Advertisement of Ovulation
Humans do not exhibit visible external manifestations of ovulation, and women are generally unaware of their period of fertility. Thus, ovulation is not obvious to most human females with the exception of individuals experiencing “Mittelschmerz”, a painful sensation indicative of ovulation. In contrast, the genital area (perineum) of chimpanzees and bonobos becomes swollen and serves as a conspicuous marker that the female is in a fertile (periovulatory) state. These features are very reduced or completely absent among gorillas and orangutans. Across primates the presence of perineal/sexual swellings correlates well with multi-male,multi-female mating systems and their absence appears to be the ancestral state (as opposed to explicitly “concealed” ovulation being a derived specialization). Among the notable exceptions are certain gibbons Hylobates lar, which are considered serially monogamous but the females of which exhibit perineal swellings. The absence of sexual swellings in orangutan and gorillas would indicate that the two species of Pan: chimpanzees and bonobos evolved such advertisement of ovulation in conjunction with their respective multi-male, multi-female mating systems and that humans retained or re-evolved the ancestral hominid-specific absence of sexual swellings. Advertisement of ovulation has been hypothesized to serve as an incentive for male-male competition.
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