Data from: Is male rhesus macaque facial coloration under intrasexual selection?

Exaggerated male traits can evolve under intra- or intersexual selection, but it remains less clear how often both mechanisms act together on trait evolution. While the males of many anthropoid primate species exhibit colorful signals that appear to be badges of status under intrasexual selection, the red facial coloration of male rhesus macaques (Macaca mulatta) appears to have evolved primarily under intersexual selection and female mate choice. Nonetheless, experiments show that red color is salient to males, raising the question of whether the signal may also be under intrasexual selection. Here, we examine whether males express this signal more strongly in competitive contexts. Facial images were collected on all 15 adult males of a free-ranging social group during the peak of the mating season, and coloration was quantified using visual models. Results show that males more similar in facial redness were more likely to interact aggressively than more dissimilar ones, suggesting that color may be involved in the assessment of rivals. Furthermore, males exhibited darker coloration on days they were observed copulating, and dominance rank predicted facial redness only on copulating days, suggesting that coloration may also advertise motivation to defend a mate. Male rhesus macaque facial coloration may thus mediate agonistic interactions with rivals during competition over reproductive opportunities, such that it is under both inter- and intrasexual selection. However, color differences were small, raising perceptibility questions. It remains possible that color variation reflects differences in male condition, which in turn alter investment towards male–male competition and mating effort.雄性的夸张性状可在性内选择（intrasexual selection）或性间选择（intersexual selection）的作用下演化，但目前学界仍未明确两种机制共同作用于性状演化的频率。尽管诸多类人猿灵长类物种的雄性均会展现出色彩信号，这类信号在性内选择框架下通常被视为地位徽章；但雄性恒河猴（Macaca mulatta）的红色面部毛色，其演化过程主要被认为受性间选择与雌性配偶选择驱动。 不过相关实验表明，红色毛色对雄性具有显著辨识度，这引出了该信号是否也处于性内选择作用下的疑问。本研究聚焦于探究雄性是否会在竞争情境下更强烈地表达该色彩信号。研究采集了某自由活动社群中全部15只成年雄性在交配高峰期的面部图像，并通过视觉模型对毛色特征进行量化分析。 结果显示，面部红色度更为相似的雄性，相较于毛色差异更大的个体，更易发生攻击性互动，这提示毛色或参与了雄性对竞争对手的评估环节。此外，雄性在被观测到交配的当日，其面部毛色更深；且仅在交配当日，优势等级才能预测面部红色度，这表明毛色或许也能传递雄性捍卫配偶的动机。 综上，雄性恒河猴的面部毛色可介导繁殖机会竞争过程中与竞争对手的对抗性互动，使其同时处于性间选择与性内选择的双重作用之下。但本次研究中毛色差异幅度较小，这引发了关于其感知可行性的疑问。毛色变异仍有可能反映雄性个体状态的差异，而个体状态又会进一步影响雄性间竞争与交配投入的资源配置。