The Costs of Reproductive Success in Male Rhesus Macaques (Macaca mulatta) on Cayo Santiago

Sexual selection acts to increase the success of males possessing advantageous traits in competition over females. In primates, interspecific variability in social and mating systems creates highly variable selective pressures on males, changing the relative strength of both intra- and intersexual selection, and the relative degree of direct vs. indirect male–male competition. Rhesus macaques are an interesting species for studying intrasexual selection and male–male competition, because they exhibit relatively low (for Papionini) body and canine size dimorphism, and exhibit large testes, suggesting reduced direct competition and strong indirect competition. We have undertaken several studies of male rhesus macaques on Cayo Santiago, from analyses of long-term life-history data to shorter term projects that combined noninvasive measures of physiological markers such as concentrations of urinary C-peptide of insulin and androgen and glucocorticoid (GC) concentrations, with measures of behavior and of sexually selected signals (male red facial coloration). We here review these studies, combining data from short-term studies with long-term mortality data to present an integrated picture of both the short- and long-term gross costs of male mating competition. We find that males exhibit many signs of the costs of indirect competition, such as energetic consequences of reduced feeding and high copulation rates. During periods of more direct contest, such as during dominance instability, males are also characterized by high androgen and GC concentrations among high-ranking individuals. Consistent with relatively weak direct male–male competition, male red skin coloration appears to be more related to female choice (intersexual selection) than the signaling of dominance status (intrasexual selection). Forty-five years of life-history data show that male mortality is higher during the mating than the birth season, a pattern we hypothesize to be linked to the costs associated with mating activity. We finish by discussing unresolved issues, such as the costs of sperm competition and the data that are needed to address them.