Same-Sex Behavior in Primates: An Ancient Strategy for Survival

Same-sex sexual behavior shows up across many primate species, and a new large comparison suggests it is not random. It appears more often in species facing tougher living conditions, heavier predation pressure, or more intense social lives. The findings point to a behavior that may help animals navigate stress, conflict, and relationships, rather than something rare or biologically pointless. The new analysis brought together evidence from hundreds of non-human primate species. The research builds on a growing body of work arguing that same-sex behavior can have evolutionary value, including by strengthening social bonds and alliances that may indirectly increase reproductive success. Behavior observed across many species Scientists have documented males or females of the same sex mounting each other or engaging in sexual stimulation across the animal kingdom. Reports exist for more than 1,500 species, and some descriptions go back as far as Aristotle. For a long time, however, researchers often treated the phenomenon as a “Darwinian paradox.” The assumption was that behaviors not directly producing offspring should be selected against. That idea has weakened as evidence has grown that same-sex behavior can be partly inherited and can carry advantages in social species, even if the acts themselves don’t result in reproduction. “Diversity of sexual behavior is very common in nature, among species and in animal societies – it is as important as caring for offspring, fighting off predators, or foraging for food,” said study co-author Vincent Savolainen, a biologist at Imperial College London. Same-sex behavior and alliances Savolainen has studied rhesus macaques in Puerto Rico for years, and his team has linked male-male mounting to social strategy. In their observations, males that mounted other males formed alliances that could help them gain access to females later on, potentially translating into more offspring over time. The same group has also reported a heritable component. In 2023, they found that macaques inherited same-sex behavior from their parents more than six percent of the time. Whether the behavior appeared, however, depended on multiple interacting factors rather than a single genetic switch. Patterns across primate species For the new study, Savolainen and colleagues assembled data on 491 non-human primate species. The researchers identified same-sex sexual behavior in 59 species, including lemurs, monkeys, and great apes across the Americas, Africa, and Asia. Because the behavior was found across diverse branches of the primate family tree, the researchers argued it likely has a long history. They described it as having a “deep evolutionary root.” The team then explored which pressures made same-sex behavior more likely in primates and other species. They examined environment, social organization, and life history traits to see what patterns emerged across species. Survival pressure shapes behavior One clear signal involved ecology. Same-sex behavior was more common in primates living in harsher conditions with limited or unstable access to food, such as Barbary macaques. It also appeared more frequently in species exposed to higher predation risk. The article gives vervet monkeys as an example, living with constant threats from predators like big cats and snakes. The researchers interpret these associations as consistent with a stress-and-cohesion idea: when life is risky, social stability becomes more valuable, and behaviors that reduce tension or strengthen bonds may offer survival benefits. Power, size, and structure The study also linked same-sex behavior to social structure. It was more common in species with strong differences in body size between males and females, such as mountain gorillas. Large size differences often go with intense competition and stricter dominance hierarchies, usually in bigger groups. By contrast, species in which males and females are closer in size often live in pairs or smaller family units. From the researchers’ perspective, this supports the idea that same-sex sexual behavior in primates may function as a flexible social strategy, used to reinforce social bonds, manage conflict, or build alliances, depending on the ecological and social pressures faced by different species. Implications for human evolution The team suggested that similar pressures could have influenced human ancestors, who also faced challenging environments and complex social lives. However, modern humans have forms of sexual orientation and preference that this primate-focused work does not attempt to explain. The study also warned against misusing its findings. It cautioned against misguided notions that social equality might eliminate same-sex sexual behavior in modern humans. Why experts are watching Isabelle Winder, an anthropologist at Bangor University in the UK who was not involved in the study, praised the work for showing what careful comparative methods can do. “It is their study’s demonstration that modern comparative methods can, for perhaps the first time, realistically illuminate some of the complexities of the evolution of ‘humanlike’ behaviors that I find most exciting,” she said. Overall, the message is not that one explanation fits every species. It’s that same-sex behavior appears often enough – and under consistent enough pressures – that it looks less like an evolutionary puzzle and more like a versatile tool for living in demanding social and ecological worlds. The study is published in the journal Nature Ecology & Evolution. —– Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com. —–