Sexual swelling
[9] Sexual swellings are concentrations of tumescent tissue, cyclically appearing on the genitalia and adjacent posterior regions of female primates.
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.
[11] 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.
It has been identified that species which exhibit exaggerated sexual swellings predominantly live in multi-male social systems, in which females mate promiscuously.
These cyclic changes in appearance of the sexual skin reflect the changes of ovarian hormones (estrogen and progestogen) during the female menstrual cycle.
[17] 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.
[22] Additionally, a study into wild white-handed gibbons showed that maximum swelling size and ovulation overlapped closely in 80% of menstrual cycles.
Younger males have a slightly increased likelihood of conceptive mounting outside of peak swelling (e.g. early in the estrous cycle).
An altered version of Holland and Rice's chase-away model is cited to explain the function of sexual swellings.
[39][40] The association found between female fertility and sexual swelling size in several species of macaque offers support for this hypothesis.
Research remains fairly consistent across animal species; female yellow baboons (Papio cynocephalus) who struggle to conceive are, on average, those that display the most prominent sexual swellings.
[41][42] Wrangham proposed the cost-of-sexual-attraction hypothesis as a result of comparing the number of sexual cycles between conceptions that are experienced by both parous and nulliparous female chimpanzees, as well as parous western and eastern chimpanzees (Pan troglodytes verus and Pan troglodytes schweinfurthii), and the size of the sexual swellings that came with these differences.
[43] For example, eastern chimpanzees who have previously produced offspring tend to experience high within-group scramble, and so are driven towards having fewer ovulatory cycles between conception.
[48] The hypothesis proposes that the swelling transforms the potential aggression that males in the new social group may show to the female into sexual urges.
Observations of common chimpanzees (Pan troglodytes) in the Tai Forest, for instance, led to the discovery that sterile adolescent females or subordinate mothers exhibit swellings upon their emigration to new communities, suggesting that the swellings do indeed function to eliminate any social stress that could be directed at them during the emigration period.
[52] However, research on olive colobus monkeys (Procolobus verus) residing in the same region showed females to emigrate without displaying sexual swellings.
[53] On the basis of this evidence, it has been suggested that the social passport hypothesis is not an appropriate explanation of the function of sexual swellings in this species.
[60] The hypothesis proposes that exaggerated swellings indicate the timing of ovulation and as a result increase paternal certainty, allowing males to assess if they have been successful in siring that female's offspring.
[33] The hypothesis proposes that sexual swellings incite competition between males for access to a female by indicating her fertility and receptivity.
[33] The hypothesis therefore proposes that sexual swellings are signals by which females, through advertising that they are receptive to mating, aim to increase their chances of high quality offspring by inciting competition between males in a group.
[42] Specifically, Pagel argued smaller swellings would be sufficient to incite competition between males since this behaviour was calculated to be an evolutionarily stable strategy.
[42] For example, it was discovered that male captive ruffled lemurs (Varecia variegata) were less likely to kill infants that they believed themselves to have sired, supporting Hrdy's second proposition.
To successfully ensure paternal confusion, Hrdy predicted that ovulation must be randomly distributed across the term where the sexual swelling is of maximum tumescence.
One example of this stems from research into sooty mangabey (Cercocebus atys); the females of this species have been found to produce sexual swellings both when they are fertile and when they are with child.
[69] Additionally, sexual swellings are only observed during the most fertile period of a female's menstrual cycle in 26-35 species of anthropoid primates, in contrast to Hrdy's prediction.
[8] In these periods before and after the peak, females still continue to mate with lower-ranking males, as their probability of ovulation is not so high, but conception is still possible.
[21] As one of the more recent hypotheses, the graded-signals hypothesis still has limited research supporting it as the evolutionary function of sexual swellings.
[8][21][74] Having reasoned that the best-male and many-males hypotheses did not fully explain why estrus must be advertised so prominently, Pagel proposed the reliable indicator hypothesis, suggesting that exaggerated swellings evolved through sexual selection due to the need for an honest signal of female quality (both their likelihood of conception and their genetic quality)[8][68][71] as a result of female-female competition to attract males.
[9] Such conditions have been described as "reversed sexual selection",[9][74] as it is the females that ultimately make the mate choice in most species,[77] and seemingly only this hypothesis that suggests the opposite.
[5] Although Pagel performed a field experiment which found support for all predictions of the hypothesis through the observation of olive baboons (Papio anubis),[5] its methodology has since been criticised,[80][81] and subsequent research has failed to find empirical evidence that sexual swellings reliably indicate female quality in this,[82] and other, species (e.g. chimpanzees,[21] mandrills,[75] and barbary macaques).
