New research explores ecological and social roots of same-sex sexual behaviour in primates

A comprehensive new study of primates reveals that same-sex sexual behaviour (SSB) is widespread and influenced by a combination of environmental and social factors.

Published in Nature Ecology and Evolution, the research analysed data from 491 non-human primate species, documenting SSB in 59 species and exploring its links to ecological conditions and social structures. The study also explored how SSB was influenced by life history traits, such as lifespan, age at maturity, and number of offspring, both in males and females. 

Lead author Chloe Coxshall of Imperial College London’s Georgina Mace Centre for the Living Planet, said: ‘Same-sex sexual behaviour is widespread across non-human primates, but we see different patterns of social behaviours and interactions across the different species. Our study shows how these patterns are influenced by environmental factors, as well as by the social systems that the primates live in.’ 

Patterns across ecology and life history 

The researchers combined species-level records of SSB with ecological and social data, examining 15 factors ranging from climate and the availability of different resources, to group structure and lifespan. They found that SSB was more likely in species living in drier climates, with scarce food resources or under greater pressure from predators. Species with noticeable differences between males and females, longer lifespans, or complex social lives were also more likely to engage in SSB. 

The findings suggest that same-sex sexual behaviour may play a role in navigating social dynamics, forming alliances, and maintaining cohesion within groups.  

Genetics and environmental context 

Previous research in rhesus macaques suggested that SSB is modestly heritable. This new study shows that, while genetic predispositions may exist, SSB is strongly influenced by ecological and social pressures. Using structural equation modelling, the researchers found that environmental conditions affect life history traits and social structures, which in turn shape SSB, while social complexity has a direct influence. 

Implications for primate evolution 

The study challenges the perception that SSB might be rare or anomalous in animals. Observed across lemurs, monkeys, and apes, the behaviour appears to have both deep evolutionary roots and multiple independent origins. 

In many species, SSB appears to be a flexible aspect of primate social life and the researchers suggest that it may help individuals navigate complex social environments and maintain group cohesion.  

Professor Vincent Savolainen, Director of the Georgina Mace Centre for the Living Planet, said: ‘Our research shows that same-sex sexual behaviour is an integral part of many non-human primate societies, and it seems to help the animals to bond and maintain group harmony. It would be interesting to explore further how different environmental and social factors have influenced how these behaviours have evolved in human societies as well in nonhuman primates.'