A new analysis of primate species indicates that same-sex sexual behavior may enhance social bonds and reproductive success, particularly in challenging environments. Researchers from Imperial College London examined data from 59 species, finding higher prevalence in areas with scarce food or predation risks. The findings challenge assumptions that such behavior is non-adaptive.
Same-sex sexual behavior is widespread among primates, observed in apes and monkeys, and a recent study proposes it serves an evolutionary purpose by aiding social climbing and increasing offspring numbers. Published in Nature Ecology & Evolution, the research by Vincent Savolainen and colleagues at Imperial College London analyzed prevalence across 59 primate species, associating it with environmental stressors like dry conditions, food shortages, and high predation pressure, as well as complex social structures.
The study builds on observations that such behavior occurs in at least 1,500 animal species, from insects to bonobos, and in 80 percent of the 20 mammal species closely studied over time. For example, in a long-term study of rhesus macaques on Cayo Santiago, Puerto Rico, three-quarters of males engage in same-sex interactions, as reported by Savolainen's team in 2023.
"Same-sex behaviour is, if you want, a currency that you can use to navigate your way in these societies," Savolainen explained. His co-author, Chloë Coxshall, added, "Same-sex behaviour may facilitate better cooperation and cohesion by strengthening social bonds, which is particularly important in these stressful environments."
While the analysis suggests adaptability—becoming more common under stress rather than less—the researchers emphasize that direct links to fitness, such as higher offspring counts, require further testing. Savolainen plans to investigate this in macaques. The work addresses the 'Darwinian paradox' of why seemingly non-reproductive behaviors persist, proposing they build coalitions for better female access.
For humans, the findings may explain the behavior's commonality but do not inform moral judgments, avoiding the naturalistic fallacy. The DOI for the paper is 10.1038/s41559-025-02945-8.