The findings add weight to the argument that homosexuality doesn’t defy nature and evolution.

Although homosexuality is documented in many animal species, the evolutionary and environmental factors behind it remain unclear.

Studies indicate ecological factors, the life history of the individual and the social structure of a population play a role in its expression.

But a collective analysis of the behaviour across multiple species and their shared drivers is limited so far.

The new study analysed data from 491 non-human primates and found same-sex intimacy in 59, with evidence of repeat occurrence of the behaviour in 23.

The study defined same-sex intimacy as any sexual behaviour directed towards an individual of the same sex – including acts like mounting, genital contact, courtship and copulation-like displays – regardless of whether reproduction was possible.

“It includes all behaviour where the intent is seen to be clearly sexual,” study author Chloe Coxshall, who studies homosexuality in rhesus macaques, said.

“This can be mounting, genital inspection and stimulation, and also fellatio – all typically considered sexual behaviours between members of the same sex,” Ms Coxshall said.

The researchers say same-sex intimacy is more common in species living in harsh or dry environments with limited food, like barbary macaques, as well as those in areas with high predation risk, like vervet monkeys.

Homosexual behaviour is also more common in species with significant differences in size or appearance between the sexes, like mountain gorillas, species that live long, like chimpanzees, and those with complex social systems and hierarchies, like baboons.

“What we find when we look at species overall is that it is more common in animals that have complex societies, like a strong hierarchy, and complex mating systems where they don’t mate in pairs but will have multiple partners,” Ms Coxshall, who is a PhD candidate at Imperial College London, said.

“So same-sex intimacy seems to be facilitating competitive scenarios, helping keep group harmony in stressful situations like when there are many predators around and making sure the group communicates well together.”

The study shows homosexuality is not straightforward, driven by genetics or any one environmental factor. Instead, scientists say it emerges from complex interactions between environmental and genetic factors. (© The Independent)