Sex at Dawn

But not all female primates have genital swellings that visually announce their ovulatory status. Meredith Small reports that only fifty-four of the seventy-eight species surveyed “experience easily seen morphological changes during cycles,” and that half of these showed “only slight pinkness.” Once again, our two closest primate cousins stand out from the pack in terms of their decidedly indiscreet sexuality, being the only primates with such extravagant, brightly colored sexual swellings. The female chimp’s red-light district comes and goes, reflecting the waxing and waning of her fertility, but as Small confirms, the bonobo’s “swellings never change much, so that bonobo females always give a signal of fertility—much as humans do.”¹⁶

Although many theories claim the human female has “hidden ovulation,” it’s not really hidden at all, if you know how and where to look. Martie Haselton and her colleagues found that men shown photographs of the same thirty women—some taken around ovulation and others not—were quite good at judging when the women were “trying to look more attractive,” which in turn corresponded to the women’s menstrual status. These authors found that women tend to dress more fetchingly when they are more likely to be fertile. “Moreover,” writes Haselton, “the closer women were to ovulation when photographed in the fertile window, the more frequently their fertile photograph was chosen.”¹⁷

Other researchers have found that men preferred women’s bodily smells near ovulation and that women tend to behave more provocatively in various ways when they’re likely to be fertile (they wear more jewelry and perfume, go out more, are more likely to hook up for casual sexual encounters, and are less likely to use condoms with new lovers).

Come Again?

Much as women’s breasts have fascinated evolution-minded theorists, the female orgasm has confounded them. Like breasts, female orgasm is a major head-scratcher for mainstream narratives of human sexual evolution. It’s not necessary for conception, so why should it exist at all? For a long time, scientists claimed that women were the only female animals to experience orgasm. But once female biologists and primatologists arrived on the scene, it became obvious that many female primates were having orgasms.

The underlying motivation for claiming that female orgasm

was unique to human beings probably lay in the role it played

in the standard narrative. According to this view, orgasm