In golden snub-nosed monkeys, sexual interactions between individuals of the same sex seem to fortify social bonds. Credit: Thomas Marent/Nature Picture Library
Same-sex sexual behaviour is part of the normal social life of some primates and could play an important part in their long-term success, a survey of nearly 500 species of apes, monkeys and other non-human primates has found.
The study, published on 12 January in Nature Ecology and Evolution, suggests that same-sex sexual behaviour in wild populations might be a response to harsh environments, predation and navigating complex social hierarchies1. Observed behaviours included mounting, genital touching and fellatio.
“A lot of people have long regarded same-sex behaviour as an accident, or rare, or only in zoo animals,” says co-author Vincent Savolainen, an evolutionary biologist at Imperial College London. But “it’s part of the normal social life of primates”.
Although a staple of wildlife documentaries, same-sex sexual behaviour in animals has only begun to receive serious study in the past few years. Explanations range from accident — behavioural spillover when reproduction is dominated by a few individuals — to adaptation, says Isabelle Winder, an evolutionary anthropologist at Bangor University in Wales. “It’s very hard to figure out what the significance is or if there is a significance.”
Strengthening social bonds
Previously, Savolainen’s team followed a colony of rhesus macaques (Macaca mulatta) in Puerto Rico for three years, discovering that same-sex sexual behaviour in males was routine and linked to later reproductive success, potentially because it fortifies social alliances2. “They fight together, they have sex together, and maybe later in life they will have access to more females,” he says.
To see if same-sex sexual behaviour had benefits in other primates, Savolainen, Chloë Coxshall — a behavioural primatologist at Imperial College London — and their colleagues scoured the scientific literature, identifying instances in 59 species among 491 surveyed, and found evidence for recurring same-sex behaviour in 23 of these.
When they compared the prevalence of same-sex sexual behaviour with environmental and other variables, the researchers found that it was most likely to occur in dry conditions and in areas where risk of predation was high, and among longer-lived species and those with strong sexual dimorphism — large size differences between males and females. Same-sex sexual behaviour was also more common in primates that live in hierarchical groups, in which individuals — especially males — must climb the social ladder to have a chance at reproducing.