Female gorillas have been documented engaging in lesbian sex for the first time.

The behaviour was observed by scientists during a research trip to the Rwandan section of the Virunga mountain range in central Africa.

The wild mountain gorillas, observed by a team led by Dr Cyril Grueter of the University of Western Australia, are believed to gain pleasure from having sex and may do it when they have been rejected by males.

While many species of male primates are well known to engage in homosexual behaviour, females have been subject to far less attention. Female gorillas have also been observed having lesbian sex in Uganda, but the data has not been published or subject to scrutiny.

The research group set out to “shed light on the evolutionary origins of homosexuality”, as gorillas are closely linked to humans. But the team were surprised to observe 44 instances of same-sex contact between female gorillas during their field study.

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Aggression between females did not occur prior to sex as the team had expected. Dr Grueter told Daily Mail: “They were obviously deriving sexual pleasure from each other.”

The scientist also explained the mechanics of lesbian gorilla sex, adding: “It usually happens that two females get on top of each other, rubbing their bellies and genitals together.”

Dr Grueter concluded that the “courtship displays” are “purely sexual behaviour” and not necessarily reflective of sexual orientation or preferences, which are believed to be fluid.

A theory to explain the phenomenon is that some of the females engaged in sex out of frustration after being rejected by a male, or after becoming aroused after witnessing sex. Other theories speculated that female gorillas have sex in order to attract males.

However, some dominant male gorillas would occasionally aggressively interrupt the proceedings, though others were unconcerned.

The researchers also noted the gorillas would try to seek privacy. The study said there was a “tendency for such copulations to take place in secluded places with dense vegetation”, probably to “avoid a negative response by the [male] silverback”.

Lesbian sex did not occur between gorillas who were closely related, although there was contact between an aunt-niece pair.

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