DOES PLASTICITY IN THE WEB BUILDING BEHAVIOR OF THE WESTERN BLACK WIDOW SPIDER, LATRODECTUS HESPERUS, AFFECT FORAGING AND DEFENSE? A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Master of Science Jacquelyn Zevenbergen August, 2007 DOES PLASTICITY IN THE WEB BUILDING BEHAVIOR OF THE WESTERN BLACK WIDOW SPIDER, LATRODECTUS HESPERUS, AFFECT FORAGING AND DEFENSE? Jacquelyn Zevenbergen Thesis Approved: Accepted: ______________________________ ______________________________ Advisor Dean of College Dr. Todd Blackledge Dr. Ronald F. Levant ______________________________ ______________________________ Committee Member Dean of the Graduate School Dr. Stephen C. Weeks Dr. George R. Newkome ______________________________ ______________________________ Committee Member Date Dr. Peter Niewiarowski ______________________________ Department Chair Dr. Bruce Cushing ii ABSTRACT The black widow spider, Latrodectus hesperus, constructs a three-dimensional cobweb that contains a tangled sheet held in place by supporting silk threads, and uses sticky gumfooted threads, instead of sticky spirals, to adhere to prey. Both the size of the sheet and the number of gumfooted threads may facilitate prey capture, while the supporting silk threads may enhance predator defense by surrounding the spider with silk barriers. We found that fasted L. hesperus constructed webs with more capture than support components compared to fed spiders, even though fed spiders typically invest more silk overall in webs. We hypothesize that fasted spiders spin webs with architectures that function better at prey capture while webs spun by fed spiders function better at defense. To test the foraging efficacy of the two types of webs, we allowed spiders to forage on crickets for three hours, videotaping them to record both successful and unsuccessful attempts to capture prey. We also attempted to compare the ability of the webs to protect spiders from predatory mud-dauber wasps, Chalybion caeruleum and Sceliphron caementarium. To eliminate spider motivation as a confounding factor, half of the individuals were placed on webs spun by spiders of the opposite foraging history. Our results show that, regardless of foraging motivation, spiders were 28% more likely to capture prey, and caught twice as many crickets, when foraging on webs spun by fasted spiders versus fed spiders. We were unable to collect enough data to test the anti- predation efficacy of the two different types of webs; however we never observed S. iii caementarium entering any cobweb during two months of observation. In contrast, we observed C. caeruleum entering theridiid webs and displaying prey mimic behavior on multiple occasions. Therefore, we conclude that hungry spiders invest their silk in the components of webs that best increase foraging success – while fed spiders increase their investment of silk in the non-foraging components that may protect them from predation. iv DEDICATION To my mother and son, Janice and Jud: for their understanding when I could not spend the time with them that they deserved. To my late father, Edward: for giving me confidence with his belief that I could accomplish anything that I was willing to work for. And most importantly, to my husband, John: I could not have realized my dreams without your sacrifice of my companionship and willingness to suffer through a chaotic home life while I focused on my education. v ACKNOWLEDGEMENTS I would like to thank my advisor, Dr. Todd Blackledge, for his support and encouragement as I have traveled my academic journey. I appreciate his patience and wonderful advice and all of the time he has spent reading, re-reading and offering numerous suggestions for my papers and presentations. The members of my committee, Dr. Stephen C. Weeks and Dr. Peter Niewiarowski, have both offered valuable perspectives on my research and shown a serious interest in my progress. I would also like to express my gratitude for the help I received from two undergraduate students, Nicki Schneider and Hannah Koppleberger. I will forever be grateful for the Biology Department office staff, Sue Robinson and Cheryl O’Dear, for their willingness to help me navigate the policies and procedures of graduate school. In addition to the staff, the faculty members of this department have been a wonderful resource; I have met almost every faculty member, and I have expanded my respect for science because of my conversations with many of them. Funding for this research project was provided by NSF, the American Arachnological Society, and the Christian Stinner Memorial Scholarship. I am grateful to Dr. Paul Martin for his investment in the Dr. Paul E. Martin Center for Field Studies and Environmental Education where I conducted most of my research and to the Crown Point Ecology Center and Hale Farm & Village where I collected wasps for my study. vi TABLE OF CONTENTS Page LIST OF TABLES………………………………………………………………………..ix LIST OF FIGURES……………………………………………………………………….x CHAPTER I. INTRODUCTION.....................................................................................................…...1 II. MATERIALS AND METHODS………………...…………………………………….8 Experiment 1: Do webs constructed by fasted spiders function better in foraging?..................................................................................................................8 Experiment 2- Do webs constructed by fasted spiders function better in defense?..………………………………………………….………………….…..12 Statistical Analysis……………………………………………………………….15 III. RESULTS…………………………………………………………………………....17 Experiment #1- Foraging..………………………………………….……………17 Experiment #2- Defense……..………………………..…………………………20 IV. DISCUSSION……..………………………………………………………………....23 Experiment #1- Foraging...………………………………………………………23 Experiment #2- Defense..……………………………..…………………………31 General Discussion……………………………………..…………………..……33 REFERENCES…………………………………………………………………………..47 vii APPENDICES………………………………………………………………….………..54 APPENDIX A. SPIDER RAW DATA.................................................................55 APPENDIX B. WEB RAW DATA..................................................................…58 viii LIST OF TABLES Table Page 1. Foraging types and predator defense strategies ………………………………..……37 ix LIST OF FIGURES Figure Page 1. Theridiid webs………….……………………………………………….……..…....38 2. Allocations of silk based on prey availability…...…………………….………....….39 3. Fed and fasted spiders construct different webs…………………………..……..….40 4. Setup for single trial of foraging experiment……………………….…………..…...40 5. Spider condition…………………………………..……………………………...….41 6. Types of silk investment…………………….……………………………….…..….42 7. Percent of trials with capture success…………………………………………..…...43 8. Frequency of number of crickets captured……...………………………………..….44 9. Time until first capture………………………….………………………………..….45 10. Comparison of number of gumfoots to the number of crickets caught……..…..…..46 11. Comparison of number of gumfoots to the time of first capture………………...….46 x CHAPTER I INTRODUCTION Most animals must balance selective pressures from successful foraging and predator defense (Jennions & Petrie, 1997; Lind & Cresswell, 2005; Sih et al., 2004b; Lima & Dill, 1990). Animals need to achieve a minimal amount of foraging success to provide the energy needed for survival and reproduction while successful predator defense is necessary for survival. Maximizing the fitness benefits of both foraging and defensive behaviors in the same context is seldom possible because the optimal behaviors for one trait are often counterproductive to maximizing the other (Blackledge & Wenzel, 1999; Lima, 1998; Lima & Bednekoff, 1999; Lima & Dill, 1990; Jennions & Petrie, 1997; Lind & Cresswell, 2005; Sih et al., 2004a). Behavioral plasticity allows animals to adjust their behavior as their needs for defense or foraging change (Blackledge & Wenzel, 1999; Li & Lee, 2004; Lima, 1998; Lima & Dill, 1990; Lind & Cresswell, 2005). Both internal changes, such as developmental shifts in energy needs or predation risk (Bilde et al., 2002; Eberhard, 2003; Higgins, 1992a; Rayor & Uetz, 1990; Rayor & Uetz, 1993; Schoener & Spiller, 1992), and external changes such as fluctuations in prey availability (Blackledge & Wenzel, 2001b; Craig et al., 2001; Herberstein et al., 2000; Pasquet et al., 1999) or predator density (Barnes et al., 2002; Persons et al., 2002; Walker & Rypstra, 2003; Wilder & Rypstra, 2004), can influence whether defense or foraging is 1 more important for fitness at different times. Other types of plasticity, such as morphological and physiological plasticity can evolve to deal with developmental or seasonal challenges (Ebert, 1996; Greene, 1996; Levitan, 1987; Morrison & Hero, 2003) but behavioral plasticity evolves because it allows individual animals to maximize their fitness in their current environment (Lima, 1998; Lima & Dill, 1990; Marchand & McNeil, 2004). Behavioral strategies, for example nocturnal activity, that successfully minimize the conflict between foraging and defense by temporally separating the costs and benefits of each can improve fitness (Cloudsley- Thompson, 1995). For example, if animals could meet their energy needs without foraging when prey was scarce (low benefit) and predation was high (high cost), and instead they foraged when prey was abundant (high benefit) and predation was low (low cost), they would enjoy greater fitness. One of the ways animals could deal with spatial variations is to move to the habitat