Bisexual trait in macaque monkeys common, heritable, and beneficial, says study

A study tracking hundreds of macaques over three years has found that same sex behaviour in males is very common, and it might even be beneficial to the species.

“We found most males were behaviourally bisexual, and that variation in same-sex activity was heritable,” said the first author Jackson Clive from Imperial College London.

“Our research therefore shows that same-sex sexual behaviours can be common amongst animals and can evolve. I hope our results encourage further discoveries in this area.”

The research has been published in Nature Ecology & Evolution.

Although there’s been plenty of reports of same-sex behaviour in the animal kingdom, usually they are just opportunistic observations in the wild, and so are classed as rare.

This study observed 236 male rhesus macaques on the Puerto Rican island, Cayo Santiago. The observations were taken from 2017 to 2020, and was accompanied by  genetic analysis, and a review of the macaques’ pedigree (or family tree) records back to the mid-1950s.

They found that 72% of the males in the study undertook same-sex mounting, while only 46% did different-sex mounting.

This at first might seem paradoxical. Sexual behaviour that is not reproductive would soon be bred out. This is called ‘Darwin’s paradox’

The team investigated whether the same-sex behaviour led a loss in genetic ‘fitness’ – meaning less offspring overall. This was not the case – they suggest that bisexual males may be more successful in reproducing.

“We also found that males that mounted each other were also more likely to back each other up in conflicts – perhaps this could be one of many social benefits to same-sex sexual activity,” says Clive.

“The behaviour can have an evolutionary underpinning.”

Using the genetic data, the team found there was a small heritable component – around 6.4%. This is the first evidence of a genetic link to primate same sex behaviour outside of humans. They also found a genetic correlation between which role – the mounter or the mountee – the males undertook.

Demographic factors such as age and social status didn’t affect the likelihood of the macaque undertaking same sex behaviour or which role they took.

The researchers say that this challenges the belief that same-sex behaviour is rare, or only a product of particular environmental conditions.

“Unfortunately there is still a belief amongst some people that same-sex behaviour is ‘unnatural,’ and some countries sadly still enforce the death penalty for homosexuality,” says senior author Professor Vincent Savolainen also from Imperial College London.

“Our research shows that same-sex behaviour is in fact widespread amongst non-human animals.”

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