Richard Collins: Why do same-sex liaisons occur among animals?

Researchers have found that same-sex behaviour is in fact widespread among non-human animals. However, the researchers caution against direct comparisons with humans

Two male Rhesus Macaques. Bisexual behaviour among male monkeys is widespread and may be beneficial to them, new research suggests. The Imperial College London researchers, [top right: Jackson Clive; centre right: Vincent Savolainen. Professor of Organismic Biology at Imperial; lower right: Ewan Flintham] say their findings challenge the beliefs of some that SSB is a rare behaviour in non-human animals or solely the product of unusual environmental conditions. Picture: Jackson Clive/Imperial College London

A mute swan nest, in County Clare during the 1980s, was said to contain 15 infertile eggs. Two females taking turns incubating them were probably lesbians.

According to the ‘Darwinian paradox’, genes which don’t promote reproduction won’t survive. No offspring can issue from homosexual relationships. Why, then, do same-sex liaisons occur among animals? One suggestion is that such encounters don’t take place at all, reported cases being unnatural aberrations, behavioural malfunctions in artificial situations. But are they?

Against Nature, an exhibition on animal homosexuality, was organised by the National Museum of Oslo in 2006, as part of a programme on taboo subjects. The organisers sought to dispel ‘the all too well-known argument that homosexuality is a crime against nature’. The show was controversial at the time — an irate member of the public warned the organisers that they would “burn in hell”.

I saw the exhibition in Stockholm, where it was renamed Rainbow Animals. Photographs and models showed creatures, from whales and giraffes to flamingos and penguins, engaged in same-sex copulation. It claimed that homosexual behaviour had been observed in more than 1,500 species, one-third of instances having been "well documented".

Exciting new study from Jackson Clive and team @ImperialLifeSci @GMaceCentre shows that same-sex sexual behaviour among male macaques is widespread and may have social benefits. 🐒https://t.co/3wj9MSXekx

— Imperial Life Sciences (@ImperialLifeSci) July 11, 2023

In a paper just published, scientists at Imperial College London provide evidence that same-sex sexual behaviour is widespread in monkeys. Over a three-year period, a team studied the behaviour of 236 male rhesus macaques living in a colony on an island off Puerto Rico. Pedigree records detailed the ancestry of the monkeys back to 1956.

All ‘social mountings’ of males on females, and males on males, were logged. Almost three-quarters of males, it was found, indulged in same-sex mountings. Less than half of them engaged in male-female ones. Clearly, homosexual behaviour in macaques can’t be dismissed as an ‘unnatural aberration’. But how is the tendency to indulge in it inherited?

Two male Rhesus Macaques. Bisexual behaviour among male monkeys is widespread and may be beneficial to them, new research suggests. Picture: Jackson Clive/Imperial College London

In 1993 geneticist Dean Harmer, who would go on to propose that a ‘God gene’ promoted ‘spiritual’ yearnings in humans, claimed to have discovered linkage between DNA markers and male sexual orientation. The markers were carried on the X chromosome, which is inherited only through the maternal line. Genetic analysis of their macaques, by the Imperial College researchers, "showed some genetic correlation between males that were more often mounters or mountees when engaging in same-sex behaviour", according to the College website.

Surprisingly, they found that males who had indulged in male-on-male mountings bred more successfully than did those confining themselves to ‘strait’ encounters. Temporary male ‘coalitions’ can confer a reproductive advantage on males of species such as cheetahs. This seems to be the case with the macaques; the research showed that "males which mounted each other were also more likely to back each other up in conflicts".

PhD student Jackson Clive is studying rhesus macaques in the wild Picture: Imperial College London

Lead author, Jackson Clive, is quoted as saying: "We found most males were behaviourally bi-sexual and that variation in sexual activity was heritable". Also "the behaviour can have an evolutionary underpinning". Male-on-male mounting encourages individuals to come to the aid of partners in challenging situations. "Our research shows that same-sex behaviour is in fact widespread among non-human animals."

The researchers caution against direct comparisons with humans.

• Jackson Clive et al. Same-sex socio-sexual behaviour is widespread and heritable in male rhesus macaques. Nature Ecology & Evolution. 2023