Same-sex sexual behaviour among non-human primates may arise as a way to reinforce bonds and keep societies together in the face of environmental or social challenges, researchers have suggested.
Prof Vincent Savolainen, a co-author of the paper from Imperial College London, added that while the work focused on our living evolutionary cousins, early human species probably experienced similar challenges, raising the likelihood they, too, showed such behaviour.
“There were many different species that unfortunately [are] all gone, that must have done this same thing as we see in apes, for example,” he said.
Writing in the journal Nature Ecology & Evolution, Savolainen and colleagues reported how they analysed accounts of same-sex sexual behaviour in non-human primates, finding it to be widespread in most major groups, with reports in 59 species including chimpanzees, Barbary macaques and mountain gorillas. That, they added, either suggested an evolutionary origin far back in the primate family tree, or the independent evolution of the behaviour multiple times.
While some studies have previously highlighted the possibility such behaviour could help reduce tensions in groups or aid bonding, the new study looked across different species to explore its possible drivers. The results reveal it to be more likely in species living in drier environments, where resources are scarce, and where there is greater risk from predators.
“Previous research has shown there is a heritable element to [same-sex sexual behaviour], however, there is also environmental influence which is often overlooked,” said Chloe Coxshall, the first author of the study.
The team found such behaviour was more likely in species that lived longer, and in species where males and females showed differences in size – a feature associated with larger social groups with intense competition. The behaviour was also more common in species with complex social systems and hierarchies.
“Same-sex [sexual behaviour] seems to be an affiliative behaviour [to] increase the bonding, decrease tension and aggression, and allow whatever species and their particular environment and society to basically navigate the challenges that they face,” said Savolainen.
The team added it appeared these different factors were interlinked, with environmental conditions influencing life history traits that in turn affected social systems.
While the researchers cautioned against misinterpretation of the findings in relation to humans – such as the idea social equality could eliminate same-sex sexual behaviour – and noted sexual orientation, preferences and identities in humans were complicated, they suggested the study raised questions.
“In humans, it may not be food scarcity or rigid social hierarchies that drive these patterns, but rather the pressures of modern social living,” they wrote, noting younger generations reported more sexual fluidity but also mental health challenges.
Prof Zanna Clay of Durham University, who was not involved in the work, said the study “clearly shows that far from being rare or atypical, same-sex behaviour is a common and important part of primate sociality, of which humans are of course part”.
Clay added: “By showing how widespread it is across primates, and its important adaptive functions, the study contributes a nice novel angle to debates on the origins of same-sex behaviour. It will be fascinating to expand this analysis to other members of the animal kingdom and test whether similar hypotheses hold.”
However, Josh Davis of the Natural History Museum in London, the author of A Little Gay Natural History, said queer behaviours were found across the natural world in a multitude of different expressions, meaning the reasons driving homosexual behaviours were likely to be equally varied.
“In addition to this, as with all studies that are looking at animal behaviour, I would be incredibly uncomfortable speculating about how these then map on to human behaviours,” Davis said. “People are complex and a result of a whole range of different factors separate from other animals, making these comparisons and extrapolations incredibly contentious.”
