5 - Sexual selection and exaggerated sexual swellings of female primates  pp. 71-89

Sexual selection and exaggerated sexual swellings of female primates

By Dietmar P. Zinner, Charles L. Nunn, Carel P. van Schaik and Peter M. Kappeler

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INTRODUCTION

Females of several species of Old World monkeys and apes exhibit enlarged perineal swellings that include the skin of the anogenital region and rump (see Fig. 5.1). Swellings are normally produced by adult females undergoing ovarian activity and they have stimulated evolutionary biologists since Darwin (1876) to think about their adaptive value and the evolutionary mechanisms responsible for their origin and maintenance. Given the association between sexual swellings and mating activity, it seems likely that some aspect of sexual selection is responsible for the evolution of this exaggerated trait. However, even today the functional significance of exaggerated swellings, as well as the processes responsible for their evolution, remain controversial (Dixson, 1983, 1998; Pagel, 1994, 1995; Radwan, 1995; Wiley & Poston, 1996; Nunn, 1999a; Stallmann & Froehlich, 2000; Domb & Pagel, 2001; Nunn et al., 2001; Domb & Pagel, 2002; Zinner et al., 2002; Snowdon, this volume).

In this chapter, we explore the role of sexual selection in the evolution of exaggerated sexual swellings. Because sexual swellings are associated with mating behaviour and competition among males for access to females, sexual selection has figured prominently among hypotheses for this exaggerated trait. Hypotheses have incorporated the two primary components of sexual selection, sometimes within the same explanation. For example, the best-male hypothesis (Clutton-Brock & Harvey, 1976) states that swellings stimulate male–male competition, improving the ability of females to identify and mate with the highest-quality males.

In recent years, our understanding of the theoretical basis for sexual selection has increased.

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