The expression of male sexual traits, which is stimulated by testosterone, entails significant costs for individuals. Consequently, natural selection is expected to favour the modulation of sexual trait development according to the balance between its costs and benefits. The proportion of rivals in a population may affect this balance by increasing or decreasing the reproductive benefits associated with the development of sex traits. Here, we explore the relationship between testosterone level and sex trait size under two populational conditions of mate competition: fenced (i.e. high male-male competition; all male age groups are present) and unfenced (i.e. low competition; most males present are juveniles). Our model species is the Iberian red deer (Cervus elaphus hispanicus), and the sex trait is the dark ventral patch that males exhibit during the rutting season. Our results showed that the positive relationship between testosterone levels and the size of the dark ventral patch depends on the environmental level of male-male competition. Only in populations where the operational sex ratio was high (i.e. high proportion of rival males), individuals with high levels of testosterone developed the sex trait. Conversely, when mate competition was low, there was no significant relationship between testosterone level and trait size. This result reinforces the idea that the effect of testosterone in promoting the development of sex traits may be mediated by the intensity of mate competition in the population, as well as the role of sexual selection in the evolution of the dark ventral patch in red deer.

中文翻译：

---

睾丸激素和雄性马鹿的深色腹面：社会环境的作用。

睾丸激素刺激的男性性特征的表达需要个人付出高昂的代价。因此，根据自然选择的成本与收益之间的平衡，人们期望自然选择有利于调节性状的发展。人口中竞争者的比例可能会通过增加或减少与性特征的发展相关的生殖利益而影响这种平衡。在这里，我们探讨了两种竞争条件下的人群睾丸激素水平与性别特征大小之间的关系：围栏（即男性-男性竞争激烈；所有男性年龄段都存在）和不设防（即低竞争；存在的大多数男性是少年） 。我们的模型物种是伊比利亚马鹿（鹿elaphus hispanicus），而性特征是男性在车辙季节出现的深色腹面斑。我们的结果表明，睾丸激素水平与黑暗腹面的大小之间呈正相关关系，这取决于男女竞争的环境水平。只有在工作性别比很高的人群中（即，相对的男性比例很高），睾丸激素水平高的人才会表现出性特征。相反，当伴侣竞争较低时，睾丸激素水平与性状大小之间没有显着关系。这一结果加强了这样一种观念，即睾丸激素在促进性状发展中的作用可能是由种群中伴侣竞争的强度以及马鹿黑暗腹面斑块的进化中性选择的作用所介导的。