Same-Sex Behavior in Primates Linked to Survival Needs

Scientists at Imperial College London have uncovered why same-sex sexual behavior happens so often among primates. Their study of 491 primate species shows this behavior helps animals survive tough conditions and stay together in groups. The work appeared this month in Nature Ecology & Evolution.

Background

People have noticed same-sex sexual behavior in animals for a long time. Reports go back to ancient times, like writings from the Greek thinker Aristotle. Over 1,500 species show this kind of activity. In primates, it includes males or females mounting each other or other forms of stimulation.

For years, experts called it a puzzle because it did not seem to help pass on genes, which goes against basic ideas from Charles Darwin. But recent work changed that view. Studies now show this behavior can be passed down from parents in some cases, and it brings real benefits.

Researchers have watched rhesus macaques in Puerto Rico for eight years. They saw males who engage in same-sex acts form strong alliances. These bonds help them get closer to females and have more young over time. In one case, this behavior showed up in over 6% of cases linked to family traits, but only under certain conditions.

This new study builds on that. The team gathered data from species around the world, from lemurs in Madagascar to apes in Africa and monkeys in Asia and the Americas. They looked at how living conditions, group life, and body traits play a role.

Key Details

The researchers checked 59 primate species where same-sex sexual behavior is documented. That is about one in eight of all non-human primates. They examined 15 factors, like weather, food supply, predator risks, group size, and how long animals live.

Harsh Environments Drive the Behavior

Primates in dry places with little food show this behavior more often. Take Barbary macaques in North Africa's mountains. Food is scarce there, especially in winter. Same-sex acts happen frequently among them.

Predator threats also matter. Vervet monkeys in Africa face lions, leopards, and snakes. In those groups, same-sex behavior helps keep everyone calm and united.

Species with big differences in male and female sizes do it more too. Mountain gorillas fit this. Males are twice the size of females and live in strict groups led by one big male. Competition is fierce, and same-sex acts ease tensions.

Social Bonds and Group Life

Groups with complex rules and hierarchies see more of this. Animals use it to make friends, settle fights, or climb the social ladder. Longer-lived species, which form bigger networks over time, engage in it too.

The study used math models to map connections. Harsh surroundings shape body traits and group setups, which then boost the behavior. But group complexity pushes it directly.

“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.” – Vincent Savolainen, Imperial College London

Vincent Savolainen leads the Georgina Mace Centre for the Living Planet at Imperial College. He points out this behavior is normal, like eating or raising young.

Another team member, Chloë Coxshall, noted different patterns across species. Some do it for alliances, others for peace during hard times.

What This Means

This work shows same-sex sexual behavior has old roots in primate history. It popped up many times on its own, not just once long ago. It acts as a tool for handling stress from nature and from living with others.

In rhesus macaques, it links to genes a bit, but surroundings matter more. Tough spots push animals to use it for survival. It keeps groups from breaking apart when food runs low or dangers rise.

For our early human relatives, similar pressures existed. They lived in changing climates, faced predators, and built big groups. This behavior might have helped them too.

But humans today are different. We have layers of feelings and choices around sex that primates do not. The study stays away from those topics. It warns against wrong ideas, like thinking equal societies would end this behavior.

Experts outside the team praise the methods. Isabelle Winder from Bangor University said it lights up how animal actions like ours evolved. Modern tools let them compare species fairly for the first time.

“Our research shows that same-sex sexual behaviour is an integral part of many non-human primate societies, and it seems to help the animals to bond and maintain group harmony.” – Vincent Savolainen

This finding changes how we see animal sex. It is not rare or odd. It fits into daily life, helping primates adapt. Future studies could check other animals or track changes over seasons.

The team plans more field work. They want to see how climate shifts today affect these patterns. In places like Puerto Rico, monkey groups face new stresses from people and weather. Watching how they respond could tell us more.

Overall, the research paints a picture of flexible strategies. Primates adjust to their world using all kinds of bonds. Same-sex behavior is one key piece in that puzzle.