New Insights in the Evolutionary Maintenance of Male Mate Choice Behaviour Using the Western Black Widow, Latrodectus Hesperus
Total Page：16
File Type：pdf, Size：1020Kb
New Insights in the Evolutionary Maintenance of Male Mate Choice Behaviour using the Western Black Widow, Latrodectus hesperus by Emily Constance MacLeod A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Ecology and Evolutionary Biology University of Toronto © Copyright by Emily C. MacLeod 2013 v New Insights into the Evolutionary Maintenance of Male Mate Choice Behaviour using the Western Black Widow, Latrodectus hesperus Emily Constance MacLeod Doctor of Philosophy Ecology and Evolutionary Biology University of Toronto 2012 Abstract Mate choice among males is relatively understudied, despite recent evidence supporting its ubiquity. Theory predicts male mate choice in response to variation in female quality, and male mating strategies that limit polygyny. However empirical research investigating these connections, particularly under natural conditions, is generally lacking. Using the Western black widow spider, Latrodectus hesperus , I investigated male mate choice, male investment, and the advantage of mating with females that differ in their potential to deliver fitness benefits to males via higher fecundity and/or reduced risk of sperm competition. Males were found to be preferentially attracted to larger, high-diet virgin females over females that were smaller due to a lower diet, or having been previously mated, or both. Through a three-year field study on females I found that males likely benefit from selectivity, as smaller, low-diet females often failed to deposit any egg sacs throughout the breeding season, likely due to their shortened web- site tenure and/or lack of bodily resources. I investigated the costs and benefits to male genital breakage in L. hesperus, a strategy typically assumed to result in male sterility while providing paternity protection. I found that genital mutilation in L. hesperus did not cause sterility and that males were capable of inseminating multiple females, likely because of the comparatively low ii amount of genital damage and the likelihood of the efficient placement of genital fragments. Evidence from double mating trials supported the efficacy of broken genital fragments as plugs, but first male sperm precedence was often maintained in cases where male genital breakage failed or fragments were positioned incorrectly, which occurred frequently. Together, these laboratory and field experiments contribute to a more complete view of mate choice. iii Acknowledgments I am very grateful for the support of my mentor Maydianne Andrade. Her guidance and direction as well as her faith in my ideas and abilities led me to be able trust myself enough to complete a project that I am truly proud of. Maydianne’s passion for her work, her intelligence, her personal integrity, and her feminist perspective on the scientific world has been inspirational to me. I am also thankful to my committee members, Peter Abrams and Deborah McLennan, for their insights and perspectives over the past five years; their comments and criticism have greatly improved this manuscript. Gard Otis introduced me to the subject of behavioural ecology, and I will be forever grateful for his guidance, friendship, and consistent support and encouragement. My academic achievements are rooted in his faith in me. I could not have completed my work without the assistance of Maria Modanu. Her piercing insight, her help, her support and her friendship over the past five years has been invaluable. I am also extremely grateful to my three field assistants: Madeline Girard, Katie Keough, and Sheena Fry. I was so fortunate to be able to spend my field seasons in the company of such intelligent, hard working, and interesting friends. I am particularly indebted to Maddie Girard and Damian Elias for both their practical assistance and friendship during my California days. I consider myself very fortunate to have had the opportunity to conduct my field research at Hastings Natural History Reservation in California, and am particularly grateful for all the support I received over four years there. Finally I would like to acknowledge my extreme privilege that having supportive friends and family has afforded me during my degree. My sister, my extended family, and especially my parents, Rod and Cathy MacLeod, have provided me with endless love and encouragement, despite their apprehension towards my study species. The patience, calmness, love and practical assistance of Simon Ford has been instrumental in my achievements, as have the friendships of Stephanie Moynagh, Viktoryah Lapp and Jennifer Code. I am also grateful for the conversation and commiseration of Tiffany Schreiver, Devin Bloom and Caroline Tucker, and the advice and support of Luciana Baruffaldi. I would like to dedicate my thesis to my grandparents, all either officially or unofficially scientists. They continue to inspire me despite their absence. iv Table of Contents Abstract ........................................................................................................................................... ii Acknowledgments .......................................................................................................................... iv List of Tables ................................................................................................................................. vi List of Figures ............................................................................................................................... vii Introduction ..................................................................................................................................... 1 Chapter 1: Male mate choice in the western black widow spider ................................................. 23 Chapter 2: Male mate choice and variable female quality in field populations ............................ 47 Chapter 3: Genital mutilation need not cause monogyny ............................................................. 90 Chapter 4: Strong first male sperm precedence despite frequent plugging failure in the western black widow spider Latrodectus hesperus .................................................................... 120 Conclusion .................................................................................................................................. 161 Appendices .................................................................................................................................. 193 v List of Tables 1.1. Differences between the weights and size of males collected from different female treatments ........................................................................................................................ 46 2.1. Summary of observations of field females .............................................................. 83 2.2. Results of GLM of field female weights and estimated sizes .................................. 85 2.3. Results of GLM of egg sac number and female size ............................................... 86 2.4. Correlations between persistence, size, web size and egg sac production ............... 87 2.5. Results of one-way ANOVA of female size each year ............................................ 88 2.6. Comparisons of females that did and did not reproduce .......................................... 89 2.7. Summary of potential predictors of polygyny ....................................................... 119 4.1. Number of wild caught females by year ................................................................ 160 vi List of Figures 1. Male and female Latrodectus hesperus ...................................................................... 21 1.1. Picture of field enclosure and cage .......................................................................... 43 1.2. Males recovered in experimental release/enclosure study ....................................... 44 2.1. Map of field study sites ............................................................................................ 74 2.2. Percent of females found within each observation period each year ....................... 75 2.3. Distribution of egg sac production among females within each year ...................... 76 2.4. Female size and egg sac production ......................................................................... 77 2.5. Egg sac production by weeks of female persistence ................................................ 79 2.6. Female size and egg sac production by field location ............................................. 80 2.7. Distribution of persistence among non-reproductive females ................................. 82 3.1. Potential positions for embolic fragments in female genitalia ............................... 114 3.2. Distribution of sclerite loss among polygynous males .......................................... 115 3.3. Distribution of polygynous matings in opportunity windows ............................... 116 3.4. Average copulation duration by mating order ....................................................... 117 3.5. Spiderling hatch by female mating order position ................................................. 118 4.1. Female reproductive organs ................................................................................... 146 4.2. Male reproductive organs ...................................................................................... 147 4.3. Distribution of number of apical sclerites found in wild-caught females .............. 148 4.4. Distribution