posted on 2021-02-23, 18:04authored byBen Joseph Vernasco
(This dataset is superseded by the one linked from DOI 10.5061/dryad.fttdz08nv and is also referenced below.) Many studies have identified the reproductive benefits of cooperative behaviors, yet few have focused on identifying the mechanisms that underlie these behaviors. Mechanistic studies can not only inform our understanding of why some individuals are more or less cooperative, but they can also identify the physiological constraints imposed upon the evolution of reproductive traits. To further our understanding of the mechanistic basis of cooperation, we measured the relationship between circulating testosterone and the cooperative courtship behaviors of territory-holding, male wire-tailed manakins. We then experimentally tested our observational results by implanting males with either a testosterone-filled or empty (i.e., control) silastic implant and measuring the effects on cooperative behavior. Because male-male cooperation occurs in the form of coordinated courtship displays in this species, our study design examined both the hormonal and social drivers of individual variation in cooperative behavior (e.g., cooperative display bouts) as well as courtship behavior more broadly (e.g., display rates). Our observational study revealed that males with higher testosterone levels performed fewer cooperative display bouts. Furthermore, in our experimental implant study, males significantly decreased the proportion of their displays that were cooperative. We found no relationship between an individual’s courtship display effort (i.e., display rate and time spent performing courtship displays) and circulating testosterone in either study. However, more cooperative males spent a greater proportion of time performing courtship displays than did less cooperative males, suggesting that testosterone may indirectly mediate courtship display behaviors by influencing a territory holder’s cooperative behavior. Overall, both our observational and experimental results suggest that reduced cooperative behavior is a cost of maintaining high levels of testosterone.