Same-sex sexual behavior among primates could be shaped in part by specific environmental and social conditions, according to a new study comparing 59 species.
Same-sex sexual behavior (SSB) in animals is increasingly recognized in the scientific community as widespread, as documented throughout the animal kingdom 1500 species. However, deciphering how and why it appeared in so many different species is challenging. The new study, published Jan. 12 in the journal Ecology and evolution of naturefound that SSB can be beneficial — at least for primates — when ecological conditions are harsh and social conditions are complex.
“[The findings] “suggest that same-sex orientation has a very strong evolutionary history and is nothing bizarre, derivative or unnatural,” said the Durham University primatologist. Fang Claywho was not involved in the new study. “In fact, it’s probably part of the evolutionary fabric of our societies.”
Primates and same-sex bonding to cope with harsh conditions
Research suggests that SSB facilitates bond formation in socially complex animals. In bonobos (Paniscus) and chimpanzees (Mr. Troglodytes), is associated with reduced voltageconflict resolution and alliance strengthening. For golden snub-nosed monkeys (Rhinopithecus roxellana), SSB and care strengthen social ties in a harsh, cold climate with limited resources.
Genes also seem to play a role. Due in 2023 studies rhesus macaques (Macaca mulatta), Vincent Savolainenbiologist at Imperial College London, found that SSB is about 6.4% heritable, meaning that the tendency to the behavior can be passed down genetically from parents to offspring. But such a small percentage leaves a lot of uncertainty about what else could be causing it.
To examine the ecological and social context, Savolainen and colleagues conducted a meta-analysis of SSB studies across primates. Of the 491 species, they found the behavior documented and prevalent in 59 species. SSB is more likely when species face drier environments, scarcer resources and many predators, according to the study. It is also more common in species with complex social systems, larger size differences between males and females, and longer lifespans.
These trends suggest that SSB may serve as a social strategy to strengthen bonds, manage conflict, or build alliances, driven by environmental and social pressures faced by the group. “Species that have particularly demanding environmental and social pressures have evolved independently of the common origin of same-sex sexual behavior as a way to cope with pressure and manage social dynamics,” Savolainen said, “forming coalitions, banding together, and helping them deal with the challenges they face.”
If predators are especially abundant, for example, it’s advantageous to have a socially close group that can trust each other with alarm calls, Savolainen said; SSB offers one way to create or maintain relationships.
Chimpanzees and bonobos are known to engage in SSB when navigating ecological challenges, Clay said. “In a situation of scarce resources, you have to work together and learn to tolerate each other,” Clay said. “If there’s a shortage of food, it’s important to have techniques to maintain and maintain social ties.”
Although the trends are striking, it is not so easy to draw parallels between species that exhibit SSB. “It points to some common explanations that can cut across deep taxonomic divisions, but there’s a risk of obscuring some of the nuances within individual lineages,” he said. Nathan Baileyevolutionary biologist at the University of St Andrews, who was not involved in the new study. “Do these behaviors occur for different functional reasons, under different selective pressures, in different lineages? They’re starting to scratch the surface.”
Savolainen said SSB has traditionally been underappreciated, so the new findings highlight its importance in the broader understanding of primate behavior. “Same-sex behavior is as important as feeding, fighting or caring for young,” he said.
The research could help create a more complete picture of social and sexual behavior in primates. “People tend to separate reproductive and social sex, when in fact I think the social element of both is very important and should be integrated,” Clay said.
But can these findings shed any light on human behavior? Our early hominin ancestors likely experienced a variety of ecological and social pressures, including those associated with SSB for primates in this study, the study authors point out—but it’s unclear whether those pressures would have contributed to the evolution of same-sex sexual orientation in hominin species in a similar way.
Generalizing the results to humans is difficult, researchers and experts say, without behavioral data from our hominin ancestors, and thinking about modern human culture and identity is thus complicated.
“I don’t think it tells us much about what’s going on in humans,” Bailey said. “There seems to be a huge variety of explanations [of SSB] across animals, even in closely related lines, so it doesn’t seem logical to me that any particular explanation in animals would map to human beings.”
The study also points to a key reason primates, including humans, have succeeded so well around the world: adaptability. “We’re not fixated on one mating system, one behavioral system,” Clay said. “For me, it’s the fact that sexual behavior can become widespread [under different conditions] it reflects the behavioral flexibility that is really important for primate success.”
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