School of Biological Sciences, Washington State University, Pullman, WA, USA; Department of Neurobiology and Behavior, Cornell University, USA; Cornell Lab of Ornithology, Ithaca, NY, USA.
Department of Ecology and Evolutionary Biology, Tulane University, New Orleans, LA, USA.
Horm Behav. 2022 Jun;142:105158. doi: 10.1016/j.yhbeh.2022.105158. Epub 2022 Apr 1.
Androgens like testosterone mediate suites of physical and behavioral traits across vertebrates, and circulation varies considerably across and within taxa. However, an understanding of the causal factors of variation in circulating testosterone has proven difficult despite decades of research. According to the challenge hypothesis, agonistic interactions between males immediately prior to the breeding season produce the highest levels of testosterone measured during this period. While many studies have provided support for this hypothesis, most species do not respond to male-male competition by elevating testosterone. As a result, a recent revision of the hypothesis ('challenge hypothesis 2.0') places male-female interactions as the primary cause of rapid elevations in testosterone circulation in male vertebrates. Here, we offer a test of both iterations of the challenge hypothesis in a tropical bird species. We first illustrate that male White-shouldered Fairywrens (Malurus alboscapulatus) differ by subspecies in plasma androgen concentrations. Then we use a social network approach to find that males of the subspecies with higher androgens are characterized by greater social interaction scores, including more time aggregating to perform sexual displays. Next, we use a controlled experiment to test whether males respond to simulated territorial intrusion and/or courtship competition contexts by elevating androgens. We found that males elevated androgens during territorial intrusions relative to flushed controls, however, males sampled during courtship competitions had greater plasma androgens both relative to controls and males sampled while defending territories. Ultimately, our results are consistent with challenge hypothesis 2.0, as sexual interactions with extra-pair females were associated with greater elevation of androgens than territorial disputes.
雄激素，如睾酮，介导了脊椎动物的一系列生理和行为特征，其在不同物种和同一物种内的循环水平存在显著差异。然而，尽管已经进行了数十年的研究，但对于影响循环睾酮水平变化的因果因素仍难以理解。根据挑战假说，繁殖季节前雄性之间的争斗互动会产生该时期内测量到的最高水平的睾酮。尽管许多研究为这一假说提供了支持，但大多数物种并不会通过提高睾酮水平来对雄性竞争做出反应。因此，该假说最近进行了修订（“挑战假说 2.0”），将雄性与雌性的互动视为雄性脊椎动物循环中睾酮快速升高的主要原因。在这里，我们在一种热带鸟类中对这两种挑战假说进行了测试。我们首先表明，白肩仙鹟（Malurus alboscapulatus）的亚种间在血浆雄激素浓度上存在差异。然后，我们使用社交网络方法发现，具有更高雄激素的亚种的雄性具有更高的社交互动得分，包括更多的时间聚集来进行性展示。接下来，我们使用控制实验来测试雄性是否会通过模拟领地入侵和/或求偶竞争的情况来提高雄激素水平。我们发现，雄性在领地入侵时的雄激素水平相对于冲洗对照有所升高，然而，在求偶竞争期间采样的雄性的血浆雄激素水平无论是相对于对照还是相对于正在保卫领地的雄性都更高。最终，我们的结果与挑战假说 2.0 一致，因为与配对外雌性的性互动与雄激素水平的更大升高有关，而不是领地争端。
