THE EVOLUTION OF LEKKING: INSIGHTS FROM A SPECIES WITH A FLEXIBLE MATING SYSTEM By KAVITA ISVARAN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2003 Copyright 2003 by Kavita Isvaran This dissertation is dedicated to my parents, Kalyani and Balkrishna Isvaran. I owe much to their love, support, and belief that I can accomplish anything I set my mind to. ACKNOWLEDGMENTS I thank my advisor, Jane Brockmann, for her involvement, support, and encouragement. I have benefited immensely from her critical and creative thinking, and have also been greatly inspired by her unflagging enthusiasm for science. I am grateful to my committee, Colin Chapman, Yadvendradev Jhala, Colette St. Mary, and Mel Sunquist, for their unstinting help. The diverse perspectives they provided enriched my work. I thank the University of Florida, the Animal Behavior Society, the American Society for Mammalogists, and Sigma Xi for funding my research. The Forest Departments of Andhra Pradesh, Gujarat, Maharashtra, Rajasthan, and Tamil Nadu in India gave me permission to work in various protected areas. I am also grateful to them for providing much logistical support and help in the field. I am immensely indebted to Yadvendradev Jhala, the USFWS funded project titled Conservation of the Indian Wolf, and the Wildlife Institute of India for making my work at Velavadar National Park, Gujarat, possible. I thank K.V.R. Priyadarshini and Bharat Jethva for help in the field. I am indebted to Elizabeth Mungall for introducing me to Texan blackbuck, for sharing her experience and knowledge, and for her interest in my work. I thank my Texan family, Wesley and Jackie Kyle, for allowing me to study blackbuck on their ranch and for making my stay with them so enjoyable. I thank Craig Osenberg, Ben Bolker, and John Eisenberg for valuable discussions of my work. I have greatly enjoyed and benefited from interactions with many graduate iv students in the Department of Zoology. I thank Laura Sirot, Amy Zanne, Manjula Tiwari, April Randle, John Paul, Nat Seavy, Kate Moran, Becca Hale, Sarah Schaack, Ben Miner, Manuel Velez, Billy Gunnels, Sarah Bouchard, Toshi Okuyama, and many many others for engrossing scientific discussions and their friendship. I am deeply grateful to my parents and sister for their unfailing and unconditional love, support, encouragement and enthusiasm. This research would not have been half as enjoyable without Suhel Quader. His intellectual support, confidence, and support during stressful times were vital to the completion of this work. v TABLE OF CONTENTS Page ACKNOWLEDGMENTS ................................................................................................. iv ABSTRACT..................................................................................................................... viii CHAPTER 1 GENERAL INTRODUCTION ....................................................................................1 2 THE ECOLOGICAL, DEMOGRAPHIC, AND SOCIAL CORRELATES OF LEKKING IN AN ANTELOPE WITH A FLEXIBLE MATING SYSTEM..............7 Introduction...................................................................................................................7 Methods ......................................................................................................................11 Results.........................................................................................................................19 Discussion...................................................................................................................24 3 WHEN SHOULD MALES LEK? INSIGHTS FROM A DYNAMIC STATE VARIABLE MODEL.................................................................................................49 Introduction.................................................................................................................49 Methods ......................................................................................................................54 Results.........................................................................................................................64 Discussion...................................................................................................................72 4 WHAT MAINTAINS THE SIZE OF MATING TERRITORIES IN A LEKKING SPECIES? MODELS AND EMPIRICAL TESTS.....................................................89 Introduction.................................................................................................................89 Models Of Territory Size............................................................................................92 Evaluating models using empirical data.....................................................................98 Results.......................................................................................................................102 Discussion.................................................................................................................104 5 MAINTENANCE OF INTRASPECIFIC VARIATION IN GROUP SIZE IN AN INDIAN ANTELOPE: INSIGHTS FROM COMPARATIVE AND ECONOMIC APPROACHES ........................................................................................................123 Introduction...............................................................................................................123 Methods ....................................................................................................................127 vi Results.......................................................................................................................135 Discussion.................................................................................................................138 6 GENERAL DISCUSSION.......................................................................................160 APPENDIX CALCULATION OF THE STANDARDIZED CLUSTERING INDEX.......................172 LIST OF REFERENCES.................................................................................................174 BIOGRAPHICAL SKETCH ...........................................................................................182 vii Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy THE EVOLUTION OF LEKKING: INSIGHTS FROM A SPECIES WITH A FLEXIBLE MATING SYSTEM By Kavita Isvaran August 2003 Chair: H. Jane Brockmann Major Department: Zoology The study of intraspecific variation can provide insights into the evolution and maintenance of behavior. To identify the conditions associated with the rare and unusual mating system, lekking, I studied variation in male mating behavior of the blackbuck, Antilope cervicapra, an Indian antelope with a flexible mating system. Blackbuck show wide variation in the distribution of mating territories, from large solitary territories to tightly clumped, classical leks. Rather than viewing lekking as a discrete mating system, I treat leks as one extreme in a range of territory distributions, and describe the mating system of a population in terms of the distribution of mating territories (the degree of clustering and territory size). I studied factors influencing male clustering and territory size in ten blackbuck populations, nine in India and one in Texas, U.S.A. I also studied one Indian population more intensively. I found that large-scale, among-population variation in territory clustering was most strongly related to the size of female groups. In turn, female group size was best explained by habitat structure and resources. viii Interestingly, I found similar patterns at a smaller spatial scale within the intensively studied population. However, my findings suggested that at finer scales, other selective factors (e.g., female choice, male competition, male harassment) are likely to shape the clustering of territories. Using a dynamic modeling approach, I found that male clustering at finer scales was favored when benefits to clustering increased nonlinearly with cluster size. Of three prominent hypothesized selective factors that I evaluated, female mating bias for larger clusters appeared most likely to satisfy the conditions favoring male clustering. Finally, I found that in addition to examining male clustering, understanding the factors leading to reduced territory sizes is essential. Models and empirical analyses of territory size suggest that a female bias for mating in the center of an aggregation (or at restricted mating sites) in combination with large cluster sizes likely favors the reduced territory sizes typically seen in classical leks. Taken together, my work suggests that multiple factors acting on different aspects of male mating behavior and at different spatial scales maintain lekking. ix CHAPTER 1 GENERAL INTRODUCTION The lek-mating system has attracted much research attention because it displays several unusual features and presents a challenge to our understanding of mating system evolution. Males fiercely defend tiny, highly clustered territories, which are devoid of resources (e.g., forage, breeding sites) attractive to females (Bradbury 1981). Females are thought to visit these territorial aggregations, called leks, for the sole purpose of mating. This form of mating system is rare and very unlike more typical mammalian and bird systems in which