Many studies have identified the reproductive benefits of cooperative behaviors, yet few have identified the mechanisms that underlie these behaviors. Mechanistic studies can inform our understanding of why some individuals are more or less cooperative, as well as identify the physiological constraints imposed upon the evolution of reproductive traits. Male wire-tailed manakins (Pipra filicauda) exhibit cooperative courtship behaviors and more cooperative territory holders have been shown to exhibit higher reproductive success. To begin to understand the proximate basis of cooperative display behaviors, we conducted both an observational study and an experimental study. Because coordinated courtship displays underlie this form of cooperation, our study also examined both the hormonal and social drivers of individual variation in courtship behavior more broadly (e.g., courtship display rates). Our observational study revealed that males with higher testosterone levels performed fewer cooperative display bouts. In addition, our experimental study demonstrated that the proportion of a male’s courtship displays that were cooperative decreased after being administered a testosterone-filled hormone implant. 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 for territory-holding males.

中文翻译：

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减少合作行为，因为在一只韭菜雀科鸟类中高睾丸激素的成本

许多研究已经确定了合作行为的生殖益处，但很少有人发现了合作行为的基础。机理研究可以帮助我们理解为什么有些人或多或少会合作，并确定施加于生殖性状进化的生理限制。雄性铁尾山雀（Pipra filicauda）表现出合作的求爱行为，并且更多的合作领土持有者已显示出更高的繁殖成功率。为了开始了解合作展示行为的基本依据，我们进行了观察性研究和实验性研究。由于协调的求爱展示是这种合作形式的基础，我们的研究还更广泛地考察了求爱行为中个体差异的激素和社会驱动因素（例如，求爱展示率）。我们的观察性研究表明，睾丸激素水平较高的男性较少进行合作性展示活动。此外，我们的实验研究表明，在接受睾丸激素填充的荷尔蒙植入后，男性具有协作性的求爱展示的比例降低了。我们发现在两项研究中，个人的求爱展示努力（即展示率和花时间进行求爱展示）与循环睾丸激素之间没有关系。但是，与较少合作的男性相比，更多的合作男性花费更多的时间进行求爱表演，暗示睾丸激素可以通过影响领土持有者的合作行为来间接地调动求爱行为。总体而言，我们的观察结果和实验结果均表明，降低合作行为是维持领地男性高水平睾丸激素的成本。