Female baboons' cries during sex are longer and livelier when their partner is a higher-ranking male, researchers have discovered.

One possible explanation for the baboon behaviour is that the female is trying to avoid being harassed by other potential suitors while she is mating with a choice male, says lead researcher Stuart Semple of the Institute of Zoology in London.

However, the reason for the spirited shouting may be purely physical. The females could simply be more stimulated by larger, dominant males, says Semple.

Primatologist Alan Dixson of the San Diego Zoological Society says that socially successful males could have a different mating style. They may mount longer or more than once, or perhaps they are more energetic.

"These vocalizations occur in any number of primate species, so what they're finding in baboons has broader implications," he adds. The baboon discovery is surprising because the opposite is true with other animals, such as Elephant seals.

Staccato Grunts

The females of several primate species including Barbary macaques, Talapoin monkeys and yellow baboons give characteristic, loud and quick staccato grunts during mating. "There are all kinds of theories about what these calls are doing and very little data," says Dixson.

To try to find out the meaning of the sex talk, Semple and his team recorded and analysed the acoustic structure of the calls of seven female yellow baboons before, during and after sex during 554 different matings in the Amboseli National Park in Kenya.

The researchers confirmed a long-held idea that one function of the baboon calls is to advertise their reproductive state. They found that the grunts are faster and more vigorous the closer a female is to ovulation, as gauged by her highly visible sexual swelling.

A popular explanation for this is that the female is inciting competition among the males so that she is more likely to mate with the highest-ranking members of the troupe during the most likely time of conception. She may also inspire more males to successfully compete for their chance to be with her, increasing sperm competition.

Other researchers have suggested that some female primates also may be revealing whether or not their partners had ejaculated. But Semple found that yellow baboons, at least, are not quite that forthcoming.

The next step is to determine whether the male baboons can perceive the call differences as well, and if they change their behavior accordingly, says Semple.

Journal Reference: American Journal of Primatology (vol 56, p 43)



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