Elucidating the mechanisms responsible for driving individual variation in behavior is a foundational question in organismal biology. Answering these types of questions is necessary for understanding how tradeoffs are mediated as well as potential constraints on evolutionary responses to selection. In Chapter I, I synthesize the evidence suggesting that testosterone plays a central role in driving individual variation in cooperative reproductive behaviors and mediating the tradeoff between cooperation and competition. The subsequent chapters of my dissertation then focus on understanding the mechanistic sources of individual variation in the cooperative courtship behaviors of male wire-tailed manakins (Pipra filicauda), a Neotropical lekking songbird. Wire-tailed manakins exhibit cooperative courtship display behaviors wherein both floater and territory-holding males perform coordinated courtship displays. Territory-holding males sire essentially all offspring and, among territory-holders, those that are more cooperative exhibit higher reproductive success. Cooperation also benefits floater males in that more cooperative floater males have a higher probability of becoming a territory-holder. In Chapter II, I detail the difficulties associated with measuring circulating testosterone in free-living animals and develop a new field technique that can improve our ability to accurately account for the effects that the stress of capture has on circulating testosterone levels in birds. In Chapter III, I quantify individual variation in the cooperative courtship behaviors of territory-holding male wire-tailed manakins using video cameras. I then use both observational and experimental approaches to show that among territory holders, high testosterone has antagonistic effects on a male's cooperative behavior. Chapter IV focuses on quantifying how an individual's cooperative behavior relates to their telomere length. Telomeres are the segments of repetitive DNA found at the end of chromosomes. Telomeres shorten in response to both physiological and environmental perturbations, are predictive of an individual's mortality risk and, because of these characteristics, are thought to reflect an individual's biological age (as opposed to chronological age). My results show that short telomeres are associated with increased cooperative behaviors and, given that a male's cooperative behavior is reflective of their reproductive investment, suggest that males with lower future reproductive potential (i.e., shorter telomeres) invest more in reproduction. My last chapter (Chapter V) focuses on synthesizing these results and suggests that future research on cooperative behaviors will need to integrate biomarkers of an individual's condition with mechanisms that reflect an individual's social competence to further understand the sources of individual variation in cooperation. / Doctor of Philosophy / Cooperative reproductive behaviors occur when multiple individuals coordinate their reproductive efforts to gain an advantage over other individuals or those individuals that attempt to reproduce in the absence of cooperation. In this context, some individuals have been found to consistently act more or less cooperative, and outstanding questions regarding the evolution of cooperative reproductive behaviors focus on attempting to understand the intrinsic differences among individuals that consistently vary in their cooperative tendencies. In this dissertation, I focus on measuring individual variation in the cooperative reproductive behavior of male wire-tailed manakins, a passerine found in the Amazon Rainforest, as well as the sources of this individual variation. Male wire-tailed manakins perform acrobatic courtship displays for females. Wire-tailed manakins are especially interesting in that multiple males will perform coordinated courtship displays, but only one male within the group typically reproduces. Individual variation in this cooperative behavior also influences how likely an individual is to reproduce. Chapter I synthesizes the evidence across species to suggest that testosterone, a hormone known for influencing competitive reproductive behaviors, influences an individual’s likelihood of acting in a cooperative or competitive manner. Chapter II describes the difficulties associated with measuring circulating testosterone in wild animals and describes techniques that can be used to overcome some of the difficulties associated with measuring circulating testosterone in free-living birds. Chapter III focuses on understanding the relationship between testosterone and individual differences in the cooperative reproductive behaviors of the wire-tailed manakin. I find that those males with higher testosterone or those with experimentally increased testosterone are less cooperative. I discuss my results in the context of understanding how this relationship could influence the evolution of circulating testosterone levels more broadly. Chapter IV examines the relationship between the cooperative reproductive behavior of male wire-tailed manakins and the repetitive segments of DNA found at the end of chromosomes called telomeres. Telomeres shorten as individuals age and the length of an individual’s telomeres has been found to be predictive of their lifespan and mortality risk. My research shows that those individuals with shorter telomeres (or a higher mortality risk) are more cooperative. Given that an individual’s cooperative behavior is predictive of their probability of successfully reproducing, my research suggests that those individuals that are more likely to perish invest more time and energy in attempting to reproduce. In Chapter V, I conclude by summarizing my results and suggesting future research that can further our understanding of the sources of consistent individual differences in cooperative behavior and the evolution of cooperation more broadly.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/102660 |
| Date | 18 September 2019 |
| Creators | Vernasco, Ben Joseph |
| Contributors | Biological Sciences, Moore, Ignacio T., Sewall, Kendra B., Ryder, Thomas B., McGlothlin, Joel W., Walters, Jeffrey R. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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