Sex between two males is extremely common in wild macaque monkeys

Homosexual behaviour appears to be widespread among male macaque monkeys in the wild and the trait may be at least partially passed down in genes from father to son. Such behaviour may provide evolutionary advantages stemming from strong male alliances, says Vincent Savolainen at Imperial College London.

“They form bonds, and they help each other in a fight,” says Savolainen. “And then the idea is that if they do this, then they might also have access to more females and, in effect, have more babies.”

Occasional same-sex touching has been observed in numerous animal species, but it is generally thought to be rare. Savolainen has often questioned a popular belief known as the “Darwinian paradox of homosexuality”, which implies that because homosexual behaviour doesn’t lead to reproduction, it has no benefit and any genes that promote it should be eliminated by natural selection.

To investigate homosexuality in other primates, Savolainen and his colleagues decided to study a colony of 1700 wild rhesus macaques (Macaca mulatta) on the Puerto Rican island of Cayo Santiago. The colony has been followed by scientists every day for the past 67 years and DNA-tested for paternity since 1992. In 2017, 2019 and 2020, the team observed 236 of the colony’s males, which belonged to two social groups, for 7 hours a day over 72 days.

The researchers found that 72 per cent of the males mounted other males, while only 46 per cent were observed mounting females. Because they didn’t want to interfere with the animals, the scientists only visually observed the animals from a distance and couldn’t always see actual penetration, although they sometimes observed sperm plugs – which form after ejaculation – in the anuses of some males.

In 16.5 per cent of cases of same-sex mounting, the two males fought together against other monkeys before engaging in sex.

Same-sex mounting became less common with age, however, providing support for a common hypothesis that it may serve as “practice” for future reproductive activity, says Savolainen.

The family history of the monkeys revealed that same-sex mounting was 6.4 per cent heritable – meaning genetics may play a small role in addition to other factors, says Savolainen. The behaviour appeared to have no negative consequences on reproductive success, he adds. On the contrary, males engaging in same-sex mounting tended to have slightly more offspring.

The males’ mounting activity is unlikely to be simply a show of dominance, says Savolainen, since the monkeys were mounting higher-ranking males nearly half the time. “They have erections sometimes; they have penetration sometimes, and they even have ejaculation sometimes,” he says. “So I think it’s enough to call it sex, and not dominance.”

While researchers can’t determine what animals are thinking as they select sexual partners, the study helps to dispel the notion that same-sex behaviour is somehow unnatural, says Jon Richardson at the University of Minnesota, who wasn’t involved in the study. “I am hopeful that we really are starting to move away from the antiquated idea that [homosexuality] in animals is an evolutionary paradox or aberration.”