Sexual swelling

Sexual swellings are enlarged areas of the perineal skin occurring in some female primates that vary in size over the course of the menstrual cycle. Thought to be an honest signal of fertility, male primates are attracted to these swellings; preferring, and competing for, females with the largest swellings.

Though heavily investigated, the ultimate function of sexual swellings remains unknown. Over the last 50 years, eight principal explanations have been proposed, each claiming to account for the function of exaggerated swellings. Alone, however, no single hypothesis is believed to account for the function of sexual swellings; a combination of these theories may be more appropriate. In line with this ideal, the most recent account regarding the function of sexual swellings (the graded-signals hypothesis) combines several existing theories in the attempt to provide a more comprehensive account of sexual swellings.

Sexual swellings are water-filled edemas of mainly the external or internal genitalia of female primates. However, the swellings can also extend to the skin of the circumanal, subcausal and paracallosal regions. Such swellings can be categorized into two groups: small and exaggerated. Small swellings are characterised by a moderate size and pinkness of the genitalia, and can be found in Old World monkeys, New World monkeys, prosimians and gibbons. In contrast, exaggerated swellings are larger in size, and their prevalence is mainly restricted to Old World primate species. For instance, they occur in all species of Cercocebus, Mandrillus, Theropithecus, Papio and Pan, and in most macaques, colobines and guenons.

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