Sexual swelling, sexual skin, or anogenital tumescence refers to localized engorgement of the anus and vulva region of 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. Sexual swelling is widespread among primates but completely absent in humans and vervet monkeys. Females of these species exhibit concealed ovulation. 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.
Characteristics
Physical characteristics and correlates Sexual swellings are concentrations of tumescent tissue, cyclically appearing on the genitalia and adjacent posterior regions of female primates. The exact reproductive purpose of sexual swellings is not fully understood, though the erogenous sensitivity of this tissue is known to motivate females to pursue sexual contact with males. The position of swellings can heavily influence the location of male focus during courtship, and of eventual penetrance. Among chimpanzees, bonobos, baboons, and many Old World monkey species, it is common for swellings to concentrate in the perineal, perianal, and coccygeal regions, rather than more ventrally in the area of the vulva. Dorsal swelling positions can therefore correlate with frequent non-conceptive, penetrant mounting, thus enhancing high volume/low efficiency female mating strategies, which guard against incidental impregnation by opportunistic low-tier males during peak fertility. Swellings can be categorized into two groups: small and exaggerated. Small swellings are characterised by a moderate size and pinkness of the anogenital tissue, 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. Researchers have attempted to determine the characteristics of the primate species displaying these exaggerated swellings. It has been identified that species which exhibit exaggerated sexual swellings predominantly live in multi-male social systems, in which females mate promiscuously. Species with such swellings have twice as many males per group than those without. Furthermore, whilst 71% of Old World primate species living in multi-male groups show exaggerated swellings, no females living within single-male groups do. In some instances, however, sexual swellings can be seen in primate societies with alternative mating systems; for example, female langur monkeys exhibit swellings but live within polygamous single-male groups. Primates with exaggerated sexual swellings also demonstrate non-seasonal breeding patterns, longer mating periods and longer ovulation cycles. Specifically, of the 23 species which are both non-seasonal breeders and live in multi-male societies, 91% have sexual swellings. Nonetheless, nonseasonal reproductive environments are not a necessary precursor for the selection of sexual swellings. Indeed, females who are seasonal breeders, such as the female Barbary macaques, also exhibit exaggerated sexual swellings.
Changes across the menstrual cycle Exaggerated sexual swellings vary both in size and focal location throughout the female's cycle, beginning after menstruation. For example, research on baboons showed that after 14 days of gradual increase, swellings peaked for 2 days before reducing. Female chimpanzees exhibit a shift in swelling prominente. These cyclic changes in appearance of the sexual skin reflect the changes of ovarian hormones (estrogen and progestogen) during the female menstrual cycle. Specifically, the increase in sexual swelling size during the follicular phase is correlated with increased estrogen levels, and the decrease in swelling size during the luteal phase is associated with rising progesterone levels. It has been shown in ovariectomized chimpanzees that swelling can be induced by estrogen and inhibited by progesterone. As a result, the peak size of the swellings often coincides with the highest potential of ovulation, although this is not a perfect association. For example, research on West African chimpanzees showed that higher probabilities of ovulation tended to occur within 7 to 9 days of the onset of maximum swelling of sexual skin. Additionally, a study into wild white-handed gibbons showed that maximum swelling size and ovulation overlapped closely in 80% of menstrual cycles. The size of sexual swellings not only varies within each cycle, but also across female cycles and across species. Specifically, the maximal swelling size increases from cycle to cycle for individual female chimpanzees and baboons. Additionally, the duration of maximal sexual swellings size varies considerably between species. Baboons for instance, have a maximal swelling lasting approximately 15.1 days, whilst the duration of maximal swelling is 10.9 days in chimpanzees. Like size, location also varies considerably across the cycle. For instance, in chimpanzees, the state of maximum dorsality correlates with the period of maximum swelling.
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