Macaque Monkeys Frequently Have Gay Sex

Male monkeys frequently engage in same-sex mounting, and in some cases, more frequently than they have sex with females.

Same-sex sexual behavior (SSB) in rhesus macaques appears to be extremely common, and even improves the reproductive output of the population. This is according to a study published in the journal Nature Ecology and Evolution on July 10.

The authors found that, of a group of 236 rhesus macaques, 72 percent of the males engaged in same-sex mounting with other males, compared to 46 percent that attempted to mount females.

"We looked at around 250 males, and 72 percent of those engaged in same-sex mounting. Most of them were bisexual," Vincent Savolainen told Newsweek. He is co-author of the paper and director of the Georgina Mace Centre for the Living Planet at Imperial College London.

In the paper, the authors describe how they studied these 236 males within a colony of 1,700 rhesus macaques living in the wild on the island of Cayo Santiago in Puerto Rico, and recorded all attempts at sexual mounting. They found that the males that had sexual contact with each other were more likely to back each other up in conflicts, showing that SSB was strongly correlated with 'coalitionary bonds'.

"[This behavior was] more common in males in rhesus macaques, [but] in another related species, the Japanese macaques, same-sex sexual behavior is more common in females," Savolainen said.

There is a wide range of other species that have been seen to copulate with members of the same sex, from albatrosses and dolphins to bisons and walruses. One study on giraffes found that up to 94 percent of observed mounting incidents were between two males.

The authors also found that this SSB was passed on from generation to generation, as pedigree data revealed that the behavior was 6.4 percent heritable. This means that the trait is able to be acted upon by natural selection, and therefore evolved into existence, rather than just being a random behavior.

"Same-sex sexual behavior is widespread in vertebrates, but our case study is unique because we had access to a pedigree dating back from 1950s. So we were able to demonstrate, for the first time in nonhuman vertebrates, that it was heritable to a degree: that is, same-sex sexual behavior has a genetic basis in these monkeys," said Savolainen.

This is therefore the first evidence of a genetic link to SSB in non-human primates.

"These findings support counter-arguments to the idea that SBB 'defies nature and evolution' (the so-called 'Darwinian Paradox')," reads a release from the authors.

The authors said they found that being a "mounter" or a "mountee" is also heritable. They added that these roles did not correlate in any way with the social position of the males within the group.

The authors also found that this SSB may be beneficial to the reproductive output of the group, rather than resulting in fewer offspring.

"We found a positive trend between amount of same-sex sociosexual behavior and number of offspring at population level, albeit not significant. We found that male sexual partnerships significantly predicted coalitionary partnerships, so we believe that this in turn give them an advantage and increased access to siring females," Savolainen said.

The authors do urge against extrapolating their results across other species. However, they say that this study therefore challenges the idea that SSB is rare in animals, or aberrant behavior. The results show that it can evolve adaptively in populations and improve the evolutionary fitness of a population.

"Unfortunately, there is still a belief amongst some people that same-sex behavior is 'unnatural,' and some countries sadly still enforce the death penalty for homosexuality," Savolainen said in the statement. "Our research shows that same-sex behaviour is in fact widespread amongst non-human animals.

"Our mission is to advance scientific understanding of same-sex behavior, including exploring the benefits it brings to nature and within animal societies. Amongst the macaques we looked at in this study, more than two-thirds displayed same-sex behavior and this behavior strengthened the bonds within the community."

The authors hope to study SSB and its genetic origins further across other primate groups where it appears to be frequent, to find if it is indeed a commonly evolved feature of primate reproductive ecology.

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