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Biologists tend to look at the animal kingdom and try to categorize things in an organized fashion. But nature has a way of defying our neat little categories. Take something as common as same-sex sexual behavior. It shows up in species from swans to elephants and from spiders to dolphins. Of course, it also shows up in humans.
But why?
Why does evolution, which selects for reproductive success, keep producing sexual behavior that doesn’t lead to offspring? This was exactly the motivation for the new study.
“We wanted to research same-sex sexual behaviour as this behaviour has previously been considered a “Darwinian paradox”, so we wanted to test why this behaviour is maintained across populations,” said Chloë Coxshall, the study’s lead author, for ZME Science.
Risky Behaviors
Coxshall and her team at Imperial College London combed through decades of research, compiling data on 491 primate species. They found records of same-sex sexual behavior (SSB) in 59 of them. They then looked at the context, trying to find patterns in everything from the weather and predators to social structures and lifespans.
“A lot of current research focuses on the genetic elements of same-sex sexual behavior, but in this study, we wanted to test which environmental factors contribute to the expression of this behavior, which can sometimes be overlooked,” Coxshall adds.
What they found was striking. Same-sex behavior wasn’t evenly distributed; instead, it seems to be shaped by environmental, life history, and social traits, all of which seem to be linked to a need for group cooperation and affiliation.
It popped up most frequently in species living in what scientists call “high-stakes” environments.
“Harsher or riskier environment may refer to intense climatic conditions, or environments where threats are higher, such as increased predation pressure or limited food resources. In these conditions, primate groups may experience increased social pressures, including strong cooperation to navigate predator-rich environments, or greater group cohesion when social bonds are strained with increased competition driven by harsh environments.
But Why?
It seems counterintuitive at first. Why waste energy on sexual behaviors that don’t produce offspring, especially when you are starving or being hunted? Because in primates, sex isn’t just about reproduction. It’s about glue. It’s a kind of social currency.
Think about it: if you are a rhesus macaque living in a place where leopards are everywhere, you need friends. You need allies who will watch your back. If the social hierarchy is rigid and brutal, you need a way to diffuse tension without getting your face bitten off. You have better returns on same-sex interaction.
This is further supported by the fact that SSB seems to be more common in species with complex social lives. If you live in a society with strict hierarchies, where everyone knows their rank and fighting for status is common, you are more likely to see same-sex behavior.
Isabelle C. Winder, a primatologist at Bangor University who wrote a News & Views piece accompanying the study, points out that this behavior is likely part of a “flexible repertoire”.
“Living in a complex long-lasting social group is potentially stressful and can incur noticeable energetic and reproductive costs,” Winder writes. “In these contexts, a flexible range of behaviours that supports the building and maintenance of strong interindividual relationships would be an advantage.”
Competition for Mates
The study also found two intriguing links to biology.
Species that show greater sexual dimorphism (where males and females look very different, usually because males are much larger) were more likely to exhibit SSB. This also makes sense in the context of conflict. High sexual dimorphism usually implies intense competition between males for mates. That competition breeds aggression. SSB might offer a pressure valve, a way to compete or bond without lethal violence.
The other link is related to average lifespan.
Interestingly, they also found that longer-lived species are more likely to do it. If you only live a year or two, you might just focus on reproducing and dying. But if you live for decades, like many monkeys and apes do, it becomes all the more important to maintain relationships over the long haul. You have to navigate shifting alliances, new leaders, and changing group dynamics. You need tools to keep the peace for twenty years, not just twenty minutes.
However, the researchers caution against drawing sweeping conclusions. The analyzed data may be biased because researchers aren’t really looking for homosexual behaviors.
“Current research into same-sex sexual behavior may be limited by a lack of recordings,” Coxshall told ZME Science. “Many researchers have seen same-sex sexual behavior, but have not recorded it, most likely as this behavior is not part of their research.”
Historically, science has had a bit of a blind spot here. A researcher in the 1950s seeing two male baboons mounting might have just ignored it, assuming it was “dominance” or “play” rather than sexual behavior, or simply deeming it “aberrant” and leaving it out of the final paper.
Simply put, same-sex sexual behavior in primates could be even more common.
What Does This Mean for Us?
It’s tempting to take a logical leap. We’re primates, we have complex hierarchies, we live in stressful environments, and we bond through sex. Humans seem to check all the boxes.
But Coxshall and her team are careful — very careful — not to draw a straight line from a macaque mounting his friend to human sexual identity.
“We must be cautious when making direct comparisons with humans, as we consider same-sex sexual behaviour as a biological behaviour, whereas with humans it is important to consider sexuality, gender, and identity.”
Human sexuality is a rich mix that blends biology with culture, psychology, and personal identity. You can’t reduce the complexity of the human LGBTQ+ experience to a simple evolutionary survival mechanism used; even a complex evolutionary survival mechanism doesn’t come close to what we humans consider to be personal identity.
However, that doesn’t mean the study is irrelevant to us. It helps destroy the argument that same-sex behavior is “unnatural.” As the numbers show, it is clearly, demonstrably natural. It has evolved and persisted in our closest relatives for millions of years because it confers an evolutionary advantage by mitigating conflict and keeping individuals safe.
If we want to find out more about our own evolution, we should start by looking at our closer relatives.
Between non-human primates and modern humans, there are a number of extinct hominid species that we could consider for future hypotheses,” Coxshall notes. “These hominid species may have lived in similar environments of social contexts to some of the non-human primates we have studied.”
In the end, this research paints a picture of same-sex sexual behavior in primates as an unlikely tool that helps individuals survive, to bond, and keep peace. It’s messy. It’s complicated. And it is perfectly natural.
The study has been published in Nature Ecology & Evolution (2026). DOI: 10.1038/s41559-025-02945-8. www.nature.com/articles/s41559-025-02945-8
