Male quality, dominance rank, and mating success in free-ranging rhesus macaques

In social mammals, condition and health are important determinants of the ability of males to achieve high dominance rank. Measures of individual condition are also predicted to affect male fitness via female preference for high-quality mates. We examined intermale variation in phenotypic quality (immune function and oxidative stress) in relation to male dominance status and mating success in a species with prominent female choice and a lack of male–female sexual coercion, the rhesus macaque (Macaca mulatta). We quantified immunity via 2 functional assays of innate immune response (bacteria killing assay and hemolytic complement assay) and measured oxidative stress via a lipid peroxidation assay in 15 adult males from 1 social group of macaques on Cayo Santiago, Puerto Rico. We then observed these males throughout the mating season to test the prediction that males in better condition achieved higher mating success. Males with more robust innate immune response and lower oxidative damage mated with a greater number of potentially fertile females. Male dominance rank, however, also correlated with our measures of quality. Higher-ranking males had more robust functional innate immune response and lower levels of oxidative damage. After accounting for rank, male quality was no longer correlated with mating success. These results demonstrate a potentially important role of male phenotypic quality in the mating system of a long-lived, group-living primate. What are the exact behavioral mechanisms via which sexual selection may operate on traits related to immunocompetence and resistance to oxidative damage in this species, however, remains an open question.