Follow meAs a female chimp nears ovulation, her rump enlarges. Such signs told researchers which chimps to follow when trying to record females’ calls during sex.Florian Moellers

When a chimp has sex in the forest, will she make a sound?

Depends in part on who’s listening, literally, says a scientist who has spent months recording chimp sex sounds in the wild.

With lots of other females within earshot, a female chimp typically doesn’t give a call, says Simon Townsend of the University of St. Andrews in Scotland. With a largely male audience, though, she’s more likely to give what primatologists call copulation squeaks or screams.

And partners matter. Even if she is not fertile, she’s more likely to vocalize when she’s with a high-ranking male than with some low ranker. The benefit of this strategy could be that she avoids attacks from other females while confusing males about who’s going to be the dad, Townsend and his colleagues propose in the June PLoS ONE.

“It’s very elegant and quite novel,” says primatologist Stuart Semple of Roehampton University in London. Previous work focused on male reaction, so documenting the effects of a female audience brings a new dimension to the research. Also the new paper finds no evidence for the standard belief that female calls incite male competition, he says.

Just what benefits might drive animals to make these calls, often among the loudest in a species’ repertoire, has long intrigued evolutionary biologists. Townsend points out that lions, elephants and plenty of other animals get noisy. An influential 1977 paper on elephant seals theorized that a loud female incited males to compete for her. Her whoops attracted attention from the rest of the males around, who vied to displace the current partner. So the calls end up, the theory goes, gaining the female the attention of the top guy in the neighborhood.

That scenario has shown up in other primate species, although there’s variability. Among rhesus macaques, it’s the males that call.

But among chimps, it’s the female that sings out. Townsend says the chimp sex call is distinctive even to human ears, a rhythmic high-pitched sound that could be spelled something like “eeeeee! eeeeee!”

To find out when the females gave the calls, Townsend and his colleagues studied an unusually female-abundant group, about 30 adult females with only five high-quality adult males, in Uganda’s Budongo Forest. As females neared ovulation, their rumps enlarged, so Townsend was able to pick promising chimps to follow through the forest. During the course of months, he observed 287 encounters of seven females. When he noticed activity, he scanned the forest for 50 meters around, the distance that the calls carry. To track the females’ hormone cycles, Townsend collected urine left on leaves and sent the samples to Germany for analysis.

The females called only 36 percent of the time, and the pattern didn’t fit the standard idea of male-incitement. Females were calling less, not more, when with lower quality-males. If the benefit was to attract the interest of top guys, Townsend says, he would have predicted the reverse. Also the females called before, during and after ovulating. So Townsend argues the females give confusing signals about paternity thus possibly enlisting the support of important males in case other females attack.

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