Homosexuality and same-sex sex have so far been considered rare exceptions in the animal kingdom – partly also as a reaction to being kept in captivity. But now British researchers have discovered that wild rhesus monkeys in Puerto Rico often have sex with members of the same sex. Of the 236 male monkeys observed over a three-year period, 72 percent repeatedly mate with other males—only a small proportion of males engaged in exclusively heterosexual mating. Same-sex behavior was not at the expense of reproductive success, but strengthened coalitions between males. According to the biologists, these observations refute the assumption that same-sex sex is contrary to nature and evolution.
Why is there homosexuality? And how “natural” is it? This question has been discussed for centuries. Because no examples of same-sex behavior from the animal kingdom were known for a long time, this was long considered “unnatural”. One of the arguments for this: Because such matings do not produce children, this contradicts the principles of evolution and “survival often the fittest”. Now, however, observations of more than a thousand animal species have shown that homosexual behavior is not a cultural trait unique to humans. Various varieties of same-sex sexuality also occur in the animal kingdom – ranging from insects to birds to mammals. However, reports of same-sex behavior are often random and anecdotal, and often involve animals living in captivity or extreme environments. It is therefore debatable to what extent such behavior is also typical and “natural” for wild animals.
Same-sex sex in 72 percent of male monkeys
To answer this question, Jackson Clive and his colleagues at Imperial College London have now conducted a comprehensive study of wild rhesus monkeys in Puerto Rico. Around 1,700 of these monkeys live freely on the 15-hectare island of Cayo Santiago, and their behavior and genealogy have been intensively researched since 1956. For their study, Clive and his team observed the behavior of males in two social groups of these rhesus monkeys over a period of three years – 2017, 2019 and 2020. They documented for each male whether and how often it copulated with conspecifics of the same or opposite sex. In rhesus monkeys, mating typically occurs from behind. The researchers determined whether there really was a sexual act based on the erection and the thrusting movements of the mounting male. In addition, they recorded with which partners the mating took place, in which situations and what social rank the participants held.
The evaluations revealed something surprising: “Overall, we found that same-sex matings were even more common than matings between males and females,” report Clive and his colleagues. 72 percent of male rhesus monkeys practiced same-sex copulation more often. “Most of these males were behaviorally bisexual,” says Clive. Closer analysis of the circumstances of these matings revealed that the social rank of the male monkeys played only a minor role. Contrary to the hypothesis that such same-sex copulations represent a mere demonstration of power by higher-ranking males, in the observed rhesus monkeys the mounted males often ranked below their male copulation partners. At the same time, however, this behavior also existed in older, high-ranking males, albeit less frequently. According to the team, social rank and dominance behavior can at most explain a small part of this same-sex behavior
No reproductive penalties, but strengthened coalitions
Next, the biologists investigated whether same-sex mating causes reproductive disadvantages for the male rhesus monkeys involved. “It is commonly believed that this behavior has a fitness cost because those who engage in same-sex mating lose reproductive opportunities,” the researchers explain. Thanks to the gene samples collected since 1956 from all animals living and newly born in these rhesus monkey groups, they were able to understand how many offspring the males with and without same-sex sex had fathered. The result: “We found a positive correlation between same-sex copulations and reproductive success,” report Clive and his colleagues. Their sex with each other did not prevent the male monkeys from mating with females and successfully fathering offspring. Instead, these males were even more successful at it.
The biologists found a possible explanation for this when they took a closer look at the circumstances of the same-sex pairings and the partners involved. It was found that 16.5 percent of these copulations occurred in situations in which the two partners involved were in conflict with other conspecifics. This is consistent with the fact that males who have had sex with each other tend to form alliances and support each other in the event of a conflict. “This could perhaps be one of the benefits of this same-sex activity,” explains Clive. Copulation among males could therefore serve to strengthen their relationship and “coalition” – and also increase their chances of reproduction. Genetic analyzes also revealed that the propensity for same-sex sex in rhesus monkeys is at least partly genetic. The team estimates the hereditary proportion in male rhesus monkeys to be 6.4 percent. “To our knowledge, this is the first evidence for a genetic basis for same-sex sexual behavior in a primate other than humans,” the researchers state.
Neither unnatural nor a rare exception
According to Clive and his team, their results disprove the notion that same-sex sex in animals is extremely rare or occurs only under unusual environmental conditions. “Unfortunately, there is still a perception among some people that same-sex sex is ‘unnatural’ and in some countries homosexuality is still punishable by death,” says Clive’s colleague Vincent Savolainen. “Our study, on the other hand, shows that same-sex behavior can also be common in non-human primates.” The biologists even suspect that this type of same-sex copulation could have developed as an evolutionary strategy, especially in primates. “In the rhesus monkeys, this behavior strengthened relationships within the community,” says Clive. The team made similar observations on mating among males in two other rhesus monkey populations in northern Thailand.
Source: Jackson Clive (Imperial College London) et al., Nature Ecology and Evolution, doi: 10.1038/s41559-023-02111-y