The Role of Females in the Evolution of Sexual Dimorphism and Mating Systems

Abstract

A hypothetical evolutionary process that may have given rise to sexual dimorphism and the mating system of pinnipeds was proposed half a century ago by Bartholomew (Evolution 24:546–559, 1970), yet it still remains the most accepted theory that links limited resources, body shape and behavior in the group (see Cassini, Chap. 6). Under this sexual selection approach, females are viewed as passive actors in the evolution of sexual characters and mating systems. The objective of this chapter is to propose a more active role of females in the evolutionary ecology of pinnipeds than that traditionally described. In this attempt to prioritize the role of females, the role of natural selection in the evolution of the sexual characteristics of these marine mammals becomes more relevant. The first section analyses the evolution of body size in pinnipeds, based on paleontological records, and describes the numerous advantages of large body size for a marine life. Each of the remaining four sections describe a different aspect of females that would have had an influence on the evolution of some unique pinniped trait. I argue that: (i) sexual size dimorphism skewed towards males can be explained by at least five different evolutionary mechanisms not related to sexual selection, (ii) that energy constraints of lactation has been an influential selective force that explains why phocids are on average significantly larger than otariids, (iii) that female avoidance of sexual disturbance is one of the most important determinant of the evolution of mating systems in pinnipeds, and (v) that mate choice by females has a more significant role in sexual selection than previously thought.