In golden snub-nosed monkeys, sexual interactions between individuals of the same sex appear to strengthen social bonds. Credit: Thomas Marent/Nature Picture Library
Homosexual behavior is part of the normal social life of some primates and can play an important role in their long-term success, a survey of nearly 500 species of monkeys, apes and other non-human primates has found.
The study, published on January 12 in Nature ecology and evolutionsuggests that homosexual behavior in wild societies may be a response to harsh environments, predation, and the navigation of complex social hierarchies.1. Observed behaviors included mounting, touching of the genitals and tongue.
“Many people have long considered homosexual behavior to be incidental, rare, or only in zoo animals,” says co-author Vincent Savolainen, an evolutionary biologist at Imperial College London. But “this is part of the normal social life of primates.”
Although a staple of wildlife documentaries, homosexual behavior in animals has only begun to receive serious study in the past few years. Explanations range from chance — behavioral spread when a small number of individuals dominate reproduction — to adaptation, says Isabel Winder, an evolutionary anthropologist at Bangor University in Wales. “It’s very difficult to know what the significance is or whether there is a significance.”
Strengthening social ties
Previously, Savolainen’s team followed a colony of rhesus macaques (macaque mulatto) In Puerto Rico for three years, he discovered that homosexual behavior in males was routine and associated with subsequent reproductive success, perhaps because it reinforced social alliances.2. “They fight together, they have sex together, and maybe later in life they’ll have access to more females,” he says.
To find out whether homosexual behavior has benefits in other primates, Savolainen, Chloe Kokshall, behavioral primatologists at Imperial College London, and their colleagues searched the scientific literature, identified cases in 59 of the 491 species they surveyed, and found evidence of frequent homosexual behavior in 23 of them.
When they compared the prevalence of homosexual behavior with environmental and other variables, the researchers found that it was more likely to occur in dry conditions, in areas where predation risk was high, and between longer-lived species and those with strong sexual dimorphism — large differences in size between males and females. Homosexual behavior was also more common in primates living in hierarchical groups, where individuals—particularly males—must climb the social ladder to have a chance at reproduction.