Female chimps tend to keep quiet during sex so their female rivals don't find out what they've been up to.

Credit: iStockphoto

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LONDON: Female chimpanzees mate with as many males as possible but keep quiet about it to boost their chances of luring the top chimps, a new study suggests.

Scientists at the University of St Andrews in Scotland studied the copulation calls – sounds made during mating – of female chimpanzees in Uganda, central Africa, to find out more about what they mean.

The team's study, published yesterday in the online journal PLoS ONE, concluded that female chimps sometimes keep quiet during sex so their female rivals don't find out what they've been up to.

Evolutionary psychologists Simon Townsend and Klaus Zuberbuhler studied chimp behaviour in Uganda's Budongo Forest over 16 months.

The team established that female chimpanzees hid their sexual activity when high-ranking females were nearby, perhaps in a bid to reduce competition for good quality males. This could also prevent higher-ranking females from turning on them.

They also found the females produced more copulation calls when high-ranking males were nearby to attract them to have sex.

Confused males

The scientists believe that by having sex with several mates, the females cause confusion among the males as to who sired their offspring. This reduces the chance that males will kill their babies, because of the possibility the offspring are their own.

The study found no evidence that males were competing to have sex with females after they produced copulation calls, and no link between a female's fertility and her use of the calls.

"Chimpanzee females adjusted their calling behaviour in flexible ways, potentially to avoid aggression from other females and possibly to secure future benefits from the socially important males," the study said.

Competitive females

The research indicated that the social pressures deriving from resource competition acted as an important selective force, shaping the copulation calling behaviour in wild chimps. "Competition between females can be dangerously high in wild chimpanzees," said Townsend.

"Our findings highlight the fact that these females use their copulation calls in highly tactical ways to minimise the risks associated with such competition," he added. "The female chimps we observed in the wild seemed to be much more concerned with having sex with as many different males as possible, without other females finding out about it, than causing male chimps to fight over them.

"We also found that the calling behaviour of copulating females was unrelated to their fertile period and therefore not linked to the likelihood of conception."

According to Zuberbuehler, "Copulation calling may be one potential strategy employed by female chimpanzees to advertise receptivity to high-ranked males, confuse paternity and secure future support from these socially important individuals."