News release

From: Springer Nature

Ecology: Social and environmental factors associated with same-sex behaviour in primates

Same-sex sexual behaviour in non-human primates is associated with ecological factors, life history and social structure, according to research published in Nature Ecology & Evolution. The findings are based on a comparative analysis of 491 species and offer insights into primate social evolution.

Same-sex sexual behaviour has been reported in many animal species. This behaviour can be heritable and may confer fitness benefits; however, this leaves other evolutionary aspects unclear as well as whether there is a link to ecological factors. In primates, previous work has proposed that same-sex sexual behaviour might help to manage relationships and group dynamics, as it has been documented in social contexts such as alliance formation and tension reduction, but cross-species analysis that might reveal shared drivers has been limited.

Vincent Savolainen and colleagues analysed data from 491 non-human primate species and found evidence of same-sex sexual behaviour in 59 of them, with evidence of repeated occurrence of the behaviour in 23. This latter result allowed them to analyse the prevalence of same-sex sexual behaviour in different contexts. The authors found that same-sex sexual behaviour was more common in species living in harsh or dry environments with limited food (e.g. Barbary macaques) and/or high predation risk (e.g. vervet monkeys). The behaviour was also more common in species that are smaller, where there are differences in size or appearance between the sexes (e.g. mountain gorilla), and/or long-lived (e.g. chimpanzees), and in those with complex social systems and hierarchies (e.g. baboons). Their analysis suggests that same-sex sexual behaviour is not simply or straightforwardly adaptive but emerges from complex context- and scale-dependent interactions between external and internal factors. Life history traits are shaped by environmental factors, which in turn shape social complexity and lead to prevalence of traits such as same-sex sexual behaviour.

Given the commonality of these drivers across the primate species studied, the authors speculate that similar factors might be implicated in explaining same-sex sexual behaviour in both ancestral hominins and even present-day humans. However, they caution that care must be taken with the interpretation of their findings, stressing that their research does not address human sexual orientation, identity or lived experience.

In an associated News & Views, Isabelle Winder notes that it is the “demonstration that modern comparative methods can, for perhaps the first time, realistically illuminate some of the complexities of the evolution of ‘humanlike’ behaviours that I find most exciting”.