The evolution of conspicuous oestrous advertisement in Old World monkeys

Abstract

Abstract. The females of many Old World primate species develop at or around the time of ovulation a prominent reddening and swelling of the sexual skin that surrounds their perineum. In most of these species, females live in social systems that typically contain more than one adult male. Accordingly, females are believed to have evolved swellings as a way to advertise conspicuously their oestrus, and thereby to incite competition among males ('best male' hypothesis) or to attract the attention of more than one male with which to mate (the 'many male' hypothesis). However, these explanations do not explain why females must advertise their oestrus so prominently, and why males should compete more over females with swellings. Here a new explanation for the evolution of oestrous swellings is offered. It is argued that prominent oestrous swellings are sexually selected traits that reliably advertise some aspect of a female's quality or condition, and which have evolved because of competition among females to attract males. An individual female benefits from a prominent swelling because it increases the males' willingness to compete, or because it draws their attention away from other females and towards herself (a form of spite). This idea explains why the trait is exaggerated, makes predictions about why some species without regular male-male competition for females none the less have oestrous swellings, and provides an explanation for the lack of prominent oestrous advertisement in monogamous species.