Same-sex sex is widespread in the animal kingdom. It is already known that genetic factors play a role. A comparative analysis of 491 primate species now shows that environmental conditions and social structure can also promote same-sex sexual behavior. According to this, 59 primate species also have sex with members of their own sex – including primarily species that live in regions with low food availability or high predation pressure. Strong social hierarchies and large physical differences between males and females can also promote homosexual behavior. Same-sex sexual intercourse could therefore play an evolutionary important role in social cohesion.
Sex between two men or two women has long been considered a purely human, “unnatural” behavior. But studies have now shown that around 1,500 animal species regularly or occasionally have homosexual intercourse. Even if such copulations do not result in offspring, they can indirectly promote the evolutionary success of a species. In many animal species, sexual interactions between members of the same sex, for example, strengthen social cohesion. Genes that have been linked to homosexuality in men may also contribute to increased fertility in women and thus persist across generations.
Social glue
“However, the evolutionary origins and ecological basis of same-sex sexual behavior remain poorly understood,” explains a team led by Chloë Coxshall from Imperial College London. In order to gain new insights into which factors can promote homosexual intercourse, the researchers compared the sexual behavior of 491 non-human primates. They found evidence of homosexual interactions for 59 of these species. In 23 of these, same-sex sex was also repeatedly observed in wild populations, so cannot be explained by unnatural behavior in captivity.
The comparisons between different primate species revealed that homosexual relationships between individuals occur primarily in species that have to contend with harsh environmental conditions. “Same-sex intercourse was more likely in species that live in dry environments with little food or that face high pressure from predators,” Coxshall and her colleagues report. This can be seen, for example, in Barbary macaques and vervet monkeys, which increasingly rely on mutual grooming and other forms of physical attention in stressful situations. “These results support the hypothesis that environmental stressors can influence the development of same-sex sexual intercourse as a flexible behavioral strategy to deal with social tensions.”
Physical and social characteristics of primates also play a role: “Species with a longer lifespan, a greater difference in size between males and females, and more complex social structures and hierarchies also show same-sex sexual behavior more often,” say the researchers. “This could support long-term social bonds across changing relationships and group dynamics.”
Parallels to humans?
The researchers emphasize that the analysis refers exclusively to non-human primates. “Nevertheless, it is difficult to ignore possible implications for our early human relatives and modern humans,” they write. It is therefore obvious that our ancestors were also exposed to adverse environmental conditions, including scarcity of resources and the threat of predators, and had to find their way in increasingly complex social structures. “These are all factors that our analysis links to same-sex intercourse in non-human primates.”
However, Coxshall and her colleagues emphasize that any attempts to extrapolate the results to people today remain purely speculative. “Evolutionary hypotheses neither determine the validity of individual identities nor do they influence their inherent value,” emphasize the researchers. “Our goal was to document factors underlying behavioral diversity in animals, not to justify or explain human identities.” However, when applied to non-human primates, the results provide valuable insights into sexual diversity and social evolution. “Our analysis shows that same-sex sexual intercourse is a context-dependent behavior that is shaped by interactions between ecological, life history and social factors,” summarizes the research team.
Source: Chloë Coxshall (Imperial College London, UK) et al., Nature Ecology & Evolution, doi: 10.1038/s41559-025-02945-8