The ontogeny of sexual dimorphism in free-ranging rhesus macaques

Objective: Reconstructing the social lives of extinct primates is possible only through an understanding of the interplay between morphology, sexual selection pressures, and social behavior in extant species. Somatic sexual dimorphism is an important variable in primate evolution, in part because of the clear relationship between the strength and mechanisms of sexual selection and the degree of dimorphism. Here, we examine body size dimorphism across ontogeny in male and female rhesus macaques to assess whether it is primarily achieved via bimaturism as predicted by a polygynandrous mating system, faster male growth indicating polygyny, or both. Methods: We measured body mass in a cross-sectional sample of 362 free-ranging rhesus macaques from Cayo Santiago, Puerto Rico to investigate size dimorphism: (1) across the lifespan; and (2) as an outcome of sex-specific growth strategies, including: (a) age of maturation; (b) growth rate; and (c) total growth duration, using regression models fit to sex-specific developmental curves. Results: Significant body size dimorphism was observed by prime reproductive age with males 1.51 times the size of females. Larger male size resulted from a later age of maturation (males: 6.8–7.8 years vs. females: 5.5–6.5 years; logistic model) and elevated growth velocity through the pre-prime period (LOESS model). Though males grew to larger sizes overall, females maintained adult size for longer before senescence (quadratic model). Discussion: The ontogeny of size dimorphism in rhesus macaques is achieved by bimaturism and a faster male growth rate. Our results provide new data for understanding the development and complexities of primate dimorphism.