According to the Challenge Hypothesis, social interactions, particularly among males, have a strong influence on circulating androgen levels. Specifically, males should respond to social challenges from conspecific males with a rapid increase in plasma androgen levels which support and stimulate further aggression. This basic tenet of the Challenge Hypothesis, an androgen increase in response to a social challenge from another male, has been tested in all vertebrate classes. While early studies generally supported the Challenge Hypothesis, more recent work has noted numerous exceptions, particularly in birds. Here, we conduct a meta-analysis of studies in fish, amphibians, non-avian reptiles, and mammals that test the prediction that circulating androgen levels of males should increase in response to an experimental challenge from another male. We found that teleost fish often increase androgens during such challenges, but other vertebrate groups show more mixed results. Why should fish be different from the other taxa? In fish with paternal care of young, the potential conflict between mating, being aggressive towards other males, and taking care of offspring is alleviated, because females typically choose males based on their defense of an already existing nest. Hence, rather than regulating the trade-off between mating, aggression, and parenting, androgens may have been co-opted to promote all three behaviors. For other taxa, increasing androgen levels only makes sense when the increase directly enhances reproductive success. Thus, the increase in androgen levels is a response to mating opportunities rather than a response to challenge from another male. To further our understanding of the role of a change in androgen levels in mediating behavioral decision-making between mating, aggression, and parenting, we need studies that address the behavioral consequences of an increase in androgens after male-male encounters and studies that test the androgen responsiveness of species that differ in the degree of paternal care.

根据“挑战假说”，社交互动，尤其是男性之间的互动，对循环雄激素水平有很大影响。具体地说，雄性应该应对同种雄性的社会挑战，使血浆雄激素水平迅速增加，从而支持并刺激进一步的侵略行为。挑战假说的基本原理是雄性激素对另一位男性的社交挑战的反应而增加，已在所有脊椎动物类别中进行了测试。尽管早期研究普遍支持挑战假说，但最近的工作注意到许多例外，尤其是鸟类。在这里，我们对鱼类，两栖动物，非禽类爬行动物和哺乳动物的研究进行了荟萃分析，以检验男性循环雄激素水平应响应另一位男性的实验挑战而增加的预测。我们发现硬骨鱼类在此类挑战中通常会增加雄激素，但其他脊椎动物群体则表现出更多的混合结果。为什么鱼应该与其他分类单元不同？在幼鱼的幼鱼的照料中，交配，对其他雄鱼的侵略和照顾后代之间的潜在冲突得以缓解，因为雌鱼通常根据对已有巢穴的防御来选择雄鱼。因此，雄激素可能没有被选择用来促进这三种行为，而不是在交配，侵略和育儿之间进行权衡取舍。对于其他类群，仅当雄激素水平直接提高繁殖成功率时，雄激素水平才有意义。因此，雄激素水平的提高是对交配机会的反应，而不是对来自另一雄性的挑战的反应。