Homosexuality is common among primates close to humans, and this trait appears to be inherited in some cases. The findings run counter to the view that homosexuality "goes against the laws of nature." Scientists said that, contrary to previous thinking, homosexuality in primate societies evolved as a behavior advantageous for adapting to environmental changes and leaving offspring.
A team led by Vincent Savolainen, a professor of biology at Imperial College London (ICL), said on the 13th in the journal Nature Ecology & Evolution that "an analysis of studies on 491 nonhuman primate species found evidence of same-sex sexual behavior in 59 species, and in some of them the trait appears to be inherited."
Savolainen said, "For a long time, many people thought same-sex sexual behavior was accidental, rare, or seen only in zoo animals," adding, "This study shows that such behavior is part of normal social life in primates." He added that it suggests same-sex sexual behavior evolved independently multiple times long ago in primates.
◇ Male homosexuality mainstream, as in ancient Greece
The history of homosexuality in human society is long. Ancient Greek society did not taboo male homosexuality and even encouraged it. Relationships between older men and boys were sometimes regarded as ideal love. It was seen less as sexual and more as a mentor-student relationship with educational and social significance.
The same was true for other primates, including humans. The team said an analysis of 96 studies on 491 nonhuman primate species found same-sex sexual behavior in 59 species. In 23 species, homosexual behavior was repeatedly observed.
As in ancient Greek society, male-male homosexuality was mainstream among monkeys. Previously, the team reported in 2023 in the same journal that a three-year follow-up of 236 rhesus macaques (Macaca mulatta) on Cayo Santiago island, Puerto Rico, found that 72% of males engaged in same-sex sexual behavior. It was actually more frequent than heterosexual intercourse (46%).
According to existing evolutionary theory, homosexuality should have been eliminated because it does not aid reproduction. The new findings differ. Males that engaged in same-sex intercourse were also active in heterosexual intercourse and, as a result, tended to spread more offspring.
The team suggested that homosexuality may not be a mere mistake or abnormality but an adaptive strategy advantageous for survival and reproduction. The scenario is as follows. Sexual contact among males strengthens bonds. Then, when rank fights occur, they can form strong alliances to help one another. Individuals with strong same-sex bonds are more likely to rise in social status, which ultimately increases opportunities to mate with females.
◇ Environmental effects are large, but genetic factors account for 6%
The team analyzed that homosexuality in primates was heavily influenced by the environment. In Barbary macaques (Macaca sylvanus), same-sex sexual behavior increased when food was scarce or drought persisted. In vervet monkeys (Chlorocebus pygerythrus), male same-sex behavior was more commonly observed when the risk of predation was high. In other words, homosexual behavior increases when survival is difficult.
That same-sex sexual behavior increases in stressful environments suggests that the behavior is a result of adaptation. If same-sex sexual behavior were performed solely for pleasure, such behavior should have decreased in stressful environments. The team said, "In golden snub-nosed monkeys (Rhinopithecus roxellana), which are adapted to cold climates, same-sex sexual behavior tends to occur alongside grooming that strengthens bonds."
Some have claimed that same-sex sexual behavior occurs because individuals cannot properly distinguish males from females. This may be true for simple animals like insects, but it was hard to apply to intelligent animals such as primates or dolphins.
In fact, the study found that primates that engage in homosexuality, like humans, clearly distinguish the sexes and have complex social structures. Homosexuality was commonly observed in species such as mountain gorillas (Gorilla beringei), where males and females differ greatly in size and appearance, chimpanzees (Pan troglodytes), which are long-lived, and baboons with complex social systems and hierarchies.
The team suggested that same-sex sexual behavior evolved not as a simple adaptive behavior but through the interplay of environmental and genetic factors. Each individual's homosexual traits are shaped by environmental factors, and as these individuals aggregate and social structures become more complex, homosexual traits are passed down to future generations. In the same vein, the team said the homosexuality of ancient humans and modern humans can be explained.
Zanna Clay, a psychology professor at Durham University, said, "This study shows that same-sex sexual behavior is not rare or abnormal but a common and important part of primate societies, including humans," adding, "It would be very interesting to extend this analysis to other members of the animal kingdom and test whether similar hypotheses hold."
However, this study did not directly discuss human sexual orientation, identity, or lived experience. In other words, it is difficult to apply the results directly to humans.
Josh Davis, a science writer at the Natural History Museum in London, said, "As with other studies of animal behavior, it is very difficult to connect the finding that homosexuality is widespread in nature to human behavior," adding, "Humans are complex and, unlike other animals, the result of a range of factors, so such comparisons and inferences are open to debate."
There were also comments that the analysis targets were limited. Isabelle Winder, a professor at Bangor University, said in a commentary in the same journal that day, "For the first time, the study demonstrates the complex evolutionary process of humanlike behaviors," but pointed out that "the results are limited to one sex and to certain primate datasets."
For example, Japanese macaques (Macaca fuscata), which are in the same genus as rhesus macaques, have females who are more active in homosexuality than males. Females choose female partners even when a male is nearby. The same is true of bonobos (Pan paniscus), primates closely related to chimpanzees. Japanese macaque and bonobo females are known to relieve tension and social conflict through homosexuality.
References
Nature Ecology & Evolution(2026), DOI: https://doi.org/10.1038/s41559-025-02945-8
Nature Ecology & Evolution(2026), DOI: https://doi.org/10.1038/s41559-025-02940-z
Nature Ecology & Evolution(2023), DOI: https://doi.org/10.1038/s41559-023-02111-y
Archives of Sexual Behavior(2024), DOI: https://doi.org/10.1007/s10508-023-02781-6
Archives of Sexual Behavior(2002), DOI: https://doi.org/10.1023/A:1014079117864