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© Cate Gillon/GettyA female gorilla (left) sends a "come hither" look to the silverback male
Female gorillas get friskier when their silverback has sex with another female, even when they themselves cannot conceive. The finding suggests they use sex to gain an advantage over competing females.

Tara Stoinski and colleagues at Zoo Atlanta in Georgia totted up how often four captive female gorillas invited sex or copulated over two years. They found that even pregnant or lactating females solicited more sex on days when other females were sexually active.

With only one male around, females could not have been trying to confuse him about his paternity, one commonly offered explanation for non-reproductive sex.

Most likely, they were offering sex to curry favour with the male, or to prevent him from impregnating other females. "With another female in the mix, the male may copulate less with the first one, or he could be depleting his sperm," says Stoinski.

In the wild, she adds, such competition between females could increase the chance that the male will protect the favoured female's offspring from danger.

Journal reference: American Journal of Primatology (DOI: link)