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LIANE HANSEN, host:

You are listening to Weekend Edition from NPR News. And now, a tale of sex and secrecy in the animal kingdom. Female primates in the wild tend to advertise their sexual encounters.

(Soundbite of animal noise)

That's a long-tailed macaque having sex. These copulation calls seem to help females attract as many sex partners as possible. A good survival strategy. But as a team of researchers discovered, female chimpanzees are inclined to be more discreet. NPR's Jon Hamilton explains.

JON HAMILTON: The Budongo Forest in western Uganda offers a place for researchers to study chimps in a natural setting. That's what Simon Townsend was doing when he noticed something odd. Sometimes female chimps produced a copulation call.

(Soundbite of chimps calling)

HAMILTON: Other times they didn't.

Dr. SIMON TOWNSEND (Evolutionary Psychologist, St. Andrews University, Scotland): They seem to be different from other primates where the females always call, and so then I started to question, well, why are they calling and why are they sometimes not calling?

HAMILTON: To answer that question, Townsend, who's from the University of St. Andrews in Scotland, spent months studying female chimps. He documented 287 sexual encounters involving seven females. But the chimps publicized only a third of those encounters. Townsend realized that the chimps were suppressing copulation calls when a high-ranking female was nearby.

Dr. TOWNSEND: They work out who's in the audience. You know, they can compute rank differences between themselves and these other females, and then from that they can decide whether to produce a call or not.

HAMILTON: He says high-ranking female chimps tend to get violent when they catch a competitor in the act, hence the secrecy. So why make copulation calls at all? Townsend says it's because the females also need to alert desirable males that they're available for breeding. A female chimp should have as many partners as possible for her children's sake.

Dr. TOWNSEND: If lots of high-ranking males mate with her, then ultimately a lot of them will be confused as to whether they're the father or not.

HAMILTON: An adult male won't kill a baby if he thinks it might be his. Scientists say female chimps may be more circumspect about their liaisons than other primates because chimps have a different social structure. Dorothy Cheney, from the University of Pennsylvania, studies baboons. She says a female baboon usually spends her entire life in a relatively harmonious group that includes her mother and other female relatives.

Dr. DOROTHY CHENEY (Biology, University of Pennsylvania): With chimpanzees, females usually come from other groups that are unrelated to many of the other females in the group. And as a result, there may be much more competition for access to males and also for protection from males.

HAMILTON: That competition could make it more dangerous to advertise a sexual encounter. The new study appears in the journal Public Library of Science I. Jon Hamilton, NPR News.

HANSEN: If you want to hear what a female chimpanzee's copulation call sounds like, go to our Web site at npr.org. This is NPR News.

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