[Apologies to readers who have been asking for more on female libido. I've been dealing with book galleys, but appreciate your interest.]

Women aren’t the only female primates who make a lot of noise in the throes of passion. British primatologist Stuart Semple found that, “In a wide variety of species, females vocalize just before, during or immediately after they mate. These vocalizations,” Semple says, “are particularly common among the primates and evidence is now accumulating that by calling, a female incites males in her group….” Precisely. There’s a good reason the sound of a woman enjoying a sexual encounter entices a heterosexual man. Her “copulation call” is a potential invitation to come hither, thus provoking sperm competition.

Semple recorded more than 550 copulation calls from seven different female baboons and analyzed their acoustic structure. He found that these complex vocalizations contained information related both to the female’s reproductive state (the vocalizations were more complex when females were closer to ovulation) and to the status of the male “inspiring” any given vocalization (calls were longer and contained more distinct sonic units during matings with higher-ranked males). Thus, in these baboons at least, listening males could presumably gain information as to their likelihood of impregnating a calling female, as well as some sense of the rank of the male they’d find with her if they approached.

Primatologist Meredith Small agrees that the copulation calls of female primates are easily identifiable: “Even the uninitiated can identify female nonhuman primate orgasm, or sexual pleasure," she says. "Females,” Small tells us, “make noises not heard in any other context but mating.” Female lion-tailed macaques use copulation calls to invite male attention even when not ovulating. Small reports that among these primates, ovulating females most often directed their invitations at males outside their own troop, thus bringing new blood into the mating mix.

Female copulatory vocalization is highly associated with promiscuous mating, but not with monogamy. Alan Dixson has noted that the females of promiscuous primate species emit more complex mating calls than females of monogamous and polygynous species. Complexity aside, Gauri Pradhan and his colleagues conducted a survey of copulation calls in a variety of primates and found that “variation in females’ promiscuity predicts their tendency to use copulation calls in conjunction with mating.” Their data show that higher levels of promiscuity predict more frequent copulation calls.

William J. Hamilton and Patricia C. Arrowood analyzed the copulatory vocalizations of various primates, including three human couples going at it. They noticed that “female sounds gradually intensified as orgasm approached and at orgasm assumed a rapid, regular (equal note lengths and inter-note intervals) rhythm absent in the males’ calls at orgasm.” Still, the authors can’t help sounding a tad let down when they note, “Neither sex [of human] … showed the complexity of note structure characteristic of baboon copulatory vocalizations.” But that’s probably a good thing, because elsewhere in their article we learn that female baboons’ copulation calls are clearly audible to even human ears from three hundred meters away.

Before you conclude that female copulatory vocalization is just a fancy phrase for a little excitement, think about the predators possibly alerted by this primate passion. Chimps and bonobos may be out of reach up in the branches, but baboons (like our ground-dwelling ancestors) live among leopards and other predators who would be quite interested in a two-for-one special on fresh primate—especially given a mating pair’s distracted, vulnerable state.

As Hamilton and Arrowood put it: “In spite of the risk of exposure of individuals and the troop to predators these baboons habitually call during copulation, [so] the calls must have some adaptive value.” What could that be? The authors (writing in Science) offered several hypotheses, including the notion that the calls may be a stratagem to help activate the male’s ejaculatory reflex, an analysis with which many prostitutes would presumably agree. Perhaps there is something to this idea, but even so, male primates are not known for needing a great deal of assistance in activating their ejaculatory reflex. If anything, the human male ejaculatory reflex tends to be too easily activated—at least from the perspective of women not being paid to activate it as quickly as possible. Especially given all the other convergent evidence, it seems far more likely that in humans, female copulatory vocalization would serve to attract males to the ovulating, sexually receptive female, thus promoting sperm competition, with all its attendant benefits—both reproductive and social.

Yet despite all the loud carrying on by women the world over, “The credo of the coy female persists,” writes Natalie Angier. “It is garlanded with qualifications and is admitted to be an imperfect portrayal of female mating strategies but then, that little matter of etiquette attended to, the credo is stated once again.”

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