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CALIFORNIA STATE UNIVERSITY, NORTHRIDGE Comparative Thermal Tolerances, Performance Eurythermy, and Temperature- Dependent Energy Demands of the World’s Southernmost Gecko with Implications for a Changing Climate A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Biology by Denita M. Weeks December 2013 The thesis of Denita M. Weeks is approved: Peter J. Edmunds, Ph.D. Date Steven R. Dudgeon, Ph.D. Date Robert E. Espinoza, Ph.D. Chair Date California State University, Northridge ii ACKNOWLEDGMENTS I would like to thank Robert E. Espinoza, Félix B. Cruz, Gabriela Perotti, Lorena B. Quiroga, Eduardo A. Sanabria, and Jason R. Warner for help in the field. I am especially grateful to Cristián S. Abdala, Félix B. Cruz, Juan Pablo Juliá, Fernando Lobo, Gabriela Perotti, Lorena B. Quiroga, Eduardo A. Sanabria, Soledad Valdecantos, and their families for their logistical support and kind hospitality. The members of my committee, Robert E. Espinoza, Peter J. Edmunds, and Steven R. Dudgeon, have been invaluable to my accomplishments at CSUN. Without their knowledge, support, and guidance, this study and all of my professional growth would not have been so productive and rewarding. Much thanks to Barbara Joos, Cristián S. Abdala, and Rocio Aguilar for providing helpful resources or feedback on the thesis and to Michael J. Angilletta for statistical advice. Additionally, I thank the following people for contribution to this thesis and making my time at California State University, Northridge so enjoyable and productive: Beck A. Wehrle, Alissa de Koning, Jason R. Warner, Mark V. Oliva, Fritz Hertel, Paul Wilson, Tim J. Karels, Dave A. Gray, and Cindy Hitchcock. I would like to acknowledge the unending personal support from family and friends. Thanks to my family, Sonya L. Weeks, Michael A. Weeks, and Patricia L. Weeks, Anita L. Gasper and Dennis R. Gasper, for the constant support, encouragement, and enthusiasm. Many thanks to my boyfriend, Eric A. R. Hernandez, for supporting me and enduring two years of my absence while I pursued my academic goals far from home. iii The IACUC of California State University, Northridge (CSUN) approved this research (0708-0002a,b) and collecting permits were authorized by the respective provincial faunal offices (APN, National Parks of Argentina: 0945/09–10 [L. Buria]; Neuquén: APN 008/10 [F. Lonac]; Chubut: 251/09 [S. Montanelli]; Río Negro: 127414– DF-2008–2010 [M. Failla]; Santa Cruz: 024/08 [L.B. Ortega). My research was supported by funding from the American Society of Ichthyologists and Herpetologists, CSUN, the National Science Foundation, Sigma Xi, and the Society for the Study of Amphibians and Reptiles. iv TABLE OF CONTENTS Signature Page .................................................................................................................... ii Acknowledgments .............................................................................................................. iii Abstract .............................................................................................................................. vi Chapter 1: Thermal Tolerances Of Homonota darwinii ..................................................... 1 Chapter 2: Sprint Performance Of Homonota darwinii .................................................... 20 Chapter 3: Temperature-Dependent Physiology And Energetics Modeling For Homonota darwinii in Current and Future Climates .......................................................................... 41 Literature Cited ................................................................................................................. 63 Appendix A: Tables .......................................................................................................... 78 Appendix B: Figures ......................................................................................................... 96 v ABSTRACT Comparative Thermal Tolerances, Performance Eurythermy, and Temperature- Dependent Energy Demands of the World’s Southernmost Gecko with Implications for a Changing Climate by Denita M. Weeks Master of Science in Biology Most aspects of the lives of ectotherms (e.g. distribution, locomotion, digestion, and reproduction) are dependent on body temperature (Tb). For ectotherms living in cool environments, the ability to cope with extreme temperatures and even achieve the warmer Tbs needed for optimal performance can be particularly challenging. Nocturnal ectotherms in such climates face an added challenge because they lack solar radiation as a heat source during periods of activity. Darwin’s marked gecko (Homonota darwinii)— the world’s southernmost gecko species—is broadly distributed from central to southern Argentina and adjacent Chile (35–52 °S latitude) where cold climates and high winds likely pose a thermal challenge for this nocturnal species. I measured the thermal tolerances and temperature-dependent sprint performance of four populations of H. darwinii spanning most of the latitudinal range of this species in southern Argentina. I also estimated rates of temperature-dependent metabolism and evaporative water loss of geckos from the same populations to characterize their physiological response to vi temperature. Temperature dataloggers were deployed at each field site to record annual operative temperatures (Te). The metabolic data were combined with the Te data in an energetic model to estimate annual energy budgets for the current climate and following predicted site-specific changes in climate. I hypothesized that the lower thermal tolerances (critical thermal minimum, CTmin) of these geckos would differ among populations such that geckos from the southernmost (and therefore presumably the coolest) populations would have lower CTmin relative to northern populations, but that the upper thermal tolerances (panting threshold, Tpant) would not differ among populations. I found that CTmin differed among populations, but not latitudinally as predicted. Remarkably, some populations had subzero CTmin suggesting freeze tolerance or supercooling ability—the first time this phenomenon has been reported for any gecko. Tpant did not differ among populations. Geckos from populations with the most thermal heterogeneity had the widest thermal tolerance ranges. I also hypothesized that the southernmost populations of this species would be the most thermally challenged, as evidenced by lower temperature-dependent sprint performance relative to northern populations. Temperature-dependent sprint performance indicated non-latitudinal shifts in performance over the geographic range of H. darwinii, yet most populations shared a similarly broad range for optimal performance indicating eurythermy. Eurythermy in performance suggests that future climate change may be neutral or even beneficial to H. darwinii. Differences in rates of resting metabolism and evaporative water loss were marginally significant among populations, yet only at higher experimental Tbs. The southernmost population of H. darwinii experiences a relatively mild, thermally homogenous climate and was estimated to have the highest annual energy demands, vii suggesting both an opportunity and need for longer daily and seasonal activity. Furthermore, my model estimates this population will be the least affected by climate warming. More northern populations that inhabit more thermally variable environments were predicted to experience similarly marginal effects of climate change based solely on abiotic considerations. viii CHAPTER 1: THERMAL TOLERANCES OF HOMONOTA DARWINII ABSTRACT Knowledge of the thermal limits of a species is essential to predicting its response to climate change. Most organisms are limited by a range of body temperatures (Tb) in which normal function of biological processes (i.e., cellular respiration, foraging, digestion) take place. These are known as thermal tolerances, and exceeding these bounds often translates to slowed performance and even death in some cases. Behavior is a widely used mechanism across taxa to avoid extreme environmental temperatures. Ectotherms, such as lizards, are especially efficient at behavioral thermoregulation because they rely almost exclusively on the abiotic environment to regulate Tb. However, sometimes, behavior is insufficient and physiological adaptations at the cellular level allow them to cope with variable, even extreme, thermal environments. Yet, challenges may still exist for nocturnal ectotherms (lack of solar radiation as a heat source during activity), particularly for those living in harsh, cold environments such as Homonota darwinii. As the world’s southernmost gecko species, it already likely faces thermal challenges in addition to being nocturnal. I measured thermal tolerances (critical thermal minimum, CTmin and panting threshold, Tpant) for four populations of Homonota darwinii spanning most of the latitudinal range of the species. CTmin differed across populations, but not latitudinally as predicted, likely because latitude was not as good a proxy for operative temperatures (Te). Some populations had subzero CTmin indicating potential supercooling or freeze tolerance—the first time either phenomenon has been reported for 1 a gecko. Tpant did not differ significantly among populations. Finally, thermal tolerance breadth appears to be correlated with thermal variability in the environment. INTRODUCTION Most multicellular organisms function over a limited range of temperatures. This range typically falls