A Proximate Perspective on the Cooperative Behavior of a Lekking Passerine

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.