Female chimps keep quiet during sex to keep other females from finding out and punishing them for mating with the best males, British researchers said on Wednesday.
The study of chimp copulation calls also found that females seem more concerned with having sex with as many mates as possible rather than just finding the strongest male as a way to confuse paternity and secure future protection for offspring.
"They are trying to make the high-ranking males think they are the father," said Simon Townsend, an evolutionary psychologist at the University of St. Andrews in Britain, who led the study. "If you confuse paternity, they are more likely to provide that female with future support."
The findings show that chimps — our closest living relatives — can use their calls flexibly in response to social factors while knowing more about the apes could help in conservation efforts, he added.
Researchers have long been interested in mating calls of different animals, especially primates. A common hypothesis is that females use such calls to advertise to prospective males they are ready to mate, which in turn incites competition that leads to the strongest partner and highest quality offspring.
But it appears the female chimps are also a touch more savvy about the opposite sex, according to the findings published in the journal PLoS One.
"The female chimps we observed in the wild seemed to be much more concerned with having sex with many different males, without other females finding out about it, than causing males to fight over them," Townsend said.
The team, which included researchers from the Max Planck Institute in Lepzig, took urine samples from females in a group of about 80 chimps under observation in the Budongo Forest in Uganda able to show when the animals were fertile.
They also recorded the mating behavior and saw that the female chimps called out for sex partners for as many as 12 days during their reproductive cycle, even though they were only fertile for about 4 days of it.
Yet they also only called out about a third of the time when mating, much less than other primates. Low-ranking females were also more likely to keep quiet during sex, probably to keep female competitors at bay, the researchers said.
"We think that by being quiet, you are less likely to incite aggression from other females," he said in a telephone interview.