Department of Biology, York University, Toronto, Ontario, Canada.
Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada.
Am J Primatol. 2025 Jan;87(1):e23711. doi: 10.1002/ajp.23711.
Secondary sexual characteristics, and the extent to which they are expressed, can convey information about the signaller. The blue scrotum and red penis of male vervet monkeys (Chlorocebus pygerythrus) make them a good species in which to examine inter- and intramale variation in signal expression. We quantified genital hue and luminance of male vervets at Lake Nabugabo, Uganda from standardized photos of male genitalia taken in May to June 2016, January to March 2019, and April to June 2019 to examine how dominance rank, fecal androgens (fARMs), fecal glucocorticoids (fGCMs), and parasitism related to achromatic (i.e., luminance) and chromatic (i.e., hue) aspects of scrotal and penile coloration, as well as how genital color related to sociosexual behaviors. We examined 182 photoshoots, 214 fecal samples for hormone analyses, and 152 for parasite analyses. Linear models indicate that genital color is linked to male dominance rank; high-ranking males had a more luminant (i.e., brighter) scrotum and a redder penis. Within males, color characteristics remained relatively stable over the short-term and changed moderately over the long-term. The direction of change was inconsistent for all color characteristics except scrotal luminance, which increased in all males over the long-term. Males with a darker penis received more mating presentations, while higher-ranking males received more mating refusals than low-ranking males, suggesting that females pay attention to penile color. We did not find support for any parasite or hormone mediation of color, and while there was a correlation between fGCM and fARMs, this was positive rather than negative as predicted by the stress-linked immunocompetence handicap hypothesis. Overall, our results indicate that the production of genital color may serve as an intra- and/or intersexual signal of male dominance rank and age in vervets.
第二性征及其表现程度可以传达有关信号发送者的信息。雄性青长尾猴（Chlorocebus pygerythrus）的蓝色阴囊和红色阴茎使它们成为研究信号表达的种间和种内变异的良好物种。我们从 2016 年 5 月至 6 月、2019 年 1 月至 3 月和 2019 年 4 月至 6 月期间对乌干达纳布加博湖的雄性青长尾猴进行了标准化的生殖器照片拍摄，并量化了其生殖器的色调和亮度，以研究优势等级、粪便雄激素（fARMs）、粪便皮质醇（fGCMs）和寄生虫与阴囊和阴茎颜色的非彩色（即亮度）和彩色（即色调）方面的关系，以及生殖器颜色与社交行为的关系。我们检查了 182 次拍摄，214 次粪便样本用于激素分析，152 次用于寄生虫分析。线性模型表明，生殖器颜色与雄性优势等级有关；高等级雄性的阴囊更明亮（即更亮），阴茎更红。在雄性内部，颜色特征在短期内相对稳定，而在长期内变化适度。除了阴囊亮度外，所有颜色特征的变化方向都不一致，所有雄性的阴囊亮度都在长期内增加。阴茎颜色较深的雄性接受了更多的交配展示，而高等级雄性比低等级雄性收到了更多的交配拒绝，这表明雌性会注意阴茎颜色。我们没有发现寄生虫或激素对颜色的任何介导作用，虽然 fGCM 和 fARMs 之间存在相关性，但与压力相关的免疫能力障碍假说所预测的相反，这种相关性是正相关而不是负相关。总体而言，我们的结果表明，在青长尾猴中，生殖器颜色的产生可能是雄性优势等级和年龄的种内和/或种间信号。
