Sexual Selection and the Evolution of Brain Size in Primates

Michael A. Schillaci,Seth G.N. Grant

doi:10.1371/journal.pone.0000062

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.

Highlights

Since Darwin’s 1871 publication [1] on the evolution of humans and sexual selection, reproductive competition among males has been considered a powerful force in the evolution of primates and other mammals
After accounting for body mass, both mating system and female promiscuity were associated significantly with the level of sperm competition estimated from testes size
Male body mass was associated with male competition for mates as measured by mass dimorphism, mating system and female promiscuity were not

Summary

Introduction

Since Darwin’s 1871 publication [1] on the evolution of humans and sexual selection, reproductive competition among males has been considered a powerful force in the evolution of primates and other mammals. Research associating increasing brain size with increasing group size and social complexity in primates predicts brain size, the size of the neocortex, will co-evolve with mating systems exhibiting social complexity [2,3,4]. In this context, larger brains are selected for because they confer greater reproductive fitness associated with increased social acuity or the ability to manipulate others within the group [2,5]. Increases in the size of the prefrontal cortex in particular, which mediates important components of complex social behavior such as planning, working memory, memory for serial order, and language may have played an important role in human brain evolution [6]

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