Chimp noises suit audience: females' sex sounds depend on eavesdroppers.

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 spent months recording chimp sex sounds in the wild.

With lots of 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 make copulation squeaks or screams, rhythmic high-pitched sounds that could be spelled "eeeeee! eeeeee!"

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 a 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 Stuart Semple of Roehampton University in London. Past work looked at male reaction, so studying the effects of a female audience brings a new dimension. Also the new paper does not support a widely expected benefit of female calling, 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. Lions, elephants and plenty of other animals get noisy. An influential 1977 paper on elephant seals theorized that a loud female incites males to compete for her, attracting the attention of the top guy in the neighborhood.

Among chimps, Townsend found, females called only 36 percent of the time, and the pattern didn't fit the standard idea of male incitement. The females called less, not more, when they were with lower quality males, and the females called before, during and after ovulating.

So Townsend argues that the females give confusing signals about paternity, possibly enlisting the support of important males in case other females attack.