Sexual selection and the evolution of brain size in primates

Abstract

Reproductive competition among males has long been considered a powerful force in the evolution of primates. The evolution of brain size and complexity in the Order Primates has been widely regarded as the hallmark of primate evolutionary history. Despite their importance to our understanding of primate evolution, the relationship between sexual selection and the evolutionary development of brain size is not well studied. The present research examines the evolutionary relationship between brain size and two components of primate sexual selection, sperm competition and male competition for mates. Results indicate that there is not a significant relationship between relative brain size and sperm competition as measured by relative testis size in primates, suggesting sperm competition has not played an important role in the evolution of brain size in the primate order. There is, however, a significant negative evolutionary relationship between relative brain size and the level of male competition for mates. The present study shows that the largest relative brain sizes among primate species are associated with monogamous mating systems, suggesting primate monogamy may require greater social acuity and abilities of deception.

Figure 1

Error-bar plots of residuals from the least-squares regressions of brain (a,b) and testis (c,d) weights on body weight, and sexual mass dimorphism values (e, f) by mating system and female promiscuity determinations. Variables were log_e–transformed prior to regression analysis. Error bars represent one standard error of the mean. Mating system: MMMF, multi-male/multi-female; PA, polyandrous; Mon, monogamous; SM single male.

Figure 2

Bivariate plot of relative brain size regressed on dimorphism. Gray oval represents the 95% confidence ellipse of the bivariate distribution. The positions of all monogamous genera (Homo, Hylobates, Aotus), chimpanzees and gorillas are labeled for reference.