Data from: Is male rhesus macaque facial coloration under intra-sexual selection?

Exaggerated male traits can evolve under intra- or inter-sexual 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 intra-sexual selection, the red facial coloration of male rhesus macaques (Macaca mulatta) appears to have evolved primarily under inter-sexual 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 intra-sexual 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 intra-sexual 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（intra-sexual selection）或性间选择（inter-sexual selection）作用下演化，但目前尚不清楚两种选择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会展现出色彩信号，这类信号在性内选择下被视为地位徽章，但雄性恒河猴（Macaca mulatta）的红色面部毛色，其演化主要由性间选择与雌性配偶选择驱动。然而实验表明，红色对雄性具有显著辨识度，这引发了该信号是否同时处于性内选择作用下的疑问。本研究旨在探究雄性是否会在竞争场景中更强烈地展现该色彩信号，研究于交配季高峰期，采集了某自由活动社群中全部15只成年雄性的面部图像，并通过视觉模型对其面部毛色进行量化分析。结果显示，面部红色度更为相似的雄性，相较于毛色差异较大的个体，更易发生攻击性互动，这表明毛色或参与了雄性对竞争者的评估过程。此外，雄性在被观察到交配的当日，面部毛色更深；且仅在交配当日，优势等级才可预测面部红色度，这提示毛色或可传递雄性捍卫配偶的动机。因此，恒河猴的面部毛色或可在繁殖机会的竞争中，介导雄性与竞争者之间的对抗性互动，使其同时受到性间选择与性内选择的作用。但本次研究中毛色差异幅度较小，这引发了关于该信号可感知性的疑问。此外，毛色差异仍有可能反映雄性个体状态的差异，而这种差异进而会改变雄性在同性竞争与交配投入上的资源分配。