Same-Sex Sexual Behavior in Primates May Be an Ancient Survival Strategy

Homosexual behaviour in primates appears to be underpinned by long-standing evolutionary processes and is more frequently seen in species facing difficult conditions, heavy predation, or more intricate social systems, scientists reported on Monday.

Across the animal kingdom, researchers have documented males or females engaging in same-sex mounting or other forms of sexual stimulation.

In total, more than 1,500 species have been observed displaying same-sex sexual behaviour, with some of the earliest recorded accounts traced back to Aristotle, the ancient Greek philosopher.

For a long time, however, this fairly widespread behaviour was dismissed within science as a "Darwinian paradox". The argument was that homosexual behaviour in animals contradicts Charles Darwin’s theory of evolution because it does not transmit genes through reproduction.

More recently, scientists have demonstrated that this behaviour can, to a degree, be inherited from an animal’s parents - and that it can also confer an evolutionary benefit.

"Diversity of sexual behaviour is very common in nature, among species and in animal societies – it is as important as caring for offspring, fighting off predators or foraging for food," Imperial College London biologist Vincent Savolainen told AFP.

Savolainen has spent eight years studying rhesus macaques in Puerto Rico. His team found that male macaques that mount one another build alliances that may help them gain access to more females - and, in turn, ultimately produce more offspring.

In 2023, the researchers also found that the macaques inherited same-sex behaviour from their parents more than 6 per cent of the time - although whether the trait was passed on varied according to a range of factors.

"Deep evolutionary root": Vincent Savolainen’s primate analysis

In his latest study, published in Nature Ecology & Evolution, Savolainen and colleagues compiled data covering 491 non-human primate species.

They detected same-sex sexual behaviour in 59 species, spanning lemurs, great apes and monkeys across the Americas, Africa and Asia.

The study said that such a broad distribution suggests the behaviour has a "deep evolutionary root".

The researchers then examined how environmental conditions, social organisation and "life history" traits influenced whether primates engaged in homosexual acts.

They reported that the behaviour was more prevalent in species living in harsh environments where food is harder to come by, such as Barbary macaques.

It was also more common in species that face a higher likelihood of predation - vervet monkeys, for instance, must evade a range of large cats and snakes in Africa.

Stress reliever?

Taken together, these findings indicate that homosexual behaviour may help groups of primates manage tension during stressful periods, the researchers said.

The behaviour was also reported more often in species where males and females differ markedly in size, including mountain gorillas.

Such pronounced size differences are frequently found in animals living in larger social groups characterised by intense competition and stricter social hierarchies. By contrast, species where males and females are closer in size are more likely to live in pairs or smaller family groups.

Same-sex sexual behaviour therefore "may function as a flexible social strategy, used to reinforce social bonds, manage conflict or build alliances, depending on the ecological and social pressures faced by different species," the study said.

The researchers also suggested that comparable pressures could have shaped behaviour in human ancestors.

"Our ancestors certainly had to face the same environmental and social complexities," Savolainen said.

"But there are things that are completely unique to modern humans, who have a complexity of sexual orientation and preference that we do not address at all," he said.

The study also cautioned against "misinterpretation or misuse of our findings," including "a misguided notion that social equality might eliminate" same-sex sexual behaviour in modern humans.

Isabelle Winder, an anthropologist at Bangor University in the UK who was not involved in the study, welcomed the work.

"It is their study's demonstration that modern comparative methods can, for perhaps the first time, realistically illuminate some of the complexities of the evolution of 'humanlike' behaviours that I find most exciting," she wrote in Nature.

© Agence France-Presse