Scientists have reported gorillas in Africa having lesbian sex for the first time ever. A team from the University of Western Australia observed 44 instances of female-on-female sex among wild mountain gorillas, based in the Rwandan section of the Virunga mountain range in central Africa.

Some of the animals "exhibited a higher propensity for same-sex acts," and some pairs, or "dyads," also exhibited a tendency toward same-sex acts, according to the PLOS study. Of all the same-sex acts noted, 30 involved two adult females, four involved two subadult females and ten involved one adult and one subadult female

It is possible that female gorillas engage in lesbian sex to satisfy an urge after experiencing sexual frustration when they are rejected by males, the scientists speculate.

The study did not find evidence clearly supporting other theories, such as sex being an expression of dominance by a higher-status female over another, or as part of a strategy to cultivate a long-term social bond.

While many species of male primates have been known for years to engage in homosexual behaviour, females have generally been the subject of far less scrutiny. Female gorillas in Uganda have also been observed engaging in same-gender sex, but the encounters have not been reviewed in a scientific journal.

The observation of same-gender sexual activity among wild animals was crucial to the early debate about gay human sex because it was evidence that such behaviour is natural and innate.

The Australian team, led by Dr Cyril Grueter, set out to "shed light on the evolutionary origins of homosexuality," because gorillas are closely linked to humans. But team members were surprised at the number of times lesbian sex was observed; most copulations first involved typical solicitation or courtship behaviour seen in heterosexual encounters.

Dominant males occasionally interrupted female-on-female sex aggressively, though others were unconcerned, the team reported.

The researchers also noted the two female gorillas tried to seek privacy. There was a "tendency for such copulations to take place in secluded places with dense vegetation," possibly to "avoid a negative response" by the dominant male silverback gorilla.

"Same-sex sexual contacts among females are clearly a component of the behavioural repertoire of mountain gorillas, albeit a relatively infrequent one," concluded the study.