Sensory exploitation in a sit-and-wait predator Exploring the functions of stabilimenta in the banded garden spider, Argiope trifasciata by SUSAN ALLISON CROWE A thesis submitted to the Department of Biology in conformity with the requirements for the degree of Master of Science Queen's University Kingston, Ontario, Canada September, 2009 Copyright © Susan A. Crowe, 2009 ii ABSTRACT Attracting prey by exploiting a visual sensory bias is a common theme in stationary predators across many taxa, particularly for obligate ambush predators, such as orb-weaving spiders, because they construct complex prey traps. Mimicry of UV-reflecting floral-guides has been suggested as the mechanism behind the tendency for spiders and silk web decorations (stabilimenta) to reflect in the UV, to attract pollinators that they then prey upon. Also, many insects are attracted to UV because it most commonly indicates open sky, or a safe flight path. My study focuses on the prey attraction function of stabilimenta, in Argiope trifasciata in eastern Ontario. Decorated webs were no more likely to contain prey than undecorated webs, but for adult spiders, longer stabilimenta were associated with increased likelihood of prey capture. For both adults and juveniles, larger webs were more likely to contain prey in undecorated webs, but for decorated webs, web size was not a predictor of prey presence. I interpret this as evidence for a trade-off between two alternative prey capture strategies: building a web with a large capture area, or building a small web with a stabilimentum. In further support of this trade-off, smaller webs were more likely to contain a stabilimentum, for both juveniles and adults. My data also suggest that close neighbours compete rather than cooperate with each other. Adult webs were spaced farther apart from each other than juvenile webs, more than would be expected based on web diameter difference. For juveniles, webs with a closer neighbour were more likely to be decorated, implying an increased need for prey attraction in the presence of a nearby competitor. For adults, prey was more likely to be found in webs that were more solitary. My results do not support the hypothesis that visually attractive spiders increased prey capture by aggregating. iii ACKNOWLEDGEMENTS Thank you to my supervisor, Bob Montgomerie, for his valuable guidance throughout this entire process and for thorough revisions to my thesis. Thank you to Lori Parker, Philina English, Sara Calhim, Fraser Cameron, and Adam Berry for providing the field assistance required to make this project possible. I am also grateful to Chris Eckert for offering valuable insights while serving on my supervisory committee, and to both Chris Eckert and Bob Montgomerie for teaching statistics courses that provided me with the foundation necessary to undertake the analyses presented in this thesis. I also want to thank Steve Lougheed for his support and guidance, for being a good mentor and friend throughout my time at Queen's, and for leading an amazing field course to Argentina that provided incredible field experiences for me and all the students on the trip. The field course was a unforgettable reminder of the fascinating diversity that exists in the sub-tropics. The project I completed on Argyrodes kleptoparasitism of Nephila webs, as well as the myriad observations we made during various hikes and excursions, gave me firsthand experience with many of the species I discuss in my general introduction. Thank you also to Caleb Hasler for engaging conversations and diverse field experiences, and for helping me to realize that statistics and evolutionary biology are two of my favourite things. I thank my grandfather Marshall Crowe for his love and support in making all of my accomplishments possible. His generosity, integrity, and achievements make him the most incredible role model a granddaughter could ever ask for. Thank you to my late grandmother, Doris Crowe, for energetically nurturing my desire to learn about the natural world. Thank you to my Grandpa and his partner Glenna Roberts for providing a peaceful thesis-writing haven for me while I was hard at work. I also thank my parents, Tom and Allison Crowe, my sister, Cathy Crowe, my aunts, Alison, Helen, Sheila, and Abigail Crowe, and my closest friends, Dana Murchison, Brenden Hurley, Creag Winacott, Jesse Toninger, Radek Zlamal, and Dave, Sam, and Jadyn Berry for their constant support despite my stress and many lengthy disappearances. Funding for this project was provided by an NSERC Canada Graduate Scholarship and Discovery Grant. iv TABLE OF CONTENTS ABSTRACT...............................................................................................................................................ii ACKNOWLEDGEMENTS......................................................................................................................iii TABLE OF CONTENTS..........................................................................................................................iv LIST OF TABLES....................................................................................................................................vi LIST OF FIGURES..................................................................................................................................ix CHAPTER 1. General Introduction..........................................................................................................1 Introduction to stabilimenta..............................................................................................15 Stabilimenta as a prey attractant.......................................................................................17 Stabilimenta as a prey attractant for facultatively aggregating spiders............................21 Stabilimenta as attractive or detractive to predators.........................................................22 Other visual hypotheses for stabilimenta function...........................................................23 Non-visual hypotheses for stabilimenta function.............................................................24 Thesis outline....................................................................................................................25 CHAPTER 2. Methods...........................................................................................................................26 Study sites and study species............................................................................................26 Monitoring prey capture, predation, and web-building behaviour...................................28 Statistical Analyses...........................................................................................................29 CHAPTER 3. Results..............................................................................................................................32 Prey types.........................................................................................................................32 Division of data into juvenile and adult subsets...............................................................34 Comparing juvenile and adult spiders..............................................................................36 v Simple correlations between web and spider variables for juveniles and adults.............39 Prediction of stabilimentum presence in juvenile and adult webs...................................41 Prediction of stabilimentum length in juvenile and adult webs.......................................45 Prediction of prey presence in juvenile and adult webs...................................................50 CHAPTER 4. Discussion........................................................................................................................60 Future Directions..............................................................................................................72 Contributions to Science...................................................................................................74 CHAPTER 5. Literature Cited................................................................................................................75 SUMMARY..............................................................................................................................................84 vi LIST OF TABLES Table 3.1 One-way ANOVAs comparing continuous spider and web variables between juvenile and adult A. trifasciata........................................................................................37 Table 3.2 Fisher exact tests comparing binary spider and web variables between juvenile and adult A. trifasciata.....................................................................................................38 Table 3.3 Correlation matrix of continuous spider and web variables for juvenile A. trifasciata.....................................................................................................................39 Table 3.4 Correlation matrix of continuous spider and web variables for adult A. trifasciata.....................................................................................................................40 Table 3.5 Multiple logistic regression using distance to closest web, web diameter, and their interaction to predict presence/absence of a stabilimentum in juvenile A. trifasciata.....................................................................................................................43 Table 3.6