The Sexual Selection Landscape and Sexually-Selected Traits of the Crested Macaque (Macaca nigra)

Sexual selection explains the evolution of traits that impact mating and reproductive success. Primates are an excellent system for studying sexually-selected traits, because they exhibit marked variation in mating systems, show evidence of intrasexual competition in both sexes, and mutual mate choice. Here, I discuss the sexual selection dynamic of the crested macaque (Macaca nigra) and its overall putative sexually-selected phenotype. Female crested macaque fertile phases are highly asynchronous, which alpha males monopolize, leading to high reproductive skew. Given the reproductive benefits to alpha status, males compete aggressively over dominance, with all observed alpha male replacements occurring via top-entry challenge competition. Infant disappearances increase after the arrival of a new alpha male, indicating potential infanticide from incoming males. Consistent with strong male-male contest competition, crested macaques exhibit marked body and canine size sexual dimorphism, with males being larger. Males exhibit small relative testis volume, indicating that indirect competition via sperm competition is not prevalent. Male-male relationships are mediated by social status signals, including loud calls and colorful genitals, and males largely avoid each other. Female-female relationships are generally affiliative with low levels of aggression. Females exhibit direct mate choice in the form of some consortship maintenance, and also exhibit a number of proceptive behaviors towards males, which increase in frequency markedly during fertile phases. Females exhibit traits that may facilitate indirect mate choice, including copulation calls and large sexual swellings. Our understanding of the species enables us to set the crested macaque into comparative context, and to characterize many of its traits as part of a holistic sexually-selected phenotype.