15
10 3.86 3.32 4.39 5
0
Estradiol Estradiol ng/gram VI DI J1 -5
-10
-15 Offspring Age
Figure 2.16. Mean+SE of estradiol concentrations from female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica. Early lactation is denoted by VI (ventral infant: 0-6 months), Middle lactation is denoted by DI (dorsal infant: 6-12 months), and late lactation is denoted by J1(Juvenile-1: 12-24 months).
52
45
40 U=-0.23, p=0.82, N=56
35
30
25
20
Cortisolng/gram 15
10
5 26.1 14.08 0 Adult Subadult
Figure 2.17. Mean +SE of cortisol concentrations in adult and subadult cycling female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
25 U=-1.94, p=0.53, N=56
20
15
10 Estradiol Estradiol ng/gram
5
14.45 6.91 0 Adult Subadult
Figure 2.18. Mean + SE of estradiol concentrations in adult and subadult cycling female black- handed spider monkeys at El Zota Biological Field Station, Costa Rica.
53
30
25
20
15
Cortisolng/gram 10
5
19.59 12.4 0 Nulliparous Parous
Figure 2.19. Mean +SE of cortisol concentrations in nulliparous versus parous female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
16
14
12
10
8
6 Estradiol Estradiol ng/gram 4
2 9.45 9.36 0 Nulliparous Parous
Figure 2.20. Mean+SE of estradiol concentrations in nulliparous versus parous female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
54
CHAPTER 3: SEASONALITY, ACTIVITY PATTERNS, AND CORTISOL IN FEMALE SPIDER MONKEYS IN A WET FOREST ENVIRONMENT
Introduction
Seasonality is a crucial factor structuring animal’s lives, seasonal patterns of temperature, daylight, and rainfall shape the ecological context, which in turn shape animal’s internal processes (Landys et al., 2006; Nelson and Drazen, 2007). Life history theories that incorporate the concept of allostasis, or stability through change, predict that animals’ physiological balance will vary based on predictable, seasonal challenges
(McEwen and Wingfield, 2003; Landys et al., 2006; Nelson and Drazen, 2007).
Because glucocorticoids (GCs), or stress hormones, serve to promote energy usage over storage by increasing availability of glucose in the bloodstream, they play a key role in mediating allostatic balance (McEwen and Wingfield, 2003; Nelson and
Drazen, 2007). Nelson and Drazen (2007:429) assert that “seasonal variation in glucocorticoid secretion is more often the rule than the exception.” In species that face greater social or energetic challenges during a mating season, GCs may rise in anticipation of these changes. For example, many birds and reptiles exhibit changes in
GC concentrations in preparation for breeding seasons (Landys et al., 2006; Nelson and
55
Drazen, 2007). Similarly, for animals that face predictable patterns of seasonal resource scarcity, GCs may elevate in anticipation of these stressors.
Species that are reliant on fruits such as spider monkeys (Ateles geoffroyi), may be affected by the predictable but nutritionally stressful periods of fruit scarcity that occur seasonally in the tropics. In addition to nutritional stress, fruit abundance can affect daily activity patterns such as the amount of time individuals can rest. Here, I examine how seasonality and activity patterns affect GC concentrations in female spider monkey feces, in order to evaluate how these factors may affect GC concentrations over the course of a year. By using non-invasive methods to measure hormones (Beehner and Whitten, 2004;
Ziegler and Wittwer, 2005), I can test hypotheses without adding acute stress through sampling.
Allostasis
While homeostasis refers to maintaining the same hormone concentrations throughout the year and over an individuals’ lifetime, allostasis refers to changing baseline concentrations in accordance with external cues, such as photoperiod or temperature, to accommodate current needs (McEwen and Wingfield, 2003). Landys and colleagues (2006) emphasize that researchers need to identify and distinguish between predictable life-history challenges and unpredictable “stressful” situations. This is crucial, because in a predictable, seasonal environment, allostasis requires altering baseline GC concentrations and thus can confound studies attempting to quantify responses to acute stress.
56
Both diurnal and seasonal changes in GCs tend to be less pronounced than GC concentrations induced by acute stressors (Landys et al., 2006). Thus, we would not expect seasonal baselines to reach the same GC peaks that arise as a result of coping with acute, unpredictable stressors, such as social aggression or predation. Landys and colleagues (2006) outline three distinct levels of physiological balance: A) a low-stress baseline, individuals face minimal predictable or unpredictable challenges, B) a challenging baseline, GCs are elevated to cope with a predictable stressor, such as seasonal food scarcity or mating season, and C) an emergency stage, GCs are extremely elevated as individuals cope with unpredictable stressors, such as predation attempts.
Seasonality in the Tropics
In tropical environments, patterns of rainfall are the most crucial factor in determining patterns of food scarcity and abundance (Levey, 1988; González-Zamora et al., 2011). However, the relationship between rainfall and fruit abundance may vary greatly across microhabitats at the same latitude. For example, in the less seasonal tropical wet forests of northeastern Costa Rica, patterns of fruiting generally follow rainfall patterns. However, in the highly seasonal tropical dry forest of northwestern
Costa Rica, fruiting peaks at the transition between the dry and wet seasons (Levey,
1988; Carnegie et al., 2011). Anthropogenic activity can also affect resource availability.
For example, in fragmented forests, spider monkeys consume more leaves due to a lower density of large trees (González-Zamora et al., 2011). Since tree size is correlated with fruit abundance (Chapman et al. 1992), fragmented forest may have lower amounts of available fruit than large contiguous forests. An exception would be when forest
57 fragments border plantations and ripe fruit such as bananas and guava provide additional food resources (personal observation).
In spider monkeys, party size is generally related to seasonal resource abundance
(Chapman 1990; Symington 1990; Chapman et al. 1995). However, spider monkeys are also characterized by high degrees of fission-fusion dynamics and may adjust party sizes based on food availability as well as social pressures (Asensio et al. 2008; Aureli et al.
2008). While resource abundance should lower the stress of finding food resources, large parties may increase the potential for competition and aggression.
Glucocorticoids and Activity Patterns
GCs play a significant role in regulating feeding behavior, locomotion, and energy metabolism (Landys et al., 2006). Activity patterns of spider monkeys can be affected by climatic variables, particularly seasonality of rainfall and ambient temperature. Additionally, correlations between GCs and feeding and locomotor behavior have been reported in studies of birds and reptiles (Landys et al., 2006). The onset of an emergency life history stage (McEwen and Wingfield, 2003; Landys et al., 2006) due to food scarcity results in a marked increase in foraging behavior. As a result, food scarcity is associated with high GC concentrations in both baboons and lemurs (Sapolsky, 1986;
Cavigelli, 1999).
A number of primate studies have recently investigated relationships between GC and activity and climatic variables. In the highly seasonal environment of Amboeseli,
Kenya, female baboons exhibit higher GCs during the dry season (Gesquiere et al.,
2008). The same pattern occurs in white-faced capuchins in the highly seasonal dry forest
58 of Santa Rosa, Costa Rica (Carnegie et al., 2011). Gesquire and colleagues (2008) found that in addition to effects of female reproductive state and individual identify, rainfall and ambient temperatures both significantly contribute to variation in fecal GC values.
Although feeding time did not predict GC concentrations, Gesquire and colleagues
(2008) note that during the dry season, females spent more time feeding, and less time resting. They suggest these patterns may account for rising GCs during the dry season.
Similarly, Weingrill and colleagues (2004) report that environmental factors accounted for most variation in cortisol concentrations in chacma baboons. Cortisol concentrations were correlated with seasonal factors, particularly amount of time engaged in rest, extreme cold temperatures, and daylight duration (Weingrill et al., 2004). Although cortisol was higher during the winter months (ie, with shorter photoperiods), daylight duration was also strongly correlated with time engaged in rest, grooming, and traveling, as well as mean monthly temperature. Weingrill and colleagues (2004) suggest that it is primarily cold temperatures that induce stress, although they were unable to tease apart the effects of daylight, activity variables, and temperature.
Cortisol in Spider Monkeys
Thus far, few studies (Ange-van Heugten et al., 2009; Rangel-Negrín et al., 2009) have examined fecal cortisol metabolites in spider monkeys. Rangel-Negrin and colleagues (2009) examined fecal cortisol metabolite concentrations across four types of wild and captive habitats. The wild habitats included “conserved habitat,” defined as continuous forests greater than 30,000 ha isolated from anthropogenic influences such as human habitation, highways, or roads, “fragmented habitat,” which were sites less than
59
200 ha in size, had heavy tourist activity, and were near human settlements, domestic animals, and roads. The captive habitats included zoo-housed individuals and privately- owned pets. They found that cortisol concentrations were lowest in conserved habitats, higher in forest fragments and zoo-housed individuals, and highest in individuals kept as pets. However, these differences were only significant when comparing conserved habitat to all other conditions. Furthermore, they found that cortisol concentrations were significantly lower during the wet season in the conserved habitat. However, no seasonal difference in cortisol concentrations were found for individuals living in fragmented forests or the zoo.
Ange-van Heugton and colleagues (2009) examined how fecal cortisol metabolites varied in zoo-housed spider monkeys that were subject to institutional differences in dietary regimes. They found that individuals housed at zoos with the highest dietary percentage of carbs, sugars, and fruit had the highest cortisol concentrations. These findings suggest that seasonality of fruit availability may affect cortisol concentrations in two contrasting ways. While high fruit abundance in the wild may reduce foraging and nutritional stress, higher rates of consumption of sugars and carbohydrates in captive individuals increase cortisol concentrations. Captive diets may contribute to high cortisol in conjunction with overnutrition, inadequate nutritional composition, and reduced activity levels. However, captive individuals’ lives vary from wild individuals in two crucial ways. First, captive diets may differ substantially from the wild fruits, leaves, and insects that comprise wild diets (Ange-van Heugten et al., 2009).
This is supported by the fact that the zoos with the most nutritionally complete diets had
60 the lowest cortisol concentrations. Second, captive individuals generally spend more time resting and socializing, and less time traveling or foraging, than wild individuals
(personal observation). As a result, the combination of high carbohydrate and sugar consumption with low activity levels may put captive individuals at risk for what
McEwen and Wingfield (2003) refer to as “Type 2” allostatic overload, which results in excess energy storage. Conversely, since wild animals consume less energy-dense food in conjunction with high activity levels, they are at risk for “Type 1” allostatic overload, in which energy usage exceeds energy income and storage. Each of these types of allostatic overload are associated with different health consequences. For wild animals, Type 1 allostatic overload, which results in low body mass and suppression of reproduction, is a more likely outcome.
Research Objective and Hypotheses
Here, I examine monthly patterns of fecal cortisol concentrations in wild female spider monkeys living in secondary forest and connected fragments at a tropical wet forest site in Costa Rica. Over a one-year period, I collected data on fruit abundance, party size, and activity variables in females from one habituated community. I hypothesized that fruit abundance, party size, and mean cortisol concentrations would significantly differ across seasons, and that fruit abundance would be the strongest predictor of monthly cortisol concentrations. Additionally, I hypothesized that increased time engaged in foraging and traveling would be associated with higher cortisol concentrations, while increased time engaged in resting would be associated with lower cortisol concentrations.
61
METHODS
Study Site
Data were collected at El Zota Biological Field Station in Costa Rica, from June 2010 to August 2011, as discussed in Chapter Two. Dense vegetation, multiple canopy layers, stilted tree roots, and evergreen buttress trees characterize this type of wet tropical forest
(Holdridge et al., 1971; Janzen, 1983). Areas that are perpetually inundated show less evidence of disturbance than drier areas, and have more large emergent trees (Lindshield,
2006). The understory of tropical wet forest is generally denser than tropical moist forest, and species richness is high (Holdridge et al., 1971). Lindshield (2006) reports that the forest canopy is dominated by Pentaclethera macroloba (oil tree), which also dominates the well-studied wet forest of La Selva Biological Field Station 40 km away. The tree canopy is 25-35m high, with emergent trees 35-60m in height (Lindshield, 2006).
Seasonally inundated swamp gaps and marshes are characterized by herbaceous peace lilies, Spathiphyllum friedrichsthalii (Lindshield, 2006). The secondary forest contains thick undergrowth consisting of palms and herbaceous vegetation, including Socretea, a type of walking palm, and Heliconia (false bird of paradise), respectively. Gallery forest is abundant due to the presence of many streams and creeks, and has an abundance of
Ficus (fig trees) and Inga (ice cream bean), similar to other riparian habitats (Lindshield,
2006) . The secondary forest has a mosaic pattern of disturbance, with harvestable monocultures of exotic Gmelina aborea (Beechwood) and native Ochroma pyramidale
(Balsa) along the main road, with scattered Carapa guianensis (crabwood), Hyeronima alchorneoides (Pílon), and Cordia alliodora (Spanish elm: Pruetz and LaDuke, 2001;
62
Lindshield, 2006). The southern portion of the station also includes plantain and banana plantation (Musa) which is left for wildlife to forage in, a vegetable and fruit garden that is perodicially used by monkeys foraging for bananas and guava, and former pastureland
(Lindshield 2006; personal observation).
Behavioral Data Collection
The focal subjects were 17 individually-recognized subadult and adult females living in the Pílon community. For full description of of community size and range, see Chapter
2. Behavioral data were collected on all adult and subadult females present in the Pilón community (Table 3.1). I conducted 10-minute scan samples (Altmann, 1974) of party size and composition and 10-minute focal samples with two minute instantaneous sampling intervals on the females between 5:30 am and 6:00 pm. Upon party encountering parties, I identified the females of the party, and selected a focal subject.
Initially, the first female identified was the subject of the first focal follow; however, after data collection became unequal, I chose the least-sampled female for the first focal follow. Subsequently, each other female in the party would be followed, and I alternated sampling each female in the party. If only one female focal subject was present, consecutive follows were conducted to maximize data collection. Although this may compromise independence of results, consecutive follows did not yield significantly different activity budgets than non-consecutive follows (feed: Wilcoxon signed rank
W=1.000, p=0.317, rest: W=10.000, p=0.961, social: W=18.000, p=0.498, travel:
W=9.500, p=0.544, N=14). Focal samples were dropped and excluded from analysis if there were more than three out-of-sight (OS) observations during the focal period.
63
During these sampling periods, the following information was collected: 1) time of day,
2) location in the forest (trail or nearest trails) 3) identity and activity of focal animal, 5) type of social interaction 6) initiator/recipient of social interaction. Activity categories include feeding/foraging, resting, traveling, social behavior, and other (Table 3.2). Social behaviors were further divided into affiliative behaviors (grooming, social play, embrace, huddle, touch, and whinny) and agonistic behaviors (avoid, displace, chase, harass, fight, and display). Immediately after any aggressive encounter (chase, harass, or fight), a 10- minute focal follow was conducted on the recipient of aggression. During this time, all- occurrences of social interactions were recorded.
Behavioral data were collected on write-in-the-rain check sheets and entered daily into Excel files. Additionally, whenever possible, predominant fruits eaten by foraging parties were recorded ad libitum. These observations were used to calculate usage of fruit resources across seasons. Approximately 715 contact hours and 186 focal hours were obtained from 17 focal animals, although the majority of behavioral data were collected on 14 adult and subadult females. Although total amount of focal hours was low compared to many primate studies, it is comparable to data obtained in other studies on spider monkeys, due to the constraints of Ateles arboreal, fast-moving lifestyle and fission-fusion dynamics. In a meta-analysis of data from 22 studies on spider monkeys,
Gonzalez-Zamora and colleagues (2009) report that the mean amount of hours per study was 188± 223. Excluding the three under-sampled individuals, the mean focal hours collected per individual in my study was 12.74, with a range of 5.2-23.2 hours. When considering only females with adequate behavioral and hormonal data (i.e., more than 30
64 complete focal samples and more than five fecal samples), the 11 females had a mean of
14.72 focal hours (range: 5:2-23.2 hrs) and 11.27 fecal samples (range: 5-18) per individual. This is comparable to focal hours per individual obtained in other studies on spider monkeys. For example, Ahumada (1992) obtained 12.4 hours per individual,
Campbell (2000) obtained 15.8 hours per individual, and Slater and colleagues (2009) obtained 13.6-14.2 hours per individual. The range of focal hours per individuals is also comparable to Campbell’s (2000) study, in which total focal hours ranged from 6.83-
20.17 hours.
Ecological data collection
Since spider monkeys are highly frugivorous, and because most research indicates that fruit availability determines grouping patterns, only fruit abundance was measured.
Diameter at Breast Height (DBH), which is at approximately 1.4 m is a commonly used measure that is accurate for estimating fruit abundance (Chapman et al. 1992). Although more detailed measures exist (Chapman et al. 1992; Pruetz 2009), DBH provides an estimate suitable for comparing relative annual variation. Vegetation plots were established using a modified version of the 0.1 ha method (Gentry, 1982). Twenty 2m by
50m plots were established throughout the secondary forest (Figure 3.1), based on randomized global positioning coordinates (GPS). In some cases, plot locations were moved from the randomized coordinates in order to adequately sample all types of forest present (swamp, gallery, plantation), and to ensure transects could be traversed in all seasons (seasonal inundation makes some areas impassable). All trees (or lianas) with
DBH >10cm were marked and numbered in each plot. In August 2010, the DBH of each
65 tree in the plots were measured and recorded. Whenever possible, the genus of the tree was identified, although many not identifiable until fruiting. A fairly comprehensive list of spider monkey feeding trees at El Zota has been compiled by Lindshield (2006) and was used to identify feeding trees. Twice a month, all marked trees in the transect plots were assessed for the presence of fruit and flowers. Biweekly fruit abundance indexes were calculated by summing the DBHs of all trees with fruit. Although the presence of all fruit and flowers were recorded, only fruit were counted in fruit abundance calculations, and fruits that were known to be inedible (Pentaclethera and Ochroma) to the spider monkeys were excluded. The year was then divided into “wet” and “dry” seasons based on rainfall patterns from Limon, Costa Rica in 2009 accessed from WorldClim (Hijmans et al., 2005). “Wet” months (Jan, April-Sept, December) were those with 250-500 mm of rainfall, while “dry” months (Feb-Mar and Oct-Nov) were those with 150-250 mm of rainfall.
Hormonal Data Analysis and Collection
Hormones were extracted from fecal samples in the field following the protocols of Ziegler and Wittwer (2005) with modifications suggested by Ehmke (pers comm).
Samples were assayed using enzyme-immunoassay (EIA) techniques at the Wisconsin
National Primate Core Assay facility by Dan Wittwer. For full descriptions of fecal collection and hormone extraction and analysis, see Chapter Two.
Statistical Analysis
Cortisol concentrations in the morning versus afternoon were compared using a
Mann-Whitney U-test. Although mean cortisol concentrations were normally distributed
66 after log-transformation, activity variables did not fit parametric assumptions after square root transformation. Consequently, non-parametric Spearman’s rank was used for an analysis of time engaged in activity variables and mean cortisol concentrations over the total study period. Because the activity variables are not independent of each other, a
Bonferroni correction was applied, and results for this test were considered significant if p>0.0125. Only the 11 individuals that were adequately sampled both behaviorally (30 or more completed focal samples) and hormonally (five or more fecal samples) were included in this analysis. A general linear mixed model (GLMM) was used to examine the effects of ecological factors and activity budget variables on mean monthly cortisol concentrations. See Chapter Two for full description of this method. Two-tailed p-values are reported for all tests.
Results
A list of fruits consumed and temporal consumption and availability is in Table
3.3. Fruits consumed were Spondias (hog plum), Hyeronima (Pílon), Ficus (fig), Socretea
(walking palm), Iriartea (stilt palm), and Diptyerx (almendra). Spondias was consumed
September-November and July-August. During these months, Spondias comprised 22-
96% of all fruit consumed (Figure 3.2). Hyeronima was consumed in September and
June-August. During these months, Hyeronima comprised 13-50% of all fruits consumed
(Figure 3.3). Ficus, which fruits asynchronously, was consumed in November, January, and May. During these months, Ficus accounted for 20-59% of total fruit consumption
(Figure 3.4). Palm fruits (Socretea and Iriartea) were consumed in November-December,
March, and July-August. During these months, these fruits accounted for 8-33% of total
67 fruit consumption (Figure 3.5). Dipteryx was consumed December-January, and accounted for 12-33% of fruit consumption for these months (Figure 3.6). Other fruits consumed included Musa (banana), Inga (ice cream bean), Pithecoctenium (monkey comb), Psidium (guava), and Virola (fruta dorada).
Fruit abundance was low September-December, but remained high January-
August (Figure 3.7). Mean fruit abundance did not significantly differ between the wet and dry seasons (Mann-Whitney U=22.000, p=0.368, N=12; Figure 3.8). Party sizes remained fairly constant September-May, but increase June-August (Figure 3.9). Party size did not significantly differ between wet and dry seasons (U=15.000, p=0.933, N=12;
Figure 3.10). Cortisol concentrations were variable over the course of the year (Figure
3.11). Mean cortisol concentration did not significantly differ between the wet and dry seasons (U=20.000, p=0.497, N=12; Figure 3.12). Cortisol concentrations were higher in the morning, but did not significantly vary between morning and afternoon (U=-1.28, p=0.20, N=134; Figure 3.13).
Individuals averaged 33.53% of their time feeding, 23.03% resting, 29.80% traveling, 6.20% socializing, and 7.54% out of sight or engaged in “Other” behaviors
(Figure 3.14). Time spent engaged in feeding ranged from 27.90% to 43.97%, resting ranged from 17.84% to 31.47%, traveling ranged from 20.27% to 39.17%, socializing ranged from 0.99% to 17.3%, and time out-of-sight or engaged in other behaviors ranged from 4% to 11.57 % (Table 3.4; Figure 3.15).
Time engaged in rest and mean cortisol concentration were negatively correlated
(Spearman rank: rs=-0.737, p=0.010. N=11; Figure 3.16). However, there was no
68 relationship between feeding (rs=0.260, p=0.440, N=11), travel (rs=0.474, p=0.474,
N=11) or social behavior (rs=0.014, p=0.968, N=11) and mean cortisol concentrations.
To examine relative effects of all variables combined, I used a GLMM test with
‘individual’ as a random factor, and monthly values of travel, rest, feed, and social behavior, along with fruit abundance, party size, and season (wet vs. dry months) as fixed factors. In the full model, only rest was statistically significant, and the model had an AIC of 635.269 (Table 3.5). Using the AIC method (see above), the best fit model incorporated only rest as a fixed factor (AIC = 77.060). Rest was significantly associated with cortisol concentrations (F1,61= 9.234, p=0.003, N = 75), with cortisol concentrations inversely related to time spent resting.
Discussion
Results of this study indicate that in this mildly seasonal environment, time engaged in rest most strongly relates to cortisol concentrations. Surprisingly, despite variation in fruit abundance and party size across seasons, neither variable was significantly associated with cortisol concentration. I suggest this because in this environment even with relatively “low” fruit abundance many choice food species remaining prevent any seasonal periods of resource scarcity. In this highly productive environment, absolute amount of fruit may be less important than types of food available, and widespread availability of several keystone foods resources may prevent food scarcity from being a major stressor. To better understand factors contributing to inter- individual variability in cortisol concentrations, one must assess factors determining resting time to explain mechanisms underlying relationships between rest and cortisol.
69
Climatic variables affecting activity budgets
Climatic variables can affect variability in activity budgets either directly or indirectly. For example, Bronikowski and Altmann (1996) found baboons relying on anthropogenic sources of food (garbage around tourist lodges) were not affected by variation in rainfall and temperature as wild-foraging baboons were. They suggest that any effects of weather result from secondary effects on food availability rather than direct effects on activity patterns.
In a meta-analysis of the relationship between activity variables and climatic variables across multiple studies of spider monkeys, Gonzalez-Zamora (2011) found that time engaged in rest had an inverse relationship with average monthly rainfall: when rainfall was increased, resting decreased. Conversely, feeding increased with rainfall, while travel time decreased with both increased rainfall and higher maximum temperatures. Overall, climactic variables best explained variation in travel time.
Additionally, they found time resting was significantly higher in seasonal forests, whereas time traveling was significantly higher in forests without pronounced seasonality. Finally, in fragmented forests, monkeys spent more time feeding and less time resting.
Gonzalez-Zamora and colleagues (2011) report average feeding times of
38.4±14.0%, average resting times of 36.6±12.8%, and average traveling times of
19.8±11.3%. The female spider monkeys of El Zota spend comparable time feeding
(33.53%) as those at these other sites. However, time resting at El Zota is on the low end of the spectrum (23.03%), while traveling is on the higher end (29.8%). The relationship
70 found between cortisol and rest in the present study may reflect that, on average, the female spider monkeys of El Zota have less available resting time compared to female spider monkeys at other sites, and thus resting time may be at a premium.
Observed activity budgets are comparable to those reported by Lindshield (2006) for female spider monkeys of the Pilón community in 2005-2006. Lindshield reports that in the disturbed (secondary habitat) of El Zota, female spider monkeys spent 48% feeding, 16% traveling, and 21% resting. Conversely, in the primary habitat, female spider monkeys spent only 24% feeding, 12% travelling, and 41% resting. Furthermore,
Lindshield (2006) observed socializing infrequently, and thus included this in her “other” category. The combined time of feeding and traveling in the present study (63.33%) and the previous study (64%) are similar, as are time engaged in rest. Lindshield (2006) reports much greater resting times for individuals in the primary forest versus the secondary forest, which supports Gonzalez-Zamora’s (2011) findings. This suggests that individuals living in the primary forest are less constrained by foraging and travel costs.
Future studies should incorporate the primary community, as comparisons of cortisol concentrations and activity variables may elucidate if individuals in the secondary habitat are subject to greater stressors due to the nature of the disturbed habitat.
Comparisons to other habitats in the literature are complicated by the fact that El
Zota represents a habitat that falls in between other researchers’ definitions. Gonzalez-
Zamora and colleagues (2011) and Rangel-Negrin and colleagues (2009) describe forests as continuous/protected vs. fragmented/unprotected. Gonzalez-Zamora and colleagues
(2011) describe two types of forests: one, fragmented forests, were all smaller than 31 ha,
71 whereas the other, continuous forest, was greater than 15,000 ha. Similarly, Rangel-
Negrin and colleagues (2009) describe two types with slightly different definitions: fragmented/unprotected forests were smaller than 200 ha, whereas conserved habitats were greater than 30,000 ha. Although El Zota has a total of 1000 ha of land, 700 ha of which is forested, the Pilon community ranges through approximately less than half of this land. Furthermore, their range includes mixture of secondary forest and is fragmented by anthropogenic activity. Lindshield (2006) notes that because of the swampy nature of the site, forest classified as secondary does include patches that are primary forest. Furthermore the forest classified as “primary” does have a history of selective logging. Thus, the divisions between primary and secondary at this site are somewhat arbitrary. Rather, it is better understood as having a continuum of disturbance
(Lindshield, 2006). Better descriptions of the levels of disturbance at each habitat may allow more accurate inter-site comparisons.
Diet and fruit abundance
Although cortisol concentrations do vary throughout the year, they are not significantly affected by differences in rainfall. In this environment, even the time periods of lowest fruit abundance offer key fruit resources. Gonzalez-Zamora and colleagues
(2011) considered tropical wet forest (greater than 2800 mm rainfall a year, sensu Gentry
1982) as “non-seasonal.” However, as data from my study indicate, fruit abundance does vary across the year. Thus, it is more accurate to consider this type of forest as “mildly seasonal” rather than “non-seasonal.” Nonetheless, in wet forest environments, seasonality is far less pronounced than in tropical moist or dry forest. This may be why
72 cortisol concentrations did not vary between the “wet” and “dry” seasons in my study.
Adequate rainfall during the driest months may facilitate the availability of choice fruit resources year-round, and prevent the periods of fruit scarcity that are experienced in highly seasonal forests.
The dietary breadth over the course of the year suggests that availability of key fruit resources may be more important than total fruit abundance. Fruit abundance was lowest in September-December, but for most of this time period Spondias provided a fairly constant food source. Because Spondias trees provide large fruit patches that gradually ripened over weeks, this species provided high-quality food resources over several months. Additionally, the secondary forest has a high density of Hyeronima, a native tree species that was originally planted for human harvesting. However, given the secondary forest was left to regenerate, this tree species was a key food resource for several months for spider monkeys.
González-Zamora and colleagues (2009) report that spider monkeys have a broad range of dietary variation from year to year. Variation in diet from year to year is similar to variation observed between different communities in neighboring forests. In previous field seasons, the female spider monkeys foraged frequently on Castilla elastica (Panama rubber tree) during the summer. However, over the course of the present study, they were not observed to utilize this resource. Similarly, in past field seasons, and at other field sites, Cercropia (guarumo) fruits are an important resource. Cercropia trees are common in this forest, and several transects frequently had fruiting Cercropia trees.
Nonetheless, the spider monkeys were not observed utilizing this resource over the
73 course of the present study. This suggests that the fruits over the course of this study do not encompass the full range of fruits consumed by the spider monkeys.
Data on fruit consumption and dietary breadth in this study should be considered preliminary. The most frequent method of recording fruit consumption in other studies is focal animal sampling. However, because social behavior and fruit abundance, rather than diet and foraging, were the major priorities of this study, I used ad libitum recording.
Thus, this data is not comparable to studies specifically focusing on diet. Furthermore, in
Campbell’s (2000) study, scan sampling of fruit eaten resulted in a much larger list of fruits eaten at Barro Colorado Island, Panama, than what is reported for other sites, where focal animal sampling was utilized. Thus, I suggest that future studies on the diet of spider monkeys at El Zota utilize both focal and scan sampling of fruit consumption, in order to broaden our understanding of which resources shape their foraging patterns and activity variables.
Gonzalez-Zamora and colleagues (2011) suggest that spider monkeys living in highly seasonal forests are subject to more stressful conditions. Thus, future research on cortisol concentrations in spider monkeys should include more seasonal environments, such as tropical moist and dry forest. I hypothesize that spider monkeys living in tropical dry forest would have the highest cortisol concentrations, whereas those living in mildly seasonal forest such as El Zota should have the lowest concentrations. Furthermore, while cortisol concentrations do not significantly different between the “dry” and “wet” seasons of El Zota, I would expect significant seasonal differences in cortisol concentrations for spider monkeys living in tropical moist and tropical dry forest.
74
Rest and Cortisol
In their study of baboons, Weingrill and colleagues (2004) found that cortisol concentrations were significantly associated with time engaged in rest. They suggest that the extra time available to engage in rest may reduce stress by improving the predictability and control of possible stressors. This may occur through multiple mechanisms. The baboons may be avoiding heat stress or decreasing chances of predation or aggression from conspecifics based on choice of resting spots. However,
Weingrill and colleagues (2004) also note that time engaged in rest may directly affect cortisol metabolism. Because cortisol is such a crucial hormone in regulating energetic balance, I suggest that this may be the most important mechanism. Landys and colleagues
(2006) describe physiological state A, the least challenging and lowest-stress baseline, as the allostatic balance of undisturbed individuals engaged at rest. Thus, by their definitions rest is the least challenging and least stressful state. Thus, it seems logical that animals that are able to engage in greater amounts of rest are able to maintain low stress levels.
Additionally, adequate resting time may indicate that individuals are not under foraging or social stress. Because cortisol is strongly associated with wakefulness (Nelson and
Drazen, 2007), it may also be that when individuals have lower cortisol concentrations, they are less motivated to seek out activity and more likely to engage in rest. However, with the correlational data used in this study, the direction of causality cannot be determined. Furthermore, because physiological regulation of hormones relies so heavily on both positive and negative feedback, it is likely that the causality may work in multiple directions.
75
When animals spend more time foraging, another behavior must be decreased to compensate. In baboons, Bronikowski and Altmann (1996) found that increased foraging time results in decreased time engaged in social behavior. Altmann (1980) suggests animals should reduce social time in response to increased foraging time,. Dunbar and
Dunbar (1988) argue they should reduce resting time, rather than social time. However,
Bronikowski and Altmann (1996) found amount of time engaged in social behavior was flexible. In my study, individuals who spent the greatest amount of time engaged in social behavior (17.3%) and the least amount of time socializing (0.99%) both engaged in similar amounts of time feeding (27.9% and 28.65%) and traveling (33.00% and
33.33%), but differed greatly in resting time (27.90% vs. 15.14%). This suggests that there are flexible trade-offs between social and resting time. For energetic and metabolic purposes, a minimum amount of rest is likely necessary to maintain health. Furthermore, social time can be decreased as necessary, and made up when there are fewer energetic demands. Given that rest seems to have a strong relationship with cortisol concentrations in the present study, increasing resting time is likely to have greater health benefits.
Conclusion
Results of this study indicate that cortisol concentrations do not vary seasonally in relation to fruit abundance or party size. Rather, time engaged in rest is the only factor significantly correlated with cortisol concentrations. I suggest that the reason that cortisol and fruit abundance are not significantly related is due to two factors: first, tropical wet forest experiences very mild seasonality; second, the temporal availability of key fruit resources, such as Spondias, may be more crucial than overall fruit abundance in
76 affecting spider monkey energetics and physiological balance. Furthermore, this study provides data on activity budget that allows comparison to data generated at other sites, and provides preliminary data on the diet of spider monkeys at El Zota Biological Field
Station. I suggest that further studies on this population utilize scan sampling to more accurately document diet and foraging behavior. Additionally, future studies should incorporate a comparison of cortisol concentrations and activity budgets between the communities located in the primary and secondary forest.
77
TABLES
ID Focals Hours Fecals AG 62 10.3 5 AS 81 13.5 17 AR 115 19.2 13 BU 74 12.3 9 DA 15 2.5 2 EV 30 5.0 1 FA 17 2.8 1 HO 79 13.2 9 IS 31 5.2 5 JI 75 12.5 8 JL 132 22.0 17 LE 140 23.2 18 MC 72 12.0 11 MI 37 6.2 2 RU 110 18.5 12 ST 17 2.8 2 ZE 31 5.2 2 Table 3.1. Distribution of focal samples, hours of observation, and fecal samples per individuals for black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
Behavior Definition Travel Movement within the crown of a tree or between the crowns of trees that is not related to food acquisition or social interaction Rest A period of inactivity Feed/forage Any localized movement within the tree crown associated with the procurement, handling of, and ingestion of food items Social Any behavior that involves interaction or contact with conspecifics or other animals Other Any behavior that does not fall into the above categories; includes observer-directed behavior and object manipulation Table 3.2. Behavioral catalogue for focal follows of female black-handed spider monkeys at El Zota Biological Field Station. Adapted from van Roosmalen & Klein (1988); MCDaniel (1994); Rodrigues (2007), and personal observations.
78
Fruits Consumed Months eaten Temporal Availability Spondias (hog plum) Sep-Nov/July-Aug Sep-Nov, April-Aug Hyeronima (pilón) Sep/June-Aug Sep/June-Aug Musa (banana/plantain) Sep-Feb/July year-round Ficus (fig) Nov/Jan/May sporadic Socretea (walking palm) Nov-Dec/July-Aug year-round Iriartea (stilt palm) Nov-Dec/July-Aug year-round Dipteryx (almendra) Dec-Jan Dec-Mar Inga (ice-cream bean) Jan-Mar unknown Pithecoctenium (monkey comb) Feb-Apr Feb-Apr Psidium (guava) March-May unknown Virola (fruta dorada) May-June May-June Epiphyte fruits (unknown genera) Jan-May year-round Table 3.3. Fruit consumption of female black-handed spider monkeys and temporal availability from Sept 2010-Aug-2011 in the secondary forest of El Zota Biological Field Station, Costa Rica. Oct-Nov and Feb-Mar are dry months (<250mm rain), whereas all other months are wet months (>250 mm rain).
79
ID Feed % Rest % Travel % Social % Other/OS % AG 35.81 19.68 32.26 3.55 8.71 AS 27.9 27.90 33.33 0.99 9.88 AR 29.39 24.87 30.61 8.77 6.54 BU 39.73 17.84 29.19 6.49 6.76 EV 31.33 26.00 36.00 2.66 4.00 HO 32.91 14.43 37.47 6.08 9.11 IS 29.67 30.20 27.10 5.81 7.10 JI 30.67 31.47 20.27 6.13 11.57 JL 34.55 23.64 28.79 7.88 5.15 LE 38.86 27.57 19.57 8.81 5.29 MC 30.28 17.78 39.17 3.61 9.17 MI 28.65 15.14 33.00 17.3 5.95 RU 35.82 21.45 28.55 6.18 8.00 ZE 43.87 24.51 21.93 2.58 7.09 Table 3.4 Individual activity budgets for female spider monkeys at El Zota Biological Field Station, Costa Rica.
Effect Numerator df Denominator df F-ratio p-value Travel 1 5 2.340 0.132 Feed 1 5 1.857 0.179 Rest 1 5 9.302 0.004 Social 1 5 1.690 0.199 FA 1 5 0.147 0.703 Party Size 1 5 0.709 0.404 Season 1 5 0.103 0.749 Table 3.5. Results of GLMM assessing effects of activity variables, fruit abundance (FA) party sizes, and season on cortisol concentrations of female black-handed spider monkeys at El Zota Biological Field Station.
80
FIGURES
Figure 3.1. Map of the secondary forest of El Zota Biological Field Station and transect locations for assessing bimonthly fruit abundance.
81
Spondias
1.2
0.96 1
0.8
0.6 0.51 0.53
0.4 0.31 0.22 0.2
Proportionoffruit tal consumed 0 0 0 0 0 0 0 0 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug
Figure 3.2. Monthly consumption of Spondias by female black-handed spider monkeys at el Zota Biological Field Station, Costa Rica, from Sept 2010-Aug 2011.
Hyeronima 0.6
0.5 0.5 0.47
0.4 0.33 0.3
0.2 0.13 0.1 Proportionconsumedoffruit 0 0 0 0 0 0 0 0 0 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug
Figure 3.3. Consumption of Hyeronima fruit by female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica, from Sept 2010-Aug 2011.
82
Ficus 0.7
0.59 0.6 0.54 0.5
0.4
0.3 0.2 0.2
0.1 Proportionconsumedoffruit 0 0 0 0 0 0 0 0 0 0 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug
Figure 3.4. Consumption of Ficus fruits by female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
Palm
0.35 0.33
0.3
0.25 0.22 0.2 0.2
0.15 0.13
0.1 0.08
0.05 Proportionconsumedoffruit 0 0 0 0 0 0 0 0 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug
Figure 3.5. Consumption of palm fruits (Socretea and Iriartea) by female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica, from Sept 2010-Aug 2011.
83
Dipteryx
0.35 0.33
0.3
0.25
0.2
0.15 0.12 0.1
0.05 Proportionfonsumedoffruit 0 0 0 0 0 0 0 0 0 0 0 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug
Figure 3.6. Consuption of Dipteryx fruit by female black-handed spider monkeys at el Zota Biological Field Station, Costa Rica.
900
800
700
600
500
400
Fruit Abundance 300
200
100
0 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug
Figure 3.7. Monthly fruit abundance in the secondary forest of El Zota Biological Field Station, Costa Rica, from Sept 2010-Aug 2011. Oct-Nov and Feb-March are dry months. Sep, Dec-Jan, and Apr-Aug are wet months.
84
700 U=22.000, p=0.368, N=12 600
500
400
300 573.61
Fruit abundance 460.16 200
100
0 Wet Dry
Figure 3.8. Mean + SE of fruit abundance in the wet versus dry seasons at El Zota Biological Field Station, Costa Rica, from Sept 2010-Aug 2011.
4.5 4 3.84 3.4 3.5 3 2.51 2.38 2.34 2.5 4.26 2 2.11 2
PartySize 2.34 1.5 2.17 1.82 2.27 1 0.5 0
Figure 3.9. Monthly mean sizes for female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica from Sept 2010-Aug 2011. Oct-Nov and Feb-Mar are dry months; all other months are wet months.
85
3 U=15.000, p=0.933, N=12 2.5
2
1.5 2.75
Partysize 2.37 1
0.5
0 Wet Dry
Figure 3.10. Party sizes between the wet and dry seasons for female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica. Wet months are Oct-Nov and Feb- Mar; all other months are wet months.
20 18.62 18.41 18 16 14.77 14.37 14 12 15.68 9.83 10 10 9.21 12.33 7.6 8 6.59
Cortisolng/gram 6 4 3 2 0
Figure 3.11. Monthly mean cortisol concentrations for female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
86
18 U=22.000, p=0.497, N=12 16
14
12
10
8 12.47
Cortisolng/gram 6 10.17 4
2
0 Wet Dry
Figure 3.12. Mean+ SE of cortisol concentrations of female black-handed spider monkeys in the wet and dry seasons. Dry months are Oct-Nov and Feb-Mar; all other months are wet months.
25
U=-1.28, p=0.20, N=134 20
15
10 17.92
5 10.99 Cortisolconcentration (ng/gram)
0 AM PM
Figure 3.13. Mean+ SE of cortisol concentrations from female black-handed spider monkeys at el Zota Biological Field Station, Costa Rica, in the morning versus afternoon.
87
7.54 6.2
33.53 Feed Rest Travel
29.8 Social Other/OS
23.03
Figure 3.14. Mean activity budgets for female black-handed spider monkeys in the secondary forest of El Zota Biological Field Station, Costa Rica.
100% 90% 80% 70% Other/OS % 60% Social % 50% Travel % 40% Rest % 30% Feed % 20% 10% 0% AG AS AR BU EV HO IS JI JL LE MC MI RU ZE
Figure 3.15. Individual activity budgets for female black-handed spider monkeys in the secondary forest of El Zota Biological Field Station, Costa Rica.
88
40
35
30 2 rs = -0.54, 25 p=0.010 20 15
10 Meancortisolng/gram 5 0 10 15 20 25 30 35 % engaged in rest
Figure 3.16. Scatterplot of mean cortisol values and time engaged in rest over the entire field season (Jun 2010-Aug 2011) for individual female black-handed spider monkeys at El Zota Biological Field Station.
30
25
20
15 Cortisol (ng/gram) 10 Rest (min/hr)
5
0 1 2 3 4 5 6 7 8 9 10 11 12 Month
Figure 3.17. Monthly cortisol concentrations and resting rates for individual female black- handed spider monkeys at El Zota Biological Field Station, Costa Rica, from Sept 2010-Aug 2011.
89
CHAPTER 4: FRIENDSHIP AMONG FEMALE SPIDER MONKEYS
Introduction
“The extent of bonding among unrelated human females is highly subjective and hotly debated, but the mere existence of female bonding among nonrelatives, whether or not it is universal, makes human females unusual among the primates”—Amy Parish (1996:64)
Bonding among female primates has generally been considered within the context of kin-biased coalitionary alliances (Wrangham, 1980; Van Schaik, 1989; Sterck et al.,
1997; Isbell and Young, 2002). Traditionally, social relationships among unrelated females are considered weak, and some theories on the evolution of human social relationships theorize that females are not “naturally” able to bond with one another due to patterns of predominantly patrilocal residence patterns (Hrdy, 1981; Ghiglieri, 1987;
Rodseth et al., 1991; Parish, 1994, 1996). Parish (1994; 1996) highlights bonding among unrelated bonobos, as well as human females, as exceptions to this general pattern.
However, the strength of female bonding in bonobos is debated, as some argue that it may an artifact of studying social relationships predominantly in captivity rather than in the wild (Stevens et al., 2006).
90
Nonetheless, while bonds among unrelated females have been considered rare, the slow life histories of primate females, in conjunction with intelligence and sociality, should foster reciprocal altruism (Trivers, 1971, 2006; Silk, 2002). While bonds among unrelated females have often been overlooked in favor of kin relationships, studies of female social relationships indicates that even unrelated females can form close bonds in a variety of primate species (Silk, 2002; Swedell, 2002; Silk et al., 2006 b; Lehmann and
Boesch, 2008, 2009). Silk (2002) defines ‘friendship” as close social relationship between unrelated individuals, characterized by high rates of association and affiliation, tolerance, and reciprocity (see also Seyfarth 1977 and Smuts 1985).
Friendship between unrelated individuals can have a number of advantages, including coalitionary support, tolerance at feeding sites, and reduced stress (Taylor et al.,
2000; Van Schaik and Aureli, 2000; Silk, 2002) Data on affiliation among human females led Taylor and colleagues (2000) to hypothesize that female primates have evolved the “tend-and-befriend” mechanism, in which females cope with stress by affiliating with other females and tending to offspring as a means of counteracting the stress-response. However, data to support this hypothesis is derived primarily from primatological literature based on matrilineal cercopithecines, psychological studies focused on Western human females in laboratory settings, and endocrinological research conducted on rodents. However, if this mechanism is indeed an evolved ancestral strategy shared by most of the primate order, rather than a more recently derived trait or artifact of cultural factors, it should be exhibited by unrelated females in phylogenetically distant species. Because female spider monkeys are characterized by female dispersal, sex-
91 segregated association patterns and some degree of choice of whom to associate with, they present an ideal case to test this hypothesis.
Spider monkeys are characterized by male philopatry, so female spider monkeys are generally unrelated (Symington, 1990; Di Fiore et al., 2009). Like female chimpanzees, female spider monkeys are often classified as “weakly bonded,” although in chimpanzees there is a range of intraspecific variation in strength of social relationships (Wrangham, 1980; Van Schaik, 1989; Isbell and Young, 2002). For example, West African female chimpanzees are reported to be more closely bonded than
East African female chimpanzees (Lehmann and Boesch, 2009). I want to expand on previous research to see if the same variation exists among female spider monkeys.
One of the most robust findings across spider monkey sites is that males initiate coalitionary aggression against females (Fedigan and Baxter, 1984; Campbell, 2003;
Gibson et al., 2008; Slater et al., 2008). Perhaps because of this aggression, female spider monkeys tend to spend much of their time alone or in the company of other females and offspring, and thus are often characterized as sex-segregated (Fedigan &
Baxter 1984; Slater et al. 2009). Thus, other unrelated adult females and their offspring are often a female’s most likely social partners. Female spider monkey social relationships represent a conundrum: are they merely associating for protection from predation or infanticide or are they “weakly bonded” and affiliating with some individuals more than others? Or, perhaps like in chimpanzees, some adult female spider monkeys selectively form some close bonds?
92
Stress and Coping Mechanisms
Physiological stress is measured using glucocorticoids (GCs), but other hormones can affect GC concentrations (Taylor et al., 2000; von der Ohe, CG, Servheen, 2002).
During pregnancy, high concentrations of estradiol can promote a marked increase in
GCs and associated binding factors. Conversely, oxytocin serves to inhibit release of
GCs. This hormone is associated with birth and lactation, and aspects of social bonding
(Uvnäs-Moberg, 1998). Engaging in social affiliation increases oxytocin levels, which consequently inhibit GC concentrations. Furthermore, research on humans indicates that vocal contact with a social affiliate can trigger release of oxytocin without tactile stimulation (Seltzer et al., 2010). Thus, I hypothesize that affiliative vocal contact should have the same effect as physical contact in spider monkeys. The most frequent vocalization in spider monkeys is the whinny, an affiliative contact call (Fedigan and
Baxter, 1984; Ramos-Fernández et al., 2011). This vocalization may potentially be used as a form of “vocal grooming,” in which individuals substitute affiliative vocalizations for more time-intensive forms of affiliation, such as grooming (Dunbar, 1993, 1998).
Research on humans demonstrates several social factors affecting stress response.
Whereas loneliness and bereavement are associated with increased GC concentrations, social support and physical contact can reduce the stress response (Taylor et al., 2000;
Heinrichs et al., 2003; Segerstrom and Miller, 2004). Humans involved in personal conflict are twice as susceptible to the common cold as counterparts without such social conflict, whereas greater social integration is associated with reduced susceptibly to colds
(Cohen, 2004).
93
Such findings have been mirrored by studies on non-human primates, both in terms of the causes of psychosocial stress and coping mechanisms that relieve it. For example, baboons that lose close kin to predators are more likely than others in the group to have increased GCs. Of females that lose a kin member, those that increase their grooming networks return more quickly to baseline levels (Engh et al., 2006 a; b). The potential for infanticide is a major stressor for female primates. Studies have documented that lactating females exhibit elevated GCs relative to other females in response to takeovers by immigrant males (Beehner et al. 2005; Engh et al. 2006b; Cristóbal-
Azkarate et al. 2007). Thus, although direct aggression raises glucocorticoid levels
(Wallner et al., 1999), for cognitively sophisticated animals, threat of aggression alone is sufficient to elevate physiological stress levels. Thus, I predict that time spent in association with males, and patterns of female-directed male aggression, will be correlated with cortisol concentrations in female spider monkeys.
Limitations of the Tend-and-befriend Hypothesis
Taylor and colleagues’ (2000) sociological model may be too simplistic. First, much of their human research is limited to situations artificially created in laboratory settings and may not be representative of how people behave in normal social settings.
Second, they draw most of their evidence from Western psychological studies. Thus, it is possible that the “befriending” that they document is simply due to sociocultural pressures favoring politeness. While Taylor and colleagues (2000) assert that cross- cultural evidence supports this hypothesis, they base this on studies demonstrating that females in various cultures are more likely than men to seek and provide help (Edwards,
94
1993). However, this evidence does not directly support their assertions because providing help is not the same as seeking social affiliation.
The tend-and-befriend hypothesis is intended to demonstrate an evolutionary basis for human female bonding and coping strategies. However, in contrast to many other primate species that form cohesive social groups, humans have a highly flexible, dispersed social organization (Geary and Flinn 2002; Aureli et al. 2008). Furthermore, the consensus on human social organization indicate that humans have a tendency toward patrilocal societies with weak social networks among females, and that this may be indicative of the ancestral hominid condition (Manson and Wrangham, 1991; Rodseth et al., 1991; Geary and Flinn, 2002; Rodseth and Novak, 2006). Under these conditions, human females would be predominantly unrelated to other females in their adult lives.
While dispersed, patrilocal societies are rare among the primate order, they are found among our closest relatives, the chimpanzees and bonobos (Goodall 1986; Stumpf
2007; Aureli et al. 2008). In both species, males reside in the natal community, while most females disperse. Bonobo females, despite being immigrants to their adult communities, form strong social bonds with other females (Parish, 1994, 1996; Parish and De Waal, 2000; Stumpf, 2007). While chimpanzee females are generally considered weakly bonded, recent evidence has indicated that there is a great deal of variation. At some sites, such as Täi National Park, female chimpanzees do exhibit strong bonds despite their lack of kinship ties (Lehmann & Boesch 2008; Lehmann & Boesch 2009).
Furthermore, female chimpanzees are often found in “neighborhoods,” which are regions
95 of the total community where female’s core areas overlap (Williams et al., 2002;
Kahlenberg et al., 2008 a; b).
Studies on chimpanzees and bonobos are valuable for providing an evolutionary context to examine human patterns, but they can only document patterns characteristic of the African apes. Furthermore, female bonding in chimpanzees may be limited by long periods of infant and juvenile dependence, in conjunction with high travel costs and risks of aggression to offspring (Otali and Gilchrist, 2005; Pontzer and Wrangham, 2006). In order to evaluate whether the tend-and-befriend pattern is part of a wider pattern, one that may be ancestral or influenced by convergent evolution, this question should be addressed in species phylogenetically distant from humans with similar patterns of philopatry and social organization. Spider monkeys are a New World species that exhibit fission-fusion social organization in conjunction with male philopatry and female dispersal. By examining this question in spider monkeys, we can focus on the selective pressures that shaped social relationships and coping responses in a convergent social organization. I predict that while most female spider monkey dyads will have weak bonds, some dyads will have close bonds, similar to what is reported for chimpanzees.
Fission-fusion Social Dynamics
Fission-fusion communities provide a unique and flexible solution to dealing with the costs and benefits of sociality (Otali and Gilchrist, 2005; Aureli et al., 2008).
Presumably, greater choice of party size and social partners should allow individuals to avoid stressful conspecifics and range with close associates. While female spider monkeys are reported to have weak social bonds, a large degree of variability has been
96 demonstrated. Some females, but not all, show close affiliative bonds with other females
(Symington, 1990). Furthermore, some research indicates that these bonds are formed after immigration, when new females travel with resident females to familiarize themselves with the new habitat (Webster and Suarez, 2008).
Based on the predictions of the tend-and-befriend hypothesis, as well as patterns of female bonding reported for chimpanzees and bonobos, I predict that female spider monkeys will: 1) exhibit variation in the strength and number of social bonds, and 2) engage in affiliative behavior as a means to cope with stressors. The relationship between affiliation and cortisol concentrations may depend on the temporal nature of affiliation.
For example, if females engaged in consistent rates of affiliation throughout the study duration, I would expect that females that exhibited higher rates of affiliation and closer social bonds should have lower mean cortisol concentrations compared to females with low rates of affiliation and weaker bonds. If this is the case, I expect that females with higher rates of affiliation and closer social bonds will have consistently low cortisol concentrations throughout the study period, while less social individuals should have chronically elevated cortisol concentrations. Alternatively, if females do not engage in consistent rates of affiliation, but rather, seek out affiliation with other females as a response to stressors, I would expect that females that have higher cortisol concentrations would engage in higher rates of affiliative behavior. If female spider monkeys follow this pattern, I would expect females to engage in varying rates of affiliation over the study period, exhibit a pattern of cortisol spikes that return to baseline values, and engage in higher rates of affiliative behavior after experiencing social stress such as aggression.
97
Methods
Details on study site, description of focal subjects and community, and methodology for data collection and hormonal analysis are discussed in Chapters 2 and 3.
Focal subjects
During my study, one female, EV and her juvenile daughter disappeared, and were last seen on 9/2/2010. Another female, AS, immigrated into the community and was first seen on 8/29/2010. Two female subadults were observed within the first months of fieldwork, but were not seen again. It is assumed that these female emigrated. One older juvenile male was found dead due to injuries on 2/13/2011. He is believed to be the offspring of female JI, who displays chronically elevated cortisol concentrations shortly after this event. Two infants were born over the study period. Female JI gave birth to a female infant between November 4, 2010- November 15, 2010. Female ST was observed with a ventral infant on April 10, 2011, although the approximate birth date is unknown because she was infrequently sampled.
Behavioral Data Collection
Activity categories include feeding/foraging, resting, traveling, social behavior, and other (Table 4.2). Social behaviors were further divided into affiliative behaviors
(grooming, social play, embrace, huddle, touch, and whinny: Table 4.3) and agonistic behaviors (avoid, displace, chase, harass, fight, and display; Table 4.4). Immediately after any aggressive encounter (chase, harass, or fight), a 10-minute focal follow was conducted on the recipient of aggression. During this time, all-occurrences of social
98 interactions were recorded. Twice-weight association indices (Ginsberg and Young,
1992; Figure 1) were calculated for each dyad, based on time spent in the same subgroup.
Statistical Analysis
I first used non-parametric statistics to test hypotheses in a univariate manner.
Spearman’s rank is used for correlations, and Mann-Whitney U was used to test difference between groups. I then used a general linear mixed model (GLMM) to examine effects of social variables on mean monthly cortisol concentrations. The methodology for this statistical test is discussed in Chapters 2 and 3. Two-tailed p-values are reported for all tests, with significance considered p<0.05, and non-significant trends considered p<0.10.
Results
Time spent affiliating with females was significantly positively correlated with mean cortisol concentrations (rs =0.829, p=0.002, N=11 females; Figure 4.2). When I examined each subtype of affiliative behavior separately, I found that time spent huddling
(rs=0.780, p=0.005), grooming (rs=0.800, p=0.003) and playing (rs=0.753, p=0.007) with other females all positively correlated with mean cortisol concentrations. Because cortisol metabolite concentrations peak up to 24 hours after a stressor (see Chapter Two), females were engaged in higher rates of affiliation on the day after experiencing stress, while GCs are still beginning to return to baseline levels (as defined by mean values over the course of the study). This is supported by patterns of affiliation on days when cortisol spikes higher than 20 ng/gram. Rates of affiliation on these days exhibited a trend toward being higher than baseline values (Wilcoxon signed rank: W=4.00, p=0.090, N=8; Figure 4.3).
99
The total amount of huddling (rs =-.264, p=0.433), grooming (rs =-0.036, p=0.915), and playing (rs=0.451, p=0.164) with all social partners (in other words, including adult males and immatures) was not correlated with mean cortisol concentrations. Offspring care behaviors (carry, nurse, bridge) were not correlated with mean cortisol concentrations (rs=0.179, p=0.702, N=7). Time spent affiliating with offspring was also unrelated to mean cortisol concentrations (rs =0.071, p=0.879).
In the initial general linear mixed model, individual was included as a random factor, and monthly values of grooming, huddling, play, embraces, total affiliation, agonism received, rest (see Chapter Two) and association with males were included as fixed factors, while cortisol concentration was the dependent variable. When individual was included as a random factor, and monthly values of huddling, total affiliation, and rest were included as fixed factors, all three variables significantly predicted cortisol concentrations (Rest: F1,58=13.269, p=0.001; Huddle: F1,58=5.229, p=0.026; Affiliation:
F1,58=4.635, p=0.035). This model had an AIC of 79.657. However, the model with the lowest AIC included only rest as a fixed factor. In this model, which had an AIC of
77.060, rest significantly predicted cortisol concentrations (F1,73=9.324; p=0.003).
Time spent in association with males ranged greatly, from 3% to 50% of females’ focal time (Table 4.6), but was not significantly related to mean cortisol concentrations
(rs=0.288, p=0.391, N=11).Time spent in parties with males versus with only females was not significantly related to mean cortisol concentrations (rs=0.290, p=0.390, N=11;
Figure 4.4). Only two females (AS and RU) engaged in grooming or huddling with adult males. These females had some of the highest association rates (21%) with males. The
100 female with the highest association rate with males (50%) did not groom or huddle with them.
Aggression directed by males ranged from 0 to 0.19 events/hr (no female initiated agonistic behaviors toward males). Females in all three reproductive conditions received aggression. Of those individual females that never received aggression during the study, all were cycling or lactating. However, aggression was not significantly related to mean cortisol concentrations (rs=-0.183, p=0.589, N=11; Figure 4.5).
Additionally, female-directed agonism initiated by males and female differed qualitatively. Agonism initiated by males was often coalitionary, incorporating two or three males, more severe, and resulted in extreme fear responses by female subjects, including screaming, carrying independent juveniles dorsally, and rapid fleeing toward the ground or away from the feeding patch. For this reason, it was characterized as
“aggression.” Agonism initiated by females was usually quicker, more subtle, and did not provoke extreme fear responses in female victims. Furthermore, only one instance of female agonism involved a coalition of two females; in this case, one of the initiators embraced the recipient before chasing her, and the other initiator embraced the recipient immediately after the chase.
Aggression and agonism received from both males and females vs. rates of embraces approached significance (rs=0.59, p=0.056, N=11; Figure 4.6). Four out of the seven females that received agonism or aggression had increased affiliation post-conflict compared to baseline (overall mean) levels, while the other three females engaged in no
101 post-conflict affiliation (Table 4.7). However, the difference between baseline and post- conflict rates of affiliation did not reach significance (W=17.000, p=0.612, N=7).
Rank was assigned as dominant, mid-ranking, submissive, or unknown based on rates of female-female agonism (Table 4.8). Individuals who initiated agonism toward other females, but received no agonism where defined as dominant. Individuals that received agonism from other females but did not initiate it were defined as subordinate. If agonism initiated and received from other females was equivalent, a female was considered mid-ranking. If no female-female agonism was observed, rank was considered unknown. Linear rankings could not be determined, which is typical for spider monkeys.
Because the relative ranks of the mid-ranking and unknown animals compared to the submissive and dominant animals was not clear, they were not included in this analysis.
Five females were identified as dominant. The two individuals that were classified as subordinate were both subadults. Although the cortisol concentrations of submissive animals was higher than dominant animals (Figure.4.7), this difference was not statistically significant (U=8.000, p=0.245, N=7 females).
Out of 135 potential dyadic associations, only 79 dyadic pairs associated together at all, and for most of these pairs, association was still rare. Association indices ranged from 0.01-0.28, with a mean of 0.05 for all dyads that associated. Number of associates per individual ranged from 3-13, and was positively correlated with cortisol concentrations (rs =0.815, p=0.0002, N=11; Figure 4.8).
Of the females that were observed in association, 42 dyads did not engage in any affiliative behavior, and their association indices ranged from 0.01-0.13 (Table 4. 9), with
102 a mean of 0.04. Thirty-one dyads did engage in affiliative behavior (Table 4.10). Their affiliation indices ranged from 0.01-0.28, with a mean of 0.07. Dyads that engaged in affiliative behavior had significantly higher association indices than those that did not engage in affiliation (U=341, p<0.0001, N=79; Figure 4.9).
Among the 32 dyads that engaged in affiliation, total affiliation (including grooming etc) and association index were significantly correlated in dyads that engaged in affiliation (rs=-0.354, p=0.047, N=32; Figure 4.10). Individuals that affiliated with other females were considered “friends.” Number of friends was not significantly related to mean cortisol concentrations (rs=0.311, p=0.352, N=11; Figure 4.11). Nineteen dyads engaged in embraces, while only eleven dyads engaged in grooming, eleven engaged in huddling, and six engaged in play. Patterns of association and types of affiliative behavior were not consistent among dyads. For example, the dyad with the highest affiliation index, JL-LE, engaged in grooming and embraces but not huddling or play.
Only two dyads engaged in all four types of affiliative behavior, and both involved subadult female HO. This female was the individual to receive the highest rates of agonism from other females. Of females that engaged in affiliative behavior, grooming ranged from 0.04-0.47 events/hr, with a mean of 0.05 events/hr. Huddling ranged from
0.03-0.38 events/hr, with a mean of 0.04 events/hr. Play ranged from 0.02-0.24 events/hr, with a mean of 0.02. Embraces ranged from 0.02-0.18 events/hr, with a mean of 0.04.
Total affiliation ranged from 0.03-0.71 events/hr, with a mean of 0.16.
Mean individual rates of whinny vocalizations were significantly correlated with mean estradiol concentrations (rs=0.818, two-tailed p=0.002, N=11; Figure 4.12). Mean
103 individual whinny rates (rs =-0.755, two-tailed p=0.007, N=11; Figure 4.13) were significantly negatively correlated with mean grooming rates. Mean estradiol concentrations (rs =-0.764, two-tailed p=0.006, N=11; Figure 4.14) were significantly negatively correlated with mean grooming rates.
Discussion
Results of this study suggest that female spider monkeys do use affiliation with other females as a coping mechanism to reduce stress. The finding that female affiliation increases on days when individuals experience spikes in cortisol concentrations supports this conclusion. In particular, the finding that it was only affiliation with female conspecifics, rather than total affiliation, suggests that this pattern is limited solely to affiliative interactions with other females. Furthermore, the high range of variability in association and affiliation patterns supports the conclusion that female spider monkeys form highly differentiated social relationships.
Thus, female spider monkeys do appear to follow some of the predictions of the tend-and-befriend hypothesis. While there evidence does not support tending of offspring as a means to reduces stress, it does support befriending among females as a coping mechanism. However, the finding that the number of associates was positively correlated with cortisol concentrations can be interpreted in multiple ways. It is possible that increasing number of associates, as opposed to friends, may increase stress. Conversely, this finding might be due to increased association with female conspecifics as a response to stress.
104
Affiliation in Spider Monkeys
Even for dyads with the highest affiliation indices, affiliation was still comparatively rare. This is similar to patterns observed in other studies of spider monkey social behavior (Fedigan and Baxter, 1984; Chapman, 1990; Symington, 1990; Ahumada,
1992; Aureli and Schaffner, 2008; Slater et al., 2009; Rebecchini et al., 2011). However, the significant relationship between affiliative behaviors and cortisol concentrations, in conjunction with the sampling regime and relationship between cortisol spikes and affiliative behavior, suggest that females are preferentially choosing affiliation when stress is high, but associating with minimal affiliative interactions when stress is low.
These patterns suggest that association with other females may likely be due predominantly to factors such as threat of male coalitionary aggression, predation, socialization benefits to offspring, or a result of random aggregation. However, the positive affiliative relationships and the patterns they exhibit suggest that females are choosing affiliation as a means to cope with stressors.
Under captive conditions, female spider monkeys often affiliate at much higher rates (Pastor-Nieto, 2001). I suggest that the low rates of affiliative interactions between female spider monkeys are a result of time constraints posed by their frugivorous diet and concomitant foraging and ranging needs. Korstens and colleagues (2006) found that time was the major constraint on subgroup size, as larger parties required more traveling time.
Thus, time sets an upper limit to the number of available social partners, and may constrain sociality. Furthermore, even within small parties, it is likely that the time necessary for foraging and travelling constrains the amount of available time individuals
105 have for resting and socializing. Slater and colleagues (2009) report that females spent significantly time more feeding, while males spent significantly more time socializing.
Because foraging and nutritional benefits are more crucial to female reproductive success
(Wrangham, 1980) females may be constrained in their available time for socializing.
Personality
Relationship strength greatly varied among dyads, and while some individuals had a comparatively large number of associates and friends, other females had fewer associates and friends. There may be a couple different underlying factors contributing to this pattern. It may be that females that experienced spikes in cortisol concentrations choose to associate and affiliate with other females at a higher rate. Thus, strong tendencies towards affiliation may be a result of experiencing more frequent stressors.
Alternatively, the differences in female sociality may be due to “personality” differences. Personality is defined as individual tendencies or traits that remain stable over time (Seyfarth et. al. 2012). Research on female personality in baboons indicates that personality affects both stress and presumably reproductive fitness (Seyfarth et al.,
2012). Using principle component analysis to analyze behavioral data, Seyfarth and colleagues (2012) identified three personality types. Individuals were grouped into three personality types: “Aloof,” “Loner,” and “Nice.” While “Aloof” and “Nice” personality types had no significant relationship with GC concentrations, “Loners” had significantly higher GC concentrations. Additionally, “Aloof” and “Nice” females had more stable social bonds over time. While “Nice” individuals had the strongest dyadic relationships, they had lower partner stability over time. Contrastingly, “Aloof” females had weaker
106 relationships, but had higher partner stability. While more data, and particularly longitudinal data, is necessary to apply similar analyses to the spider monkeys, this may be a productive research avenue in the future to better determine the factors affecting GC concentrations in the female spider monkeys.
Patterns of grooming, embraces, and vocalizations
The role of grooming in New World monkeys has been debated. In particular,
Schaffner and Aureli (2005) argue that embraces have a more crucial role in regulating spider monkey social relationships. Female spider monkeys with young infants receive more embraces after the birth of their offspring, and Slater and colleagues (2007) compare this finding to biological markets documented in baboons. Furthermore, embraces are significantly more likely to occur during fusion events (Aureli and
Schaffner, 2007). Additionally, Rebecchini and colleagues (2011) report that embraces are associated with risk.
In my study, more dyads engaged in embraces than any other type of affiliative behavior. However, the closest dyads did exhibit other affiliative behaviors, including grooming, huddling and play. The finding that these three affiliative behaviors co-vary, and exhibit a relationship with cortisol concentrations, suggests that they play a different role than embraces in maintaining social relationships. I found that rates of embraces have a positive relationship with rates of agonism, and this finding approached significance. I suggest that embraces serve to mediate tense situations such as fusion events, resolve conflicts, and test bonds. This is supported by the finding that embraces
107 were the only affiliative behavior that did not inter-correlate with the other predominant affiliative behaviors, grooming, huddling, and play.
While some studies have documented stronger grooming relationships between female-male dyads than between female-female dyads in spider monkeys (Fedigan and
Baxter, 1984; Symington, 1990; Slater et al., 2009) others, such as my study have documented stronger grooming relationships between female-female dyads compared to male-female dyads (Ahumada, 1992). Furthermore, studies have also documented that embraces and arm-wrapping are preferentially directed to same-sex social partners (Slater et al., 2009). Rates of embraces are reported to be higher for male-male dyads, which has been interpreted as a result of the high level of risk in male relationships (Rebecchini et al., 2011).
The relationships between grooming rates, whinny rates, and estradiol concentrations may provide insight into the proximate factors that influence vocalizations, and may provide support for Dunbar’s (1993;1996) “vocal grooming” hypothesis. In my study, I found that whinny rates are significantly positively correlated with mean estradiol concentrations. This suggests that estradiol may play a role in regulating these vocalizations. Whinnies are an affiliative contact call, and previous studies have indicated a pronounced sex difference in whinny production rates (Fedigan and Baxter, 1984; Ramos-Fernández, 2005; Slater et al., 2009). Estradiol is a hormone associated with mate-seeking behavior and female reproductive cycling. However, whinny rates did not differ across reproductive states. If this vocalization was specifically associated with attracting mates, we would expect cycling females to engage in higher
108 rates of this vocalization. I suggest instead that whinnies may provide a form of “vocal grooming” and can be substituted for more time-intensive forms of affiliation. The inverse relationship between grooming rates and whinny rates support this conclusion.
Seltzer and colleagues (2010) found that in humans, vocal contact can produces similar oxytocin increases to those that occur with tactile stimulation. It is possible affiliative primate vocalizations, such as whinnies, can provide a similar effect. Thus, female spider monkeys may engage in higher rates of whinny vocalizations as a substitute for grooming and other affiliative behaviors. The relationship between estradiol and whinny rates also suggests that this hormone may play a role in regulating whinny production, and could potentially underlie the pronounced sex differences observed in other studies. However, more research is needed on the relationships between reproductive hormones and whinny production rates in both sexes in order to evaluate this hypothesis.
Social Bonds in Species with Female Dispersal
Swedell (2002) argues that the major reason that female hamadryas baboons were considered to have “undeveloped and undifferentiated” relationships was simply because female social interactions had never been a focal point for previous studies. Furthermore, group sizes may also play a role, as she notes that previous study sites had smaller one- male-units (OMUs) and thus females at those sites had limited social opportunities. One of her findings was that female sociality was stronger in OMUs with more females, simply because there were opportunities for this relationships to emerge (Swedell, 2002).
However, even within OMUs of the same size, there was a range of variation in the strength of female social relationships.
109
In many respects, the bonds between female spider monkeys converge with the female bonds observed in chimpanzees. In particular, patterns of male coalitionary aggression and sex-segregation are most similar to the social factors that shape chimpanzee societies. Like female hamadryas baboons female chimpanzees were initially considered to be asocial and weakly bonded (Goodall, 1986; Ghiglieri, 1987; Stumpf,
2007; Lehmann and Boesch, 2008). However, in captivity, as well as at some field sites, female chimpanzees display some evidence of female bonding, and female-female associations are as strong as male-male associations (Baker and Smuts, 1994; Lehmann and Boesch, 2008, 2009; Langergraber et al., 2009). Lehmann and Boesch (2008) suggest that the conclusion that female chimpanzees were “asocial” emerged in part because males were more frequently focal subjects, due to both habituation differences and project focus. Additionally because male dyads exhibited higher rates of the parameters studied, such as grooming, female relationships were considered weak in comparison
(Lehmann and Boesch, 2008).
This may also be the case for spider monkeys. Although several studies have focused on social relationships, and particularly in specifically comparing male bonds versus female bonds (Fedigan and Baxter, 1984; Slater et al., 2009; Rebecchini et al.,
2011), previous studies have not specifically examined variation in female-female relationships. Most research on social bonds in spider monkeys focus on comparing female-female and male-male bonds (Fedigan and Baxter, 1984; Slater et al., 2009).
Given the likely kin relationships and coalitionary behavior exhibited by males, it is unsurprising that in comparison, females are considered “weakly bonded.” However, the
110 findings of this study, in conjunction with other studies that report sex-segregated association patterns (Fedigan and Baxter, 1984; Chapman, 1990; Hartwell, 2010) indicate that female-female associations are far stronger than female-male associations for this species. In this study, female spider monkeys spent comparatively little time with their male conspecifics.
These patterns pose an interesting scenario, because most theories of primate social bonds focus exclusively on either kinship or intersexual bonds. For example, most adaptive explanations for bonding among female bonobos have focused on intersexual bonds (Parish, 1996). Essentially, female bonds were considered a by-product of the need to simultaneously attract males, rather than as adaptive in their own right. However
(Parish, 1994, 1996) demonstrates strong female bonds among captive female bonobos.
These bonds are interpreted to have a number of advantages unrelated to attracting and affiliating with males. Based on comparisons of demographic data between captive bonobos and wild chimpanzees, Parish (1996) hypothesizes that female bonds in bonobos provides greater access to food resources that facilitates earlier age of first birth, and consequently, higher lifetime reproductive success.
The bonds between female spider monkeys have some notable differences compared to female bonobos. In particular, female bonobos have strong dominance hierarchies, while dominance hierarchies are largely undifferentiated among female spider monkeys. Although several studies, including the current one, have identified high-ranking versus low-ranking females the dispersed nature of spider monkey communities decreases contest competition and the formation of linear dominance
111 relationships (Van Roosmalen and Klein, 1988; Chapman, 1990). The only differentiated dominance relationships are likely between established resident females and new immigrants or natal subadults (Asensio et al., 2008; Aureli and Schaffner, 2008).
Furthermore, female coalitionary behavior is crucial to bonobo society, while female coalitions are less common among spider monkeys. In addition to typical indicators of relationship strength, such as grooming and close proximity patterns, female bonobos also engage in sexual behavior, and this has traditionally been considered a major factor in female bonding for this species.
Factors affecting Inter-site Variation
Like hamadryas baboons and chimpanzees, inter-site differences in environmental pressures and demographic composition may underlie some variation in strength of female social relationships at different sites (Swedell, 2002; Lehmann and Boesch, 2008,
2009). Most research on spider monkey social relationships come from a single field site,
Punta Laguna, Mexico. At this site, female social relationships are reported to be weaker, while male-female association rates and interactions were higher than I found in my study (Aureli and Schaffner, 2007, 2008; Slater et al., 2007, 2009; Ramos-Fernández et al., 2009; Rebecchini et al., 2011). Additionally, at this site, rates of embraces are relatively high, while grooming rates are comparatively low (Slater et al., 2007, 2009;
Rebecchini et al., 2011).
Because party size and composition is so strongly affected, and in some cases, limited, by ecological factors (Symington, 1990; Chapman et al., 1995; Korstjens et al.,
2006; Aureli and Schaffner, 2008), we would expect social opportunities, and thus the
112 formation of social bonds, to vary with these factors. This factor likely accounts for variation in grooming relationships between sites and species. Skewed sex ratios across sites may also be a factor in variability between sites. Most sites report a female-biased sex ratio, with the ratio of females to males increasing from birth to adulthood (Chapman et al., 1989 a; Rebecchini et al., 2011). Higher mortality rates for juvenile and subadult males, sometimes as a result of intra-community aggression, are presumed to account for these differences (Chapman et al., 1989 b; Campbell, 2006; Valero et al., 2006; Gibson et al., 2008). In the present study, the body of a juvenile-3 male was found with injuries, which is consistent with patterns reported at other sites. As a result of higher mortality rates, the presence of more females than males in adulthood increases the potential for female-female relationships. Ramos-Fernandez and colleagues (Ramos-Fernández et al.,
2009) found that females have higher and more stable association indices than males, and formed the core of spider monkey societies. However, female association indices were similar to what would expected by random aggregation. Thus, they suggest that female associations are driven largely by passively aggregating around food sources (Ramos-
Fernández et al., 2009). Furthermore, only new immigrants were peripheral to the core female social network, and associated at equal rates with male and female partners.
Potential Stressors
Surprisingly, association with males and aggression received from males had no significant relationship to cortisol concentrations. Furthermore, patterns of post-conflict affiliation were mixed. Some females that received agonism engaged in high rates of post-conflict affiliation, whereas some female engaged in no post-conflict affiliation.
113
While the causes of spikes in cortisol concentrations could not be determined, potential stressors include threat of male coalitionary aggression, female-female aggression, presence of predators, and/or threats to their offspring. Chapman and colleagues (1989) report that at Santa Rosa, Costa Rica, female spider monkeys direct aggression toward their female conspecifics juvenile male offspring. Furthermore, subadults females are reported to face the highest rates of agonism from female conspecifics (Asensio et al.,
2008). In this study, the two females that were classified as subordinate and received the highest rates of agonism from female conspecifics were both subadults.
Overall, male association and aggression had no relationship to mean cortisol concentrations. However, it is possible that incidents of female-directed aggression were missed. Because these attacks occur so quickly, often the female recipients were not located until after screams and distress vocalizations alerted observers to their presence.
Furthermore, often females would engage in distress vocalizations for no apparent reason. It is possible that these vocalizations were the result of previous aggression prior to leaving the subgroup, or threats of predators that were not observed.
While tayras (Eira barbara) would likely pose little threat to a full-sized adult spider monkey, they may pose a threat to infants and juveniles. For example, at this site, an observer witnessed an adult female (LE) chase a tayra away from two immatures that were playing (her own juvenile and the infant son of her close associate JL: A. Sukiennik, pers. comm.). Furthermore, at the nearby site of La Suerte Biological Field Station, another observer saw a tayra aggressively pursue an immature spider monkey and her mother (A. Nekaris, pers. comm). Tayra predation attempts have also been documented
114 in howler monkeys (Asensio and Gómez-Marín, 2002; Camargo and Ferrari, 2007).
Thus, it is likely that, if mothers and other individuals are not vigilant, tayra can pose a lethal threat to immatures. Additionally, over the course of the study, jaguar tracks were intermittently observed throughout the range of the Pilon community, and a young jaguar was observed twice by a researcher conducting night surveys (M. Hoffman, pers. comm).
I also may have heard the vocalizations and movements of a jaguar during the day, and the spider monkeys I was observing immediately emitted alarm calls. Successful predation of adult spider and howler monkeys by jaguar have been documented at sites in
Venezuela and Colombia (Peetz et al., 1992; Matsuda and Izawa, 2008). For example at
Guri Lake in Venezuela, jaguar predation resulted in the deaths of five howler monkeys out of a group of six (Peetz et al., 1992). At La Macarena in Columbia, a jaguar killed an adult spider monkey directly in front of a human observer (Matsuda and Izawa, 2008).
Although such predation attempts are rarely documented, jaguars likely pose the biggest historical threat to spider monkeys at El Zota.
Implications for Primate and Human Evolution
The findings of the current study have significance for understanding inter- specific and intra-specific variation and re-evaluating traditional theories of primate socioecology. Parish and de Waal (2000) note that research across field sites increases the continuum of intraspecific variation in behavior observed, and “species-typical” behavior must be re-evaluated. While initial models of primate socioecology focused on differences between species as a result of dispersal patterns, predation, and infanticide risk, variation across habitats within the same species can further illuminate our
115 understanding of how demographics, ecological pressures, and kinship affect primate social organization. Furthermore, most theories of female bonding in primates focus primarily on feeding competition (Wrangham, 1980; Van Schaik, 1989; Sterck et al.,
1997; Isbell and Young, 2002). However, in spider monkeys, as well as chimpanzees and bonobos, subgroup fissioning and solitary foraging are likely the best strategy for access to food resources. While scramble competition was considered to reduce the need for female bonding, the focus on contest competition for food resources may have obscured consideration of the other factors that influence female bonding. Unlike traditionally
“female-bonded” primates, or even female bonobos, in which female relationships can increase access to food resources (Parish, 1996) female spider monkeys do not appear to gain nutritional benefits from associating or affiliating.
The findings from a number of studies indicate that unrelated female primates do form strong, differentiation social relationships, even in the absence of kinship (Silk,
2002; Swedell, 2002; Silk et al., 2006 a; b; Lehmann and Boesch, 2008, 2009;
Langergraber et al., 2009). Rodseth and Novak (2006) assert that bonding among unrelated females may be unique to humans. However, the evidence from a number of primate species counters this assumption. Even among female-bonded cercopithecines, where the emphasis has been on kin relationships, strong friendships bonds have been documented among unrelated females (Silk et al., 2006 a; b). These findings require a shift from focusing specifically on kinship to considering both immediate and ultimate benefits to social bonds among unrelated individuals. While there may be multiple benefits to bonding among unrelated females, such as decreased competition and
116 socialization for offspring, the tend-and-befriend hypothesis articulated by Taylor and colleagues (2000) posits one clear proximate mechanism. The benefits of coping with stressors through affiliation have previously been supported by evidence in humans and matrilineal primates (Taylor et al., 2000; Silk et al., 2003; Engh et al., 2006 a). However the findings of my study, as well as the growing body of data on social relationships among unrelated females in other species, suggests that this mechanism is a result of ancestral tendencies of the primate order. This finding supports Taylor and colleagues
(2000) assertion that this mechanism can account for the selective pressures that favored female bonding in hominid evolution, despite the strong tendencies toward male philopatry.
In sum, human females cannot be considered unique in their ability to form strong female bonds in the absence of kinship. Rather than being an artifact of human cognition or cultural influences, bonds among unrelated females can form in a number of primate species. These findings in fact contradict antiquated notions that female bonding is
“unnatural” in humans reported by Parish (1994, 1996) in her review of several theories regarding human evolution and social relationships.
117
TABLES
ID Focals Hours Fecals AG 62 10.3 5 AS 81 13.5 17 AR 115 19.2 13 BU 74 12.3 9 DA 15 2.5 2 EV 30 5 1 FA 17 2.8 1 HO 79 13.2 9 IS 31 5.2 5 JI 75 12.5 8 JL 132 22 17 LE 140 23.2 18 MC 72 12 11 MI 37 6.2 2 RU 110 18.5 12 ST 17 2.8 2 ZE 31 5.2 2 Table 4.1. Distribution of Focal samples, hours, and fecal samples per individuals female black- handed spider monkey at El Zota Biological Field Station.
Behavior Definition Travel Movement within the crown of a tree or between the crowns of trees that is not related to food acquisition or social interaction Rest A period of inactivity Feed/forage Any localized movement within the tree crown associated with the procurement, handling of, and ingestion of food items Social Any behavior that involves interaction or contact with conspecifics or other animals Other Any behavior that does not fall into the above categories; includes observer-directed behavior and object manipulation Table 4.2. Behavioral catalogue used for focal sampling of female spider monkeys. Adapted from van Roosmalen & Klein (1988); McDaniel (1994); Rodrigues (2007), and personal observations.
118
Behavior Definition Grooming Parting of a conspecific’s hair and picking out foreign objects out of hair with hands or mouth Social play Includes grappling, wrestling, lunging, and chase games; may be accompanied by head-shaking, play-faces, panting, and play grunts Embrace Clasping arms around conspecifics; often in conjunction with leaning in and sniffing the pectoral region of a conspecifics Huddle Sitting in contact, may include tail-or limb-wrapping around Conspecific Touch Reaching out and making gentle contact with a conspecific Whinny Contact call with a wave-like frequency Table 4.3. Affiliative behaviors recorded for female spider monkeys. Adapted from Eiesenberg (1976); Van Roosmalen and Klein (1988); Fedigan &Baxter (1984); Schaffner & Aureli (2005); Ramos-Fernandez (2005); Rodrigues (2007); and personal observations.
Behavior Definition Avoid Retreat from a conspecific’s approach Displace Assuming a conspecifics’s spatial position, forcing the conspecifics to move Chase Following or lunging at a retreating conspecifics Harass Hitting, poking, or tail-pulling a conspecific; may resulting in escape behavior and distress vocalizations Fight Biting, wrestling, and slapping that results in screaming and fear Responses Display Aggressive visual signals that may include branch-shaking, head-shaking, arm-swaying, and arm-wrapping; may be accompanied by open-mouth threats and pilo-erection Table 4.4. Agonistic behaviors recorded in female spider monkeys. Adapted from Eiesenberg (1976); Van Roosmalen and Klein (1988); Fedigan &Baxter (1984); Schaffner & Aureli (2005); Ramos- Fernandez (2005); Rodrigues (2007); and personal observations.
119
ID Association % Aggression Groom JL 3 0 0 IS 6 0 0 BU 11 0 0 JI 12 0 0 AR 13 0.10 0 LE 13 0.17 0 AG 14 0.19 0 HO 18 0.15 0 AS 21 0 0.22 RU 21 0.11 0.05 MC 50 0 0 Mean 16.55 0.07 0.02 Table 4.5. Association and interaction with males for female black-handed spider monkeys at El Zota Biological Field Station. Association is expressed in percent, aggression received and grooming are expressed in event/hr.
BL PC BL PC BL PC ID Affiliation Affiliation Embrace Embrace Groom Groom AG 0.88 1.52 0.29 1.52 0.10 0 AR 1.61 6.00 0.21 0 1.35 1.50 BU 1.87 0 0.16 0 0.73 0 HO 2.88 4.54 0.76 4.54 1.14 0 JI 1.20 0 0.24 0 0.96 0 LE 2.67 12.00 0.17 0 2.07 9 RU 2.36 0 0.05 0 1.84 0 Table 4.6. Post-conflict affiliation for female black-handed spider monkeys at El Zota Biological Field Station. BL=Baseline, PC=Post-Conflict. All values are expressed in events/hr.
120
ID Received Initiated Cortisol Rank AG 0 0.10 5.93 D IS 0 0 6.09 U JI 0 0.16 9.23 D AS 0 0.07 9.38 D LE 0 0 12.55 U AR 0.10 0.10 13.27 M JL 0 0.14 13.83 D RI 0 0 16.95 U HO 0.38 0 17.08 S BU 0.16 0 23.33 S MC 0 0.08 34.67 D Mean 0.06 0.06 14.76 Table 4.7. Agonism (events/hr), cortisol, and rank for female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica. D=Dominant, M=Mid-ranking, U=Unknown, S=Subordinate.
121
Dyad AI Dyad AI FA-MI 0.13 EV-MC 0.03 BU-JL 0.12 HO-IS 0.03 HO-MI 0.11 HO-ZE 0.03 AG-HO 0.09 LE-RU 0.03 AR-LE 0.07 MC-EV 0.03 AG-BU 0.06 MI-RU 0.03 JL-MI 0.06 AG-MI 0.02 JL-RU 0.06 AR-DA 0.02 RU-ZE 0.06 BU-MC 0.02 EV-ZE 0.05 LE-MI 0.02 AS-FA 0.04 MC-DA 0.02 AS-RU 0.04 RU-DA 0.02 BU-JI 0.04 AG-JI 0.01 BU-MI 0.04 AG-ZE 0.01 EV-HO 0.04 AS-MI 0.01 JI-ZE 0.04 AS-ST 0.01 JL-MC 0.04 AR-ZE 0.01 AG-MC 0.03 BU-DA 0.01 AS-JL 0.03 BU-EV 0.01 AR-MC 0.03 BU-IS 0.01 EV-DA 0.03 BU-RU 0.01 Table 4.8. Association indices (AI) for female spider monkey dyads that did not engage in affiliative interactions.
122
Dyad AI Groom Huddle Play Embrace TA AR-EV 0.1 0 0 0 0.08 0.08 HO-JL 0.1 0.08 0.04 0 0.06 0.18 AR-BU 0.01 0 0 0 0.03 0.03 AR-JL 0.01 0.05 0 0 0 0.05 EV-JL 0.01 0 0 0 0.04 0.04 IS-JI 0.02 0 0.06 0 0.11 0.17 AS-LE 0.03 0 0 0.03 0 0.03 HO-JI 0.03 0 0.04 0 0 0.04 AG-RU 0.04 0 0.03 0 0 0.03 EV-RU 0.04 0.09 0 0 0 0.09 JI-MC 0.04 0 0 0 0.06 0.06 AG-JL 0.05 0 0 0 0.09 0.09 AG-LE 0.05 0 0 0 0.06 0.06 AS-BU 0.05 0.04 0 0 0 0.04 AS-JI 0.05 0 0 0 0.08 0.08 AR-JL 0.05 0 0.38 0 0.02 0.40 MC-RU 0.05 0 0 0.07 0 0.07 AG-AS 0.06 0 0 0 0.08 0.08 AS-MC 0.06 0.08 0 0 0 0.08 AR-HO 0.08 0.19 0.31 0 0 0.50 AS-HO 0.09 0 0 0.04 0 0.04 AR-RU 0.09 0 0.34 0 0 0.34 FA-ST 0.09 0 0 0 0.18 0.18 HO-LE 0.09 0 0 0 0.05 0.05 HO-MC 0.09 0.04 0.08 0.04 0.12 0.28 BU-LE 0.11 0 0 0 0.03 0.03 HO-RU 0.11 0.47 0.06 0 0.03 0.56 LE-MC 0.12 0 0 0.03 0 0.03 MI-ST 0.14 0.22 0.22 0 0 0.44 BU-HO 0.16 0.27 0.12 0.24 0.08 0.71 JL-LE 0.28 0.11 0 0 0.07 0.18 Mean 0.07 0.05 0.05 0.02 0.04 0.16 Table 4.9. Affiliation indices (AI) and rates of affiliative behaviors for female spider monkey dyads that engaged in affiliation. TA refers to total affiliation. Dyads greater than the mean are in bolded italics.
123
FIGURES
Figure 4.1. Equation to calculate twice-weight association index. x= number of observations in which individuals A and B are observed in the same subgroup, Ta=number of observations in which individual A is observed in a subgroup without individual B, and Tb=number of observations in which individual B is observed in a subgroup without individual A.
40 35
rs²=0.687
30 p=0.002 25 20 15 Cortisolng/gram 10 5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Time engaged in affiliation (min/hr)
Figure 4. 2. Affiliation and cortisol concentrations in individual female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
124
3 W=4.00, p=0.090, N=8
2.5
2
1.5 2.44
1 1.9 Affiliation Affiliation events/hr
0.5
0 Spike Baseline
Figure 4.3. Mean + SE for rates of affiliation on days with cortisol spikes versus baseline values for female black-handed spider monkeys at El Zota Biological Field Station.
25 rs²=0.084
20 p=0.390
15
10 Cortisolng/gram 5
0 0 0.05 0.1 0.15 0.2 0.25 % time in association with males
Figure 4.4. Scatterplot of ssociation with males and mean cortisol concentration for female black-handed spider monkeys at El Zota Biological Field Station. One outlier is removed to allow closer scale.
125
40
35 rs²=0.033 p=0.589
30 25 20 15 Cortisolng/gram 10 5 0 0 0.05 0.1 0.15 0.2 Aggression received from males (events/hr)
Figure 4. 5. Scatterplot of agonism received from males and mean cortisol concentrations for black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
0.8 rs²=0.348
0.7 p=0.0560
0.6 0.5 0.4 0.3
Embraces(events/hr) 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 Agonism (events/hr)
Figure 4.6. Scatterplot of rates of agonism received and embraces for female spider monkeys at El Zota Biological Field Station, Costa Rica.
126
25 U=8.000, p=0.245, N=7
20
15
10 20.21 14.61
Meancortisol(ng/gram) 5
0 S D Rank
Figure 4.7. Mean+ SE for mean cortisol concentrations of female spider monkeys of different rank at El Zota Biological Field Station. S=Subordinate, D=Dominant.
14
12
10
8
6 rs²=0.664
4 p=0.0002 NumberofAssociates 2
0 0 5 10 15 20 25 30 35 40 Mean Cortisol ng/gram
Figure 4.8. Scatterplot of mean cortisol concentrations versus number of associates for female black- handed spider monkeys at El Zota Biological Field Station, Costa Rica.
127
0.09
0.08
0.07
0.06
0.05
0.04 0.074
0.03 Axssociation IndexAxssociation 0.02 0.0346 0.01
0 None Affiliation
Figure 4.9. Association indices for females that engaged in no affiliative behavior versus females who did among female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
0.8 rs²=0.125
0.7 p=0.047
0.6 0.5 0.4 0.3
0.2 Affiliation Affiliation (events/hr) 0.1 0 0 0.05 0.1 0.15 0.2 0.25 0.3 Association Index
Figure 4.10. Scatterplot of association index and affiliation in dyads of female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
128
9 8
7
6 5 4
3 rs²=0.124 Numberoffriends p=0.352 2 1 0 0 5 10 15 20 25 30 35 40 Cortisol concentration (ng/gram)
Figure 4.11. Scatterplot of mean cortisol concentrations versus number of friends for female black- handed spider monkeys at El Zota Biological Field Station, Costa Rica.
20 18 2 16 rs =0.669, p=0.002 14
12
E2 10 8 6 4 2 0 0 2 4 6 8 10 12 Whinny rates (events/hr)
Figure 4.12. Scatterplot of whinny rates versus estradiol concentrations in female black-handed spider monkeys at El Zota Biological Field Station Costa Rica.
129
12
10
2 8 rs =-0.570 p=0.007 6
4 Whinny(events/hr) 2
0 0 0.5 1 1.5 2 2.5 3 Groom (events/hr)
Figure 4.13. Scatterplot of whinny rates versus grooming rates in individual female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
20
18 2 rs =-0.584 16 p=0.006 14
12
E2 10 8 6 4 2 0 0 0.5 1 1.5 2 2.5 3 Groom event/hr
Figure 4.14. Scatterplot of estradiol concentrations versus grooming rates in individual female black- handed spider monkeys at El Zota Biological Field Station, Costa Rica.
130
CHAPTER 5: CONCLUSIONS
Research Objective
The major objective of this study is to examine how different ecological and social settings affect stress levels in female black-handed spider monkeys and to test hypotheses regarding coping mechanisms that may apply to female primates across the order. Stress responses are an adaptive strategy that prepares animals to adequately deal with physical, social and environmental challenges; however, chronic stress has well- documented detrimental effects that can reduce individual health and reproductive fitness. Understanding the coping mechanisms used by individuals to modulate stress is necessary to understand how behavior and physiology interact. The “tend-and-befriend” mechanism, as hypothesized by Taylor and colleagues (2000), refers to female affiliation as an adaptive strategy specifically used by female primates to cope with stress. They propose that this mechanism is a widespread strategy throughout the primate order, and one that underlies patterns of female bonding observed in humans. Although predictions of this hypothesis have been supported among matrilineal primates characterized by female kinship bonds, no similar tests have been conducted on the role of female affiliation in stress reduction in female dispersing species where females are generally unrelated. Since our hominid ancestors are presumed to be male-philopatric, testing this hypothesis should lend insight into the evolution of female-female bonding in our hominid ancestors and Homo sapiens.
131
Major findings
In this study, I examined patterns of reproductive state, ecological variables, and female-female affiliative behavior on cortisol concentrations among female black-handed spider monkeys. Behavioral, hormonal, and ecological data were collected in a wild habituated community. I found that reproductive state and ecological variables do not significantly affect cortisol concentrations. However, I found that time engaged in rest had an inverse relationship with cortisol concentrations: essentially, female spider monkeys are less stressed when they are able to engage in more rest, or engage in more rest when they are not stressed. Finally, I found that female affiliative behavior is significantly correlated with cortisol concentrations, and that this is due to females specifically engaging in female-female affiliation when they are experiencing stress.
Thus, the predominantly unrelated female spider monkeys of this community do appear to “befriend”, as predicted by Taylor and colleagues (2000). However, the evidence does not support the “tend” aspect of this hypothesis. These results suggests two evolutionary possibilities – either such a coping method is an ancestral strategy within the primate order, or the reliance on social affliation to reduce stress in the Atelines and apes is the result of convergent evolution.
Limitations of this study
Unfortunately, this study does have some unavoidable limitations, and future studies should find ways to address these. First, the project originally proposed included microsatellites to assess whether all the females in the community were truly unrelated.
Due to financial and practical limitations, this could not be accomplished, and
132 conclusions that female-female bonding occurs among unrelated females are based on evidence from other studies indicating female dispersal is the norm among spider monkeys (Di Fiore et al., 2009). Given the fragmented nature of the surrounding habitat and the fact that there is one other spider monkey community living in contiguous forest, it is possible that there are repeated migrations from the same community, which would result in familiar, potentially related female-female dyads existing within this community.
However, Di Fiore and colleagues (2009) note that in their studies, females often traveled great distances to immigrate rather than transferring the closest community, and sometimes temporarily joined different communities. Nonetheless, it would strengthen the conclusions of this study if future research on this community included microsatellite analysis to determine relatedness. Furthermore, even in a species in which female dispersal is the norm, not all females necessarily disperse (Pusey, 1997; Stumpf, 2007).
The best-known case of this is the chimpanzee named Fifi at Gombe, Tanzania, who remained in her natal group and whose offspring have been high-ranking within their community (Goodall, 1986). In the spider monkey community that I studied, at least a few individual females are likely natal. For example, a subadult, BU, is a female that was previously studied as a juvenile (Rodrigues, 2007) and I suspect that MC is a natal female, given her strong resemblance in facial markings to one of the dominant males,
CS.
Additionally, in a community characterized by high levels of fission-fusion social dynamics, it is often hard to reliably locate individuals to sample both behaviorally and hormonally on a consistent basis. This is further complicated by the rapid travel of spider
133 monkeys, their arboreal nature, and ecological barriers, such as swamps. Furthermore, individuals in my population differed in personality and, given their dispersed social system, in levels of habituation. All of these factors can bias sampling efforts. For example, certain individuals seemed naturally extroverted toward both other monkeys and human observers. Others tended to be shy and elusive, and resisted my attempts to follow them until well into the field season.
It is unclear what methods can be taken to overcome these obstacles, beyond extending the trail system, building bridges across swamps, or radio-collaring individuals. While radio-collaring may optimize the ability to reliably locate and follow certain individuals, it likely would cause stress to the animals and may counteract habituation efforts. Nonetheless, it is likely that continued study and monitoring of this population will increase habituation of individual members, and increase our knowledge of the community’s social dynamics.
Theoretical significance
Traditional socioecological models of primate sociality, such as those of
Wrangham (1980), Van Shaik (1989) and Isbell and Young (2002) rely on ecological factors to explain patterns of dispersal and sociality among female primates. For example, how predation pressures and ability to defend clumped resources might cause females to be weakly vs. strongly bonded within social groups. However, despite revisions and modifications, these models have come under scrutiny for inadequately explaining the range of social behaviors and organization found across primate species
(Thierry 2008; Janson 2000; Clutton-Brock & Janson 2012). Thus, one of the next steps
134 in understanding social organization and relationships among different primates is in specifically quantifying inter-specific and intra-specific variation among individuals in different populations and among closely phylogenetically related or convergent species.
Spider monkey remains one of the animals that do not necessarily fit standard socioecological models yet this is partially because female-female bonding has only rarely been studied in this species.
The findings of my study suggest that female affiliation and bonding are driven by factors beyond ecological constraints, or relatedness. Essentially, there appear to be benefits of female affiliative behaviors when individuals are experiencing high stress levels. Thus, in some cases, female tolerance and affiliation is a form of mutualism, in which two individuals can both benefit from reciprocal altruism. Furthermore, given the longevity and slow life history of these species, female relationships may be particularly crucial in providing socialization to offspring. Because there is a strongly documented pattern of female-directed coalitionary aggression by males in other spider monkey communities (Fedigan and Baxter, 1984; Campbell, 2003; Gibson et al., 2008; Slater et al., 2008), it is likely that other females are a more attractive option as non-threatening social partners. While females do initiate agonism toward other females, female coalitionary aggression is rare. Furthermore, females rarely initiate agonism toward adult males. Thus, male coalitionary aggression is one of the most likely social stressors that female spider monkeys are likely to face.
Because of female-biased dispersal and the fission-fusion grouping patters, spider monkeys have often been described as having very weak female-female social bonds.
135
The “friendships,” or close affiliative relationships, observed in this study are similar to some female-female relationships reported in chimpanzees and bonobos. For example, even in the classically “weakly-bonded” female chimpanzees of Gombe, Goodall (1986) describes variation in the strength of individual female bonds among dyads, with some female-female dyads exhibited close friendships However, further research indicates that only about half of the females at Gombe disperse (Pusey et al 1997). Furthermore, research on the West African chimpanzees, especially at Taï National Forest in the Côte
D’Ivoire, suggest that female chimpanzees are capable of forging close bonds (Lehmann
& Boesch 2008; Lehmann & Boesch 2009). While chimps exhibit greater variation in female-female social bonds, it is well-established that unrelated female bonobos develop close bonds, and this is why female bonobos are able to prevent the kind of male coalitionary aggression that characterizes chimpanzees (Stumpf, 2007).
The factors that influence strength of social relationships remain unclear. Because relatedness and kin selection have been considered the prime movers for social bonds, unrelated individuals bonds are generally explained as long-term bonds characterized by reciprocal altruism (“friends”) or short-term opportunistic bonds based on individuals’ current social utility (“business partners” :Barrett and Henzi, 2002). However, long-term bonds can only be evaluated through longitudinal studies, particularly over several years.
For example, Lehmann and Boesch (2009) report that female chimpanzee social bonds are more stable over time than male bonds. While males may need to direct affiliative behavior at higher rates to keep up with changing alliances, females may be under less pressure to do so. Lehmann and Boesch (2009) suggest that for this reason, association
136 patterns may be more indicative of long-term bonds, while grooming relationships may be more flexible and reflective of short-term goals.
The friendships in my spider monkey study seem to fall in between the two definitions. Females seem to be actively seeking affiliation when they experience high stress levels, suggesting that they are take advantage of social partners utility when it is beneficial. However, the same dyads that engage in affiliative behavior on on days when they experience high cortisol concentrations are often found ranging together with minimal social interactions on days when cortisol concentrations are low. Anecdotally, over the course of fieldwork, it became obvious that certain dyads were close “friends,” which later quantitative data analysis has supported. However, associations and different affiliative behaviors are still highly variable among dyads.
I suggest that activity budget constraints are the primary reason that female spider monkey generally engage in low rates of affiliative behavior. Because female health and reproductive success is closely tied to adequate nutrition, the energetic demands of foraging and traveling put both rest and social behavior at premiums.
Bronikowski and Altmann (1996) report that there is a flexible trade-off between rest and social behavior. While Dunbar (1988) argues that socializing is more crucial to primates’ lives to maintain social cohesion, Bronikowski and Altmann (1996) argue that resting may be more crucial when activity patterns are constrained. The findings of this study, particularly the relationship between cortisol concentrations and rest, and cortisol concentrations and affiliative behavior, suggest support the assertion that there is a flexible trade-off between these activities. Individuals’ stress levels, in conjunction with
137 the types of stressors the individuals experience may play a role in whether they choose to rest alone, or huddle, groom or play with other females. If ecological conditions cause nutritional stress or require increased travel and foraging time, individuals may be more inclined to spend their available time resting. However, when individuals experience social stressors, such as aggression, they make seek affiliation with close associates to cope with these challenges.
While the results of this study support the tend-and-befriend hypothesis, I was not able to support Taylor and colleagues (2000) notion that this is an ancestral state in primates. A few possibilities exist. First, this could be a case of ecological convergence between the Ateline primates and apes. These two clades are phylogenetically distant, but exhibit similar social dynamics, particularly between spider monkeys and chimpanzees, muriquis and bonobos, and howler monkeys and gorillas (Chapman et al., 1995; Aureli et al., 2008). Though an ancestral tendency toward female affiliation is likely more parsimonious than behavioral convergence in select species, this hypothesis should be tested in other primate species that are characterized by female dispersal. Costa Rican squirrel monkeys (Saimiri oerstedii) and red colobus monkeys (Piliocolobus spp) may be ideal taxa to further test this hypothesis. Additionally, this hypothesis could be tested in species characterized by bisexual dispersal, such as howler monkeys (Alouatta sp.) and gorillas (Gorilla sp.) Squirrel monkeys may be an ideal genus to test the effects of ecology and dispersal on female social relationships (Boinski et al., 2001). While Costa
Rican squirrel monkeys (Saimiri oerstedii) are characterized by male residence and female dispersal, Peruvian squirrel monkeys (S. boliviensis) are characterized by female
138 residence and male dispersal. Surinamese squirrel monkeys (S. sciureus) differ from both of these species due to a predominant pattern of bisexual dispersal. Boinski and colleagues (2001) argue that these differences are a result of varying ecological pressures throughout their range. Comparisons of female social relationships among these three species may help elucidate the factors that underlie variation in female relationships with regards to ecology and dispersal patterns.
If these other species exhibit the tend-and-befriend strategy, this would strengthen
Taylor and colleagues (2000) argument that it is an ancestral tendency that occurs across the primate order. However, patterns of female affiliation in response to stressors may not be unique to the primate order. The hormonal underpinnings of this hypothesis are derived largely from rodents, which also exhibit reduced stress due to activation of oxytocin through affiliation (Taylor et al. 2000). Given the hormonal similarities across female mammals, particularly those characterized by slow life histories and complex social relationships, it is possible that this pattern is shared by a number of other mammals. Thus, further research on a variety of mammalian taxa may be necessary to determine if this is an ancestral mammalian trait, or one that evolved separately in species with convergent ecological and social pressures. Nonetheless, the results of the present study indicate that the tend-and-befriend strategy is not unique to human females or a result of cultural conditioning, but an adaptive pattern that helps females cope with stress.
Applied significance
The validation of a glucocorticoid assay for female spider monkey fecal metabolites has great potential for evaluating the welfare of captive animals as well as
139 evaluating health and welfare of wild populations. Building on the work of Rangel-
Negrín and colleagues (2009), fecal sampling can be a valuable tool in assessing health of spider monkey within different ecosystems. Such assessments can be used to identify priority conservation areas. For example, evaluation of cortisol concentrations between populations constrained by fragmented habitat and anthropogenic disturbances may help us evaluate populations that are at risk. Additionally, these methods can be applied to evaluate captive welfare, as well as for evaluating the success of reintroduction projects for individual monkeys.
The findings of this study also highlight the value of female social partners for female spider monkeys, and suggest that opportunity to fission in captive environments may help offset stress. Many zoos have trouble housing more than a few male spider monkeys due to potential for aggression. I suggest that zoos may have more success in housing spider monkeys if they try to maintain the natural dispersal patterns, by keeping related patrilines intact, and transferring females to other institutions about as subadults or early adults. This may reduce the potential of male aggression, while females should still be able to develop and maintain friendships as they do in the wild.
Future directions
Building on the findings of this project, there are a several future research avenues that should be evaluated. First, future studies on the spider monkeys of El Zota should specifically examine diet and feeding behavior in order to quantify how the ecological factors vary between the spider monkey communities in the primary versus secondary forest. Research on feeding ecology and activity budgets in both these communities can
140 help us to identify if spider monkeys living in secondary forest have less available time for resting and socializing due to foraging and traveling demands. In particular, studies utilizing scan sampling may be the most valuable for documenting the full range of fruits consumed, and if there are differences between these two contiguous habitats (Campbell,
2000 a). Second, future studies should include a comparison of cortisol concentrations between the communities located in the primary and secondary forests. Spider monkeys were once thought to be limited to primary forest, but clearly do occupy secondary forest and fragments. However, more research is needed to determine whether there are energetic consequences to living among these different types of forest. Third, future studies of cortisol on wild spider monkeys should incorporate comparison across a range of habitat types, including tropical moist and dry forest.
The relationship between cortisol and reproductive state on a finer temporal level may be a valuable avenue for future research. This topic may be best studied by utilizing either captive or radio-collared animals, in order to consistently sample more frequently than was possible in the current study. Future research is necessary to investigate how cortisol and estradiol concentrations vary on a weekly basis throughout pregnancy and early lactation. Additionally, more research is needed to determine how cortisol concentrations are affected by the resumption of cycling. Assays of progesterone in addition to estradiol and cortisol may be necessary to further investigate this question.
Additionally, one interesting finding of this study is the positive correlation between whinny vocalizations and estradiol concentrations, both of which inversely related to grooming rates. While I hypothesize that whinny vocalizations may play a
141 greater role in facilitating female-female contact and affiliation than in mate attraction, both of these hypotheses should be tested. It is unclear whether the relationship between whinny rates and estradiol are a result of variation in female cycling, or reflect individual-level differences in hormone concentrations. Further studies should include an examination of the acoustic properties of whinny vocalizations, frequency of whinny vocalizations, grooming rates, and reproductive hormones in both male and female spider monkeys.
Finally, studies of hormonal correlates of female bonding in captive primates may be a productive avenue to elucidate the hormonal underpinnings of female bonding between unrelated females. Captive chimpanzees and bonobos may be ideal taxa to examine this question, due to both the socioecology and dispersal patterns of these species in the wild, in conjunction with the greater ease in collecting urine samples in captivity. In particular, the relationship between female affiliation, cortisol concentrations, and oxytocin concentrations in captive bonobos may help illuminate the proximate mechanisms which underlie female bonding and affiliation.
Conclusions
This study is the first to examine the relationship between female social relationships and endocrinology in spider monkeys. Additionally, in order to investigate to investigate this question, I validated a cortisol enzyme-immunoassay (EIA) for female spider monkeys, and evaluated the effects of reproductive state and ecology on cortisol concentrations. I found that fecal cortisol metabolites can be used as a reliable indicator of behavioral stress, and that there is a twenty-four hour lag between the experience of a
142 stressor and the rise in fecal cortisol metabolites. I further determined that although cortisol and estradiol do have a significant relationship in lactating females, estradiol concentrations do not significantly confound measurement of cortisol concentrations. The lack of significant variation between females of different reproductive states, particularly between cycling and lactating females, indicates that female spider monkeys experience normal cortisol responsiveness during lactation. This supports others assertions that primates do not experience the decrease in cortisol responsiveness that rodents experience during lactation. Individual factors such as social and ecological stressors may play a greater in modulating cortisol concentrations. In particular, both individual and monthly variation in rest is significantly associated with cortisol concentrations.
Furthermore, female spider monkeys do exhibit differentiated social relationships that are characterized by association as well as affiliative behaviors such as grooming. These affiliative behaviors occur at higher rates than normal on days following the experience of a stressor and a rise in cortisol concentration. I suggest that this pattern supports the predictions of the tend-and-befriend hypothesis, and supports Taylor and colleagues
(2000) assertions that this may be an ancestral tendency within the primate order.
143
REFERENCES
Ahumada J. 1992. Grooming behavior of spider monkeys (Ateles geoffroyi) on Barro Colorado Island, Panama. International Journal of Primatology 13:33–49.
Albuquerque SR, Sousa MBC, Santos HM, and Ziegler TE. 2001. Behavioral and Hormonal Analysis of Social Relationships Between Oldest Females in a Wild Monogamous Group of Common Marmosets (Callithrix jacchus). 22:631–645.
Almond RE, Ziegler TE, and Snowdon CT. 2008. Changes in prolactin and glucocorticoid levels in cotton-top tamarin fathers during their mate’s pregnancy: The effect of infants and paternal experience. American journal of Primatology 70:560–5.
Altemus M. 1995. Neuropeptides in Anxiety Disorders: Effects of Lactation. Annals of the New York Academy of Sciences 771.
Altmann J. 1980. Baboon Mothers and Infants. Cambridge: Harvard University Press.
Ange-van Heugten KD, Van Heugten E, Timmer S, Bosch G, Elias A, Whisnant S, Swarts HJM, Ferket P, and Verstegen MW a. 2009. Fecal and Salivary Cortisol Concentrations in Woolly (Lagothrix ssp.) and Spider Monkeys (Ateles spp.). International Journal of Zoology 2009:1–9.
Asensio N, and Gómez-Marín F. 2002. Interspecific interaction and predator avoidance behavior in response to tayra (Eira barbara) by mantled howler monkeys (Alouatta palliata). Primates 43:339–41.
Asensio N, Korstjens AH, Schaffner CM, and Aureli F. 2008. Intragroup aggression , fission – fusion dynamics and feeding competition in spider monkeys. Behaviour 145:983–1001.
Aureli F, and Schaffner C. 2008. Social Interactions, Social Relationships, and the Social System of Spider Monkeys. In: Campbell C, editor. Spider Monkeys: Behavior, Ecology, and Evolution of the Genus Ateles. Cambridge: Cambridge University Press. p 236–265.
Aureli F, and Schaffner CM. 2007. Aggression and conflict management at fusion in spider monkeys. Biology Letters 3:147–9.
144
Aureli F, Shaffner C, Boesch C, Bearder S, Call J, Chapman C, Connor R, Di Fiore A, Dunbar RIM, Henzi SP, Holekamp K, Korstjens AH, Layton R, Lee P, Lehmanna J, Manson JH, Ramos-Fernandez G, Strier KB, and van Schaik CP. 2008. Fission-fusion dynamics: New research frameworks. Current Anthropology 49:627–654.
Baker K, and Smuts B. 1994. Social relationships of female chimpanzees: Diversity between captive groups. In: Wrangham R, McGrew W, de Waal F, Heltne P, editors. Chimpanzee Cultures. Cambridge, Massachusetts: Harvard University Press. p 227–243.
Bales KL, French JA, Hostetler CM, and Dietz JM. 2005. Social and reproductive factors affecting cortisol levels in wild female golden lion tamarins (Leontopithecus rosalia). American Journal of Primatology 67:25–35.
Barrett L, and Henzi SP. 2002. Constraints on relationship formation among female primates. Behaviour, 139 2:263–289.
Beehner JC, Bergman TJ, Cheney DL, Seyfarth RM, and Whitten PL. 2005. The effect of new alpha males on female stress in free-ranging baboons. Animal Behaviour 69:1211–1221.
Beehner JC, and McCann C. 2008. Seasonal and altitudinal effects on glucocorticoid metabolites in a wild primate (Theropithecus gelada). Physiology & Behavior 95:508–14.
Beehner JC, and Whitten PL. 2004. Modifications of a field method for fecal steroid analysis in baboons. Physiology & Behavior 82:269–77.
Boinski S, Sughrue K, Selvaggi L, Quatrone R, Henry M, and Cropp S. 2001. An expanded test of the ecological model of primate social evolution: Competitive regimes and female bonding in three species of squirrel monkey (Saimiri oerstedii, S. boliviensis, and S. sciureus). Behaviour 139:227–261.
Bolker BM, Brooks ME, Clark CJ, Geange SW, Poulsen JR, Stevens MHH, and White J-SS. 2009. Generalized linear mixed models: A practical guide for ecology and evolution. Trends in Ecology & Evolution 24:127–35.
Bronikowski AM, and Altmann J. 1996. Foraging in a variable environment: Weather patterns and the behavioral ecology of baboons. Behavioral Ecology and Sociobiology 39:11–25.
Brunton P, Russell J, and Douglas A. 2008. Adaptive responses of the maternal hypothalamic- pituitary-adrenal axis during pregnancy and lactation. Journal of Neuroendocrinology 20:764–76.
Brunton P, and Russell J. 2003. Hypothalamic-pituitary-adrenal responses to centrally administered orexin-A are suppressed in pregnant rats. Journal of Neuroendocrinology 15:633–7.
Camargo CC, and Ferrari SF. 2007. Interactions between tayras (Eira barbara) and red-handed howlers (Alouatta belzebul) in eastern Amazonia. Primates 48:147–50. 145
Campbell C. 2006. Lethal Intragroup Aggression by Adult Male Spider Monkeys (Ateles geoffroyi). American Journal of Primatology 1201:1197–1201.
Campbell CJ, Shideler SE, Todd HE, and Lasley BL. 2001. Fecal analysis of ovarian cycles in female black-handed spider monkeys (Ateles geoffroyi). American Journal of Primatology 54:79–89.
Campbell CJ. 2000a. The reproductive biology of black-handed spider monkeys (Ateles geoffroyi): Integrating behavior and endocrinology. PhD Thesis. University of California- Berkeley. 1–183.
Campbell CJ. 2000b. Fur rubbing behavior in free-ranging black-handed spider monkeys (Ateles geoffroyi) in Panama. American Journal of Primatology 51:205–8.
Campbell CJ. 2003. Female-directed aggression in free-ranging Ateles geoffroyi. International Journal of Primatology 24:223–237.
Carnegie SD, Fedigan LM, and Ziegler TE. 2011. Social and environmental factors affecting fecal glucocorticoids in wild, female white-faced capuchins (Cebus capucinus). American Journal of Primatology 73:861–9.
Carpenter C. 1935. Behavior of red spider monkeys in Panama. Journal of Mammalogy 16:171– 180.
Cavigelli S. 1999. Behavioural patterns associated with faecal cortisol levels in free-ranging female ring-tailed lemurs, Lemur catta. Animal Behaviour 57:935–944.
Chapman CA, Chapman LJ, Wangham R, Hunt K, Gebo D, and Gardner L. 1992. Estimators of fruit abundance of tropical trees. Biotropica 24:527–531.
Chapman CA, Chapman LJ, and Wrangham R. 1995. Ecological constraints on group size: An analysis of spider monkey and chimpanzee subgroups. Behavioral Ecology and Sociobiology 36:59–70.
Chapman CA, and Chapman LJ. 1990. Reproductive biology of captive and free-ranging spider monkeys. Primates 9:1–9.
Chapman CA, Fedigan LM, Fedigan L, and Chapman LJ. 1989a. Post-weaning resource competition and sex ratios in spider monkeys. Oikos 54:315–319.
Chapman CA. 1990. Association patterns of spider monkeys: The influence of ecology and sex on social organization. Behavioral Ecology and Sociobiology 26:409–414.
Chrousos GP, Renquist D, Brandon D, Eil C, Pugeat M, Vigersky R, Cutler GB, Loriaux DL, and Lipsett MB. 1982. Glucocorticoid hormone resistance during primate evolution: Receptor- mediated mechanisms. Proceedings of the National Academy of Sciences of the United States of America 79:2036–40. 146
Clutton-Brock T, and Janson C. 2012. Primate socioecology at the crossroads: Past, present, and future. Evolutionary anthropology 21:136–50.
Cohen S, Kaplan JR, Cunnick JE, Manuck SB, and Rabin BS. 1992. Chronic social stress, affiliation, and cellular immune response in nonhuman primates. Psychological Science 3:301.
Cohen S. 2004. Social relationships and health. The American psychologist 59:676–84.
Cook C. 1997. Oxytocin and prolactin suppress cortisol responses to acute stress in both lactating and non-lactating sheep. Journal of Dairy Research 64:327–339.
Cristóbal-Azkarate J, Chavira R, Boeck L, Rodriguez-Luna E, and Vea J. 2007. Glucocorticoid levels in free ranging resident mantled howlers: a study of coping strategies. American Journal of Primatology 876:866–876.
Davis N, Schaffner CM, and Wehnelt S. 2009. Patterns of injury in zoo-housed spider monkeys: A problem with males? Applied Animal Behaviour Science 116:250–259.
Downs JL, and Urbanski HF. 2006. Neuroendocrine changes in the aging reproductive axis of female rhesus macaques (Macaca mulatta). Biology of Reproduction 75:539–46.
Dunbar R, and Dunbar P. 1988. Maternal time budgets of gelada baboons. Animal Behaviour 36:970–980.
Dunbar R. 1993. Co-evolution of neocortical size, group size, and language in humans. Behavioral Brain Sciences 16:681–735.
Dunbar R. 1998. Grooming, gossip, and the evolution of language. London: Faber and Faber.
Edwards C. 1993. Behavioral sex differences of children of diverse cultures: The case of nurturance to infants. In: Pereira M, Fairbanks L, editors. Juvenile Primates: Life History, Development, and Behavior. New York: Oxford University Press. p 327–338.
Ehmke EE. 2010. Stress and affiliation among wild female primates: Effects of group size, risk, and reproductive condition in a dynamic forest community. PhD Dissertation. University of Florida. 1–254.
Engh AL, Beehner JC, Bergman TJ, Whitten PL, Hoffmeier RR, Seyfarth RM, and Cheney DL. 2006a. Behavioural and hormonal responses to predation in female chacma baboons (Papio hamadryas ursinus). Proceedings. Biological Sciences / The Royal Society 273:707–12.
Engh AL, Beehner JC, Bergman TJ, Whitten PL, Hoffmeier RR, Seyfarth RM, and Cheney DL. 2006b. Female hierarchy instability, male immigration and infanticide increase glucocorticoid levels in female chacma baboons. Animal Behaviour 71:1227–1237.
147
Fedigan LM, and Baxter MJ. 1984. Sex differences and social organization in free-ranging spider monkeys (Ateles geoffroyi). Primates 25:279–294.
Fedigan LM. 2003. Impact of Male Takeovers on Infant Deaths , Births and Conceptions in Cebus capucinus at Santa Rosa , Costa Rica. International Journal of Primatology 24:723– 741.
Di Fiore A, Link A, Schmitt CA, and Spehar SN. 2009. Dispersal patterns in sympatric woolly and spider monkeys: integrating molecular and observational data. Behaviour 146:437–470.
Geary DC, and Flinn M V. 2002. Sex differences in behavioral and hormonal response to social threat: Commentary on Taylor et al. (2000). Psychological Review 109:745–750.
Gentry A. 1982. Patterns of Neotropical plant-species diversity. Evolutionary Biology 15:1–85.
Gesquiere LR, Khan M, Shek L, Wango TL, Wango EO, Alberts SC, and Altmann J. 2008. Coping with a challenging environment: effects of seasonal variability and reproductive status on glucocorticoid concentrations of female baboons (Papio cynocephalus). Hormones and behavior 54:410–6.
Ghiglieri P. 1987. Sociobiology of the great apes and the hominid ancestor. Journal of Human Evolution 16:319–357.
Gibson KN, Vick LG, Palma AC, Carrasco FM, Taub D, and Ramos-Fernández G. 2008. Intra- community infanticide and forced copulation in spider monkeys: A multi-site comparison between Cocha Cashu, Peru and Punta Laguna, Mexico. American journal of primatology 70:485–9.
Ginsberg JR, and Young TP. 1992. Measuring association between individuals or groups in behavioural studies. Animal Behaviour 44:377–379.
González-Zamora A, Arroyo-Rodríguez V, Chaves OM, Sánchez-López S, Aureli F, and Stoner KE. 2011. Influence of climatic variables, forest type, and condition on activity patterns of Geoffroyi’s spider monkeys throughout Mesoamerica. American Journal of Primatology 73:1189–98.
González-Zamora A, Arroyo-Rodríguez V, Chaves OM, Sánchez-López S, Stoner KE, and Riba- Hernández P. 2009. Diet of spider monkeys (Ateles geoffroyi) in Mesoamerica: current knowledge and future directions. American Journal of Primatology 71:8–20.
Goodall J. 1986. The Chimpanzees of Gombe: Patterns of Behavior. Cambridge: Bellknap Press.
Gore AC, Windsor-Engnell BM, and Terasawa E. 2004. Menopausal increases in pulsatile gonadotropin-releasing hormone release in a nonhuman primate (Macaca mulatta). Endocrinology 145:4653–9.
148
Goymann W, Möstl E, Van’t Hof T, East ML, and Hofer H. 1999. Noninvasive fecal monitoring of glucocorticoids in spotted hyenas, Crocuta crocuta. General and Comparative Endocrinology 114:340–8.
Groer MW, Davis MW, and Hemphill J. 2002. Postpartum stress: Current concepts and the possible protective role of breastfeeding. Journal of Obstetric, Gynecologic, and Neonatal Nursing 31:411–7.
Hartwell KS. 2010. Sexual Segregation in Spider Monkeys in Belize.Masters Thesis. University of Calgary.
Heinrichs M, Baumgartner T, Kirschbaum C, and Ehlert U. 2003. Social support and oxytocin interact to suppress cortisol and subjective responses to psychosocial stress. Biological Psychiatry 54:1389–1398.
Heintz MR, Santymire RM, Parr LA, and Lonsdorf E V. 2011. Validation of a cortisol enzyme immunoassay and characterization of salivary cortisol circadian rhythm in chimpanzees (Pan troglodytes). American Journal of Primatology 73:903–8.
Heistermann M, Palme R, and Ganswindt A. 2006. Comparison of different enzymeimmunoassays for assessment of adrenocortical activity in primates based on fecal analysis. American Journal of Primatology 273:257–273.
Hernández-López L, Cerda-Molina A L, Chavira-Ramírez R, and Mondragón-Ceballos R. 2010. Age-dependent changes in fecal 17beta-estradiol and progesterone concentrations in female spider monkeys (Ateles geoffroyi). Theriogenology 73:468–73.
Hijmans RJ, Cameron SE, Parra JL, Jones PG, and Jarvis A. 2005. Very high resolution interpolated climate surfaces for global land areas. International Journal of Climatology 25:1965–1978.
Holdridge L, Grenke W, Hatheway WH, Liang T, and Tosi Jr J. 1971. Forest environments in tropical life zones: A pilot study. Oxford: Pergamon Press.
Hrdy S. 1981. The woman that never evolved. Cambridge: Harvard University Press.
Isbell LA, and Young TP. 2002. Ecological models of female social relationships in primates: Similarities, disparities, and some directions for future clarity. Behaviour 139:177–202.
Janson C, and Robinson J. 1987. Capuchins, squirrel monkeys, and Atelines: Socioecological convergence with Old World monkeys. In: Smuts B, DL C, Seyfarth R, Wrangham RW, Struhsaker T, editors. Primate Societies. Chicago: University of Chicago Press. p 69–82.
Janson CH. 2000. Primate socio-ecology: The end of a golden age. Evolutionary Anthropology: Issues, News, and Reviews 9:73–86.
Janzen D. 1983. Costa Rica Natural History. Chicago: University of Chicago Press. 149
Kahlenberg SM, Emery Thompson M, and Wrangham RW. 2008a. Female Competition over Core Areas in Pan troglodytes schweinfurthii, Kibale National Park, Uganda. International Journal of Primatology 29:931–947.
Kahlenberg SM, Thompson ME, Muller MN, and Wrangham RW. 2008b. Immigration costs for female chimpanzees and male protection as an immigrant counterstrategy to intrasexual aggression. Animal Behaviour 76:1497–1509.
Kammerer M, Adams D, Castelberg B Von, and Glover V. 2002. Pregnant women become insensitive to cold stress. BMC Pregnancy and Childbirth 5:2–6.
Khan MZ, Altmann J, Isani SS, and Yu J. 2002. A matter of time: Evaluating the storage of fecal samples for steroid analysis. General and Comparative Endocrinology 128:57–64.
Korstjens AH, Verhoeckx IL, and Dunbar RIM. 2006. Time as a constraint on group size in spider monkeys. Behavioral Ecology and Sociobiology 60:683–694.
Kung H-C, Hoyert DL, Xu J, and Murphy SL. 2008. Deaths: final data for 2005. National vital statistics reports : from the Centers for Disease Control and Prevention, National Center for Health Statistics, National Vital Statistics System 56:1–120.
Landys MM, Ramenofsky M, and Wingfield JC. 2006. Actions of glucocorticoids at a seasonal baseline as compared to stress-related levels in the regulation of periodic life processes. General and Comparative Endocrinology 148:132–49.
Langergraber K, Mitani J, and Vigilant L. 2009. Kinship and social bonds in female chimpanzees (Pan troglodytes). American Journal of Primatology 71:840–51.
Lehmann J, and Boesch C. 2004. To fission or to fusion: effects of community size on wild chimpanzee (Pan troglodytes verus) social organisation. Behavioral Ecology and Sociobiology 56:207–216.
Lehmann J, and Boesch C. 2008. Sexual Differences in Chimpanzee Sociality. International Journal of Primatology 29:65–81.
Lehmann J, and Boesch C. 2009. Sociality of the dispersing sex: The nature of social bonds in West African female chimpanzees, Pan troglodytes. Animal Behaviour 77:377–387.
Levey DJ. 1988. Spatial and Temporal Variation in Costa Rican Fruit and Fruit-eating Bird Abundance. Ecological Mongraphs 58:251–269.
Lindshield SM. 2006. The density and distribution of Ateles geoffroyi in a mosaic landscape at El Zota Biological Field Station, Costa Rica. Masters Thesis. Iowa State University. 1–142.
Lockwood C, Radunovic N, and Nastic D. 1996. Corticotropin-releasing hormone and related pituitary-adrenal axis hormones in fetal and maternal blood during the second half of pregnancy. Journal of Perinatal Medicine 24:243–251. 150
Maestripieri D, Hoffman CL, Fulks R, and Gerald MS. 2008. Plasma cortisol responses to stress in lactating and nonlactating female rhesus macaques. Hormones and Behavior 53:170–6.
Manson JH, Gros-Louis J, and Perry S. 1994. Three Apparent Cases of Infanticide by Males in Wild White-faced Capuchins (Cebus Capucinus). Folia Primatologica 75:104–106.
Manson JH, and Wrangham RW. 1991. Intergroup Aggression in Chimpanzees and Humans. Current Anthropology 32:369.
Martínez-Mota R, Valdespino C, Rebolledo JAR, and Palme R. 2008. Determination of Fecal Glucocorticoid Metabolites to Evaluate Stress Response in Alouatta pigra. International Journal of Primatology 29:1365–1373.
Martínez-Mota R, Valdespino C, Sánchez-Ramos M a., and Serio-Silva JC. 2007. Effects of forest fragmentation on the physiological stress response of black howler monkeys. Animal Conservation 10:374–379.
Matsuda I, and Izawa K. 2008. Predation of wild spider monkeys at La Macarena, Colombia. Primates 49:65–8.
McEwen BS, and Wingfield JC. 2003. The concept of allostasis in biology and biomedicine. Hormones and Behavior 43:2–15.
Mezzacappa ES, Guethlein W, Vaz N, and Bagieila E. 2000. A preliminary study of breast- feeding and maternal symptomology. Annals of Behavioral Medicine 22:71–79.
Mezzacappa ES, Kelsey RM, Myers MM, and Katkin ES. 2001. Breast-feeding and maternal cardiovascular function. Psychophysiology 38:988–97.
Milton K. 1981. Food Choice and Digestive Strategies in Two Sympatric Species. American Naturalist 117:496–505.
Nelson RJ, and Drazen DL. 2007. Seasonal changes in stress responses. In: Fink G, editor. Encylopedia of stress. Vol. 3. Second Edi. Oxford: Academic Press. p 402–8.
Nguyen N, Gesquiere L, Wango E, Alberts S, and Altmann J. 2008. Late pregnancy glucocorticoid levels predict responsiveness in wild baboon mothers (Papio cynocephalus). Animal Behaviour 75:1747–1756.
Von der Ohe, CG, Servheen C. 2002. Measuring stress in mammals using fecal glucocorticoids: opportunities and challenges. Wildlife Society Bulletin 30:1215–1225.
Otali E, and Gilchrist JS. 2005. Why chimpanzee (Pan troglodytes schweinfurthii) mothers are less gregarious than nonmothers and males: The infant safety hypothesis. Behavioral Ecology and Sociobiology 59:561–570.
151
Parish A, and De Waal FB. 2000. The other “closest living relative”. How bonobos (Pan paniscus) challenge traditional assumptions about females, dominance, intra- and intersexual interactions, and hominid evolution. Annals of the New York Academy of Sciences 907:97–113.
Parish AR. 1994. Sex and food control in the “uncommon chimpanzee”: How Bonobo females overcome a phylogenetic legacy of male dominance. Ethology and Sociobiology 15:157– 179.
Parish AR. 1996. Female relationships in bonobos (Pan paniscus): Evidence for bonding , cooperation , and female dominance in a male-philopatric species. Human Nature 7:61–96.
Pastor-Nieto R. 2001. Grooming, kinship, and co-feeding in captive spider monkeys (Ateles geoffroyi). Zoo Biology 20:293–303.
Peetz A, Norconk MA, and Kinzey WG. 1992. Predation by jaguar on howler monkeys (Alouatta seniculus) in Venezuela. American journal of Primatology 28:223–228.
Pontzer H, and Wrangham RW. 2006. Ontogeny of ranging in wild chimpanzees. International Journal of Primatology 27:295–309.
Pruetz J, and LaDuke T. 2001. New Field Site: Preliminary census of primates at El Zota Biological Field Station, Costa Rica. Neotropical Primates 9:22–23.
Pruetz J. 2009. The socioecology of adult female patas monkeys and vervets in Kenya. Upper Saddle River, NJ: Pearson Prentice Hall.
Pusey A. 1997. The influence of dominance rank on the reproductive success of female chimpanzees. Science 277:828–831.
Ramos-Fernández G, Boyer D, Aureli F, and Vick LG. 2009. Association networks in spider monkeys (Ateles geoffroyi). Behavioral Ecology and Sociobiology 63:999–1013.
Ramos-Fernández G, Pinacho-Guendulain B, Miranda-Pérez A, and Boyer D. 2011. No evidence of coordination between different subgroups in the fission–fusion society of spider monkeys (Ateles geoffroyi). International Journal of Primatology 32:1367–1382.
Ramos-Fernández G. 2005. Vocal communication in a fission-fusion society: Do spider monkeys stay in touch with close associates? International Journal of Primatology 26:1077–1092.
Rangel-Negrín A, Alfaro J, Valdez R, Romano M, and Serio-Silva J. 2009. Stress in Yucatan spider monkeys: Effects of environmental conditions on fecal cortisol levels in wild and captive populations. Animal Conservation 12:496–502.
Rebecchini L, Schaffner CM, and Aureli F. 2011. Risk is a component of social relationships in spider monkeys. Ethology 117:691–699.
152
Rodrigues MA. 2008. The emergence of sex-segregated association patterns in juvenile spider monkeys. American Journal of Physical Anthropology S46:181–182.
Rodrigues MA. 2007. Sex Differences in the Social Behavior of Juvenile Spider Monkeys (Ateles geoffroyi). Masters Thesis. Iowa State University. 1-143.
Rodrigues, MA, Lindshield, SM, Walz, JT, Palmer, M, Larsen T. 2009. Parties in the rainforest: Subgroup size and composition of black-handed spider monkey at a wet site in Costa Rica. American Journal of Physical Anthropology S48:223.
Rodseth L, and Novak S. 2006. The impact of primatology on the study of human society. In: Barkow JH, editor. Missing the revolution: Darwinism forS Social Scientists. New York: Oxford University Press. p 187–220.
Rodseth L, Wrangham R, Harrigan A, and Smuts B. 1991. The human community as a primate society. Current Anthropology 32:221–241.
Van Roosmalen M, and LL K. 1988. The Spider Monkey, Genus Ateles. In: Mittermeier R, Rylands A, Coimbra-Filho A, da Fonseca G, editors. Ecology and Behavior of Neotropical Primates, Vol 2. Washington, DC: World Wildlife Fund. p 455–539.
Sanford R, Paaby P, Luvall J, and Phillips E. 1994. Climate, Geomorphology, and Aquatic Systems. In: McDade L, Bawa K, Hespenheide H, Hartshorn G, editors. La Selva: Ecology and Natural History of a Tropical Rainforest. Chicago: University of Chicago Press. p 19– 33.
Sapolsky R. 1992. Neuroendocrinology of the stress-response. In: Becker J, Breedlove S, Crews D, editors. Behavioral Endocrinology. 1st ed. Cambridge: Massachusetts Institute of Technology. p 287–324.
Sapolsky RM. 1986. Endocrine and behavioral correlates of drought in wild olive baboons (Papio anubis). American Journal of Primatology 11:217–227.
Sapolsky RM. 2009. Why Zebras Don’t Get Ulcers. New York: Henry Holt and Company.
Van Schaik C. 1989. The ecology of social relationships amongst female primates. In: Standen V, Foley R, editors. Comparative Socioecology The Behavioural Ecology of Humans and other Mammals. Blackwell Scientific Publications.
Van Schaik CP, and Aureli F. 2000. The natural history of valuable relationships in primates. In: Aureli F, editor. Natural Conflict Resolution. Berkeley: University of California Press. 307–333.
Segerstrom SC, and Miller GE. 2004. Psychological stress and the human immune system: aAmeta-analytic study of 30 years of inquiry. Psychological Bulletin 130:601.
153
Seltzer LJ, Ziegler TE, and Pollak SD. 2010. Social vocalizations can release oxytocin in humans. Proceedings Biological Sciences / The Royal Society 277:2661–6.
Selye H. 1973. The evolution of the stress concept: The origintor of the concept traces its development in 1936 of the alarm reaction to modern therapeutic application of syntoxic and catatoxic hormones. American Scientist 61:692–699.
Setchell JM, Smith T, Wickings EJ, and Knapp LA. 2008. Factors affecting fecal glucocorticoid levels in semi-free-ranging female mandrills (Mandrillus sphinx). American Journal of Primatology 70:1023–32.
Seyfarth RM, Silk JB, and Cheney DL. 2012. Variation in personality and fitness in wild female baboons. Proceedings of the National Academy of Sciences of the United States of America:1–6.
Shanks N, Windle RJ, Perks P, Wood S, Ingram CD, and Lightman SL. 1999. The hypothalamic- pituitary-adrenal axis response to endotoxin is attenuated during lactation. Journal of Neuroendocrinology 11:857–65.
Silk JB, Alberts SC, and Altmann J. 2003. Social bonds of female baboons enhance infant survival. Science 302:1231–4.
Silk JB, Alberts SC, and Altmann J. 2006a. Social relationships among adult female baboons (Papio cynocephalus) II. Variation in the quality and stability of social bonds. Behavioral Ecology and Sociobiology 61:197–204.
Silk JB, Altmann J, and Alberts SC. 2006b. Social relationships among adult female baboons (Papio cynocephalus) I. Variation in the strength of social bonds. Behavioral Ecology and Sociobiology 61:183–195.
Silk JB. 2002. Using the’F'-word in primatology. Behaviour 139:421–446.
Slater K, Schaffner C, and Aureli F. 2007. Embraces for infant handling in spider monkeys: evidence for a biological market? Animal Behaviour 74:455–461.
Slater KY, Schaffner CM, and Aureli F. 2008. Female-directed male aggression in wild Ateles geoffroyi yucatanensis. International Journal of Primatology 29:1657–1669.
Slater KY, Schaffner CM, and Aureli F. 2009. Sex differences in the social behavior of wild spider monkeys (Ateles geoffroyi yucatanensis). American Journal of Primatology 71:21–9.
Smith TE, Schaffner CM, and French JA. 1997. Social and developmental influences on reproductive function in female wied ’s black tufted-ear marmosets (Callithrix kuhli). Hormones and Behavior 168:159–168.
Sterck EHM, Watts DP, and Van Schaik CP. 1997. The evolution of female social relationships in nonhuman primates. Behavioral Ecology and Sociobiology 41:291–309. 154
Stevens JMG, Vervaecke H, De Vries H, and Van Elsacker L. 2006. Social structures in Pan paniscus: Testing the female bonding hypothesis. Primates 47:210–7.
Strier KB, Lynch JW, and Ziegler TE. 2003. Hormonal changes during the mating and conception seasons of wild northern muriquis (Brachyteles arachnoides hypoxanthus). American Journal of Primatology 61:85–99.
Strier KB, and Ziegler TE. 2005. Variation in the resumption of cycling and conception by fecal androgen and estradiol levels in female Northern Muriquis (Brachyteles hypoxanthus). American Journal of Primatology 67:69–81.
Strier KB. 1994. Myth of the typical primate. American Journal of Physical Anthropology 37:233–271.
Stumpf R. 2007. Chimpanzees and Bonobos: Diversity within and between species. In: Campbell C, Fuentes A, MacKinnon C, Panger M, Bearder S, editors. Primates in Perspective. Oxford: Oxford University Press.
Swedell L. 2002. Affiliation among females in wild hamadryas baboons (Papio hamadryas hamadryas). International Journal of Primatology 23:1205–1226.
Symington M. 1990. Fission-fusion social organization in Ateles and Pan. International Journal of Primatology 11:47–61.
Taylor S, Klein L, and Lewis B. 2000. Biobehavioral responses to stress in females: Tend-and- befriend, not fight-or-flight. Psychological Review 107:411–429.
Taylor SE. 2006. Biobehavioral Bases of Affiliation Under Stress. Current Directions in Psychological Science 15:273–277.
Thierry B. 2008. Primate socioecology, The lost dream of ecological determinism. Evolutionary Anthropology 17:93–96.
Tilbrook S, Turner A, Ibbott MD, and Clarke IJ. 2006. Activation of the hypothalamo-pituitary- adrenal axis by isolation and restraint stress during lactation in ewes: Effect of the presence of the lamb and suckling. Endocrinology 147:3501–9.
Touma C, and Palme R. 2005. Measuring fecal glucocorticoid metabolites in mammals and birds: the importance of validation. Annals of the New York Academy of Sciences 1046:54–74.
Trivers R. 1971. The Evolution of Reciprocal Altruism. The Quarterly Review of Biology 46:35– 57.
Trivers R. 2006. Reciprocal altruism: 30 years later. In: Kappeler P, van Schaik C, editors. Cooperation in Primates and Humans: Mechanisms and Evolution. Berlin: Springer-Verlag. p 67–83.
155
Tu MT, Lupien SJ, and Walker C-D. 2005. Measuring stress responses in postpartum mothers: perspectives from studies in human and animal populations. Stress 8:19–34.
Uvnäs-Moberg K. 1998. Oxytocin may mediate the benefits of positive social interaction and emotions. Psychoneuroendocrinology 23:819–35.
Valero A, Schaffner C, Vick LG, Aureli F, and Ramos-fernandez G. 2006. Intragroup lethal aggression in wild spider monkeys. American Journal of Primatology 737:732–737.
Walker SL, Smith RF, Jones DN, Routly JE, and Dobson H. 2008. Chronic stress, hormone profiles and estrus intensity in dairy cattle. Hormones and Behavior 53:493–501.
Wallner B, Möstl E, Dittami J, and Prossinger H. 1999. Fecal glucocorticoids document stress in female Barbary macaques (Macaca sylvanus). General and Comparative Endocrinology 113:80–86.
Wasser SK, Hunt KE, Brown JL, Cooper K, Crockett CM, Bechert U, Millspaugh JJ, Larson S, and Monfort SL. 2000. A generalized fecal glucocorticoid assay for use in a diverse array of nondomestic mammalian and avian species. General and Comparative Endocrinology 120:260–75.
Webster TH, and Suarez S. 2008. Transfer behavior and association patterns of female spider monkeys (Ateles belzeburth belzebuth) in Yasuni National Park, Ecuador. American Journal of Physical Anthropology S46:219.
Weingrill T, Gray DA, Barrett L, and Henzi SP. 2004. Fecal cortisol levels in free-ranging female chacma baboons: relationship to dominance, reproductive state and environmental factors. Hormones and Behavior 45:259–69.
Whitten PL, Stavisky R, Aureli F, and Russell E. 1998. Response of fecal cortisol to stress in captive chimpanzees (Pan troglodytes). American Journal of Primatology 44:57–69.
Wiesenfeld AR, Malatesta CZ, Whitman PB, Granrose C, and Robin U. 1985. Psychophysiological response of breast- and bottle-feeding mothers to their infant’s signals. Psychophysiology 22:79–86.
Williams JM, Pusey AE, Carlis J V, Farm BP, and Goodall J. 2002. Female competition and male territorial behaviour influence female chimpanzees’ ranging patterns. Animal Behaviour 63:347–360.
Windle R, Wood S, Shanks N, Perks P, Conde G, Da Costa a P, Ingram CD, and Lightman SL. 1997. Endocrine and behavioural responses to noise stress: Comparison of virgin and lactating female rats during non-disrupted maternal activity. Journal of Neuroendocrinology 9:407–14.
Wolfe JD, and Ralph CJ. 2009. Correlations between El Niño–Southern Oscillation and changes in nearctic–neotropic migrant condition in Central America. The Auk 126:809–814. 156
Wrangham R. 1980. An ecological model of female-bonded primate. Behaviour 75:262–300.
Ziegler T, Scheffler G, and Snowdon C. 1995. The relationship of cortisol levels to soical environment and reproductive functioning in female cotton-top tamarins, Saguinus oedipus. Hormones and Behavior 29:407–424.
Ziegler TE, Santos CV, Pissinatti A, and Strier KB. 1997. Steroid excretion during the ovarian cycle in captive and wild muriquis, Brachyteles arachnoides. American Journal of Primatology 42:311–321.
Ziegler TE, and Sousa MBC. 2002. Parent–daughter relationships and social controls on fertility in female common marmosets, Callithrix jacchus. Hormones and Behavior 42:356–367.
Ziegler TE, and Wittwer DJ. 2005. Fecal steroid research in the field and laboratory: Improved methods for storage, transport, processing, and analysis. American Journal of Primatology 67:159–74.
157
Appendix A: Focal Subject Tables
Code Individual Class Offspring AG Agata Lactating Amaya AS Anne SA cycling AR Ariadne Lactating Aaron BU Buttercup SA cycling DA Dawn cycling Dylan EV Evelyn Lactating Elsa FA Fanta Lactating Frito HO Houdini SA cycling IS Isela cycling Ines JI Jill pregnant/lactating Julian, Jordan JL Jlo Lactating Judah LE Leila Lactating/cycling? Lorelai MC MC Mindy cycling MI Missy cycling Moe RU Rudy Lactating/cycling? Rosie ST Strawberry pregnant/lactating Shortcake ZE Zelda Lactating Zander, Zeke A.1 Focal subjects, reproductive stage, and offspring for female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
158
Code Focals Hours Fecals Mean Cortisol Mean Estradiol AG 62 10.3 5 5.93 4.66 AS 81 13.5 17 9.38 10.32 AR 115 19.2 13 13.27 5.06 BU 74 12.3 9 23.33 4.61 DA 15 2.5 2 10.6 5.84 EV 30 5 1 2.38 5.47 FA 17 2.8 1 4.45 0.75 HO 79 13.2 9 17.08 2.5 IS 31 5.2 5 6.09 3.2 JI 75 12.5 8 9.23 9.11 JL 132 22 17 13.83 3.4 LE 140 23.2 18 12.55 3.97 MC 72 12 11 34.67 18.15 MI 37 6.2 2 23.8 3.08 RU 110 18.5 12 16.95 8.44 ST 17 2.8 2 9.29 4.48 ZE 31 5.2 3 2 14.84 A.2. Number of focals, hours, fecals, and mean cortisol and estradiol concentrations for all female black-handed spider monkeys at El Zota Biological Field Station, Costa Rica.
159
Appendix B: Hormone Profiles
9 8
7
6 5
4 Cortisol 3
Cortisolng/gram Estradiol 2 1 0
B.1. Cortisol and estradiol profiles for Agata (AG), an adult female black-handed spider monkey at El Zota Biological Field Station, Costa Rica.
160
90 80
70
60 50 40 Cortisol
30 Estradiol Cortisolng/gram 20 10 0
B.2. Cortisol and estradiol profile for Anne Shirley (AS), a subadult female black-handed spider monkey at El Zota Biological Field Station.
45 40
35 30 25 20 Cortisol 15
Cortisolng/gram Estradiol 10 5
0
2/19/2011 8/19/2010 9/19/2010 1/19/2011 3/19/2011 4/19/2011 5/19/2011 6/19/2011 7/19/2011
11/19/2010 12/19/2010 10/19/2010 B.3. Cortisol and estradiol profile for Ariadne (AR), an adult female black-handed spider monkey at El Zota Biological Field Station.
161
160 140
120 100 80 60 Cortisol
Cortisolng/gram 40 Estradiol 20
0
8/17/2010 9/17/2010 1/17/2011 2/17/2011 3/17/2011 4/17/2011 5/17/2011 6/17/2011 7/17/2011
12/17/2010 11/17/2010 10/17/2010 B.4. Cortisol and estradiol profile for Buttercup (BU), a subadult female spider monkey at El Zota Biological Field Station, Costa Rica.
50 45 40
35 30 25 20 Cortisol
Cortisolng/gra 15 Estradiol 10 5 0
B.5. Cortisol and estradiol profile for Houdini (HO), a subadult female black-handed spider monkey at El Zota Biological Field Station, Costa Rica.
162
12
10
8
6 Cortisol 4
Cortisolng/gram Estradiol 2
0
B.6. Cortisol and estradiol profile for Isela (IS), an adult female black-handed spider monkey at El Zota Biological Field Station, Costa Rica.
45 40
35
30 25 20 Cortisol
15 Estradiol Cortisolng/gram 10 5 0
B.7. Cortisol and Estradiol profile for Jill (JI), an adult female black-handed spider monkey at El Zota Biological Field Station. Jill gave birth to an infant during the week of 11/6/2010-11/15/2010.
163
60
50
40
30 Cortisol 20
Cortisolng/gram Estradiol 10
0
3/2/2011 7/2/2011 9/2/2010 1/2/2011 2/2/2011 4/2/2011 5/2/2011 6/2/2011 8/2/2011
11/2/2010 12/2/2010 10/2/2010 B.8. Cortisol and estradiol profile for Jlo (JL), an adult female spider monkey at El Zota Biological Field Station, Costa Rica.
70
60
50
40
30 Cortisol
Cortisolng/gram 20 Estradiol 10
0
9/15/2010 5/15/2010 6/15/2010 7/15/2010 8/15/2010 1/15/2011 2/15/2011 3/15/2011 4/15/2011 5/15/2011 6/15/2011 7/15/2011
11/15/2010 12/15/2010 10/15/2010 B.9. Cortisol and estradiol profile for Leila (LE), an adult female black-handed spider monkey at El Zota Biological Field Station, Costa Rica.
164
300
250
200
150 Cortisol 100
Cortisolng/gram Estradiol 50
0
8/25/2010 9/25/2010 1/25/2011 2/25/2011 3/25/2011 4/25/2011 5/25/2011 6/25/2011 7/25/2011
11/25/2010 12/25/2010 10/25/2010 B.10. Cortisol and estradiol profile for Muttonchop Mindy (MC), an adult female black-handed spider monkey at El Zota Biological Field Station, Costa Rica.
70
60
50
40
30 Cortisol
Cortisolng/gram 20 Estradiol 10
0
6/30/2011 7/30/2010 8/30/2010 9/30/2010 1/31/2011 2/28/2011 3/31/2011 4/30/2011 5/31/2011 7/31/2011
11/30/2010 12/31/2010 10/31/2010 B.11. Cortisol and estradiol profile for Rudy (RU), an adult female black-handed spider monkey at El Zota Biological Field Station, Costa Rica.
165