A pair of male macaques in Cayo Santiago, Puerto Rico
Chloe Coxshall
Homosexual behavior is commonly observed among male macaque monkeys in their natural habitat, and it is believed to have a genetic basis that can be passed down from father to son. Vincent Savolainen, a researcher at Imperial College London, suggests that such behavior may provide evolutionary advantages through the formation of strong male alliances.
These alliances help the male macaques in various ways, such as providing assistance in fights. The idea is that by forming these alliances, the monkeys have a better chance of accessing more females and, consequently, have more offspring.
Although occasional same-sex behavior has been documented in various animal species, it is generally considered to be rare. This leads to a popular belief known as the “Darwinian paradox of homosexuality,” which suggests that since homosexual behavior does not directly lead to reproduction, it should not have any evolutionary benefit and any genes promoting it should be eliminated by natural selection.
To investigate same-sex behavior in primates, Savolainen and his colleagues conducted a study on a colony of 1,700 wild rhesus macaques (Macaca mulatta) in Cayo Santiago, Puerto Rico. This colony has been monitored by scientists for the past 67 years, and DNA testing for paternity has been conducted since 1992. The research team observed 236 male macaques from two social groups for 7 hours a day over a period of 72 days in 2017, 2019, and 2020.
The findings of the study revealed that 72% of the male macaques engaged in same-sex mounting, while only 46% were observed mounting females. The researchers refrained from interfering with the animals and only visually observed them from a distance, so the actual penetration was not always visible. However, they did observe sperm plugs, which form after ejaculation, in the anuses of some males.
In 16.5% of the instances of same-sex mounting, the males fought together against other monkeys before engaging in sexual activity.
The study also found that same-sex mounting became less common with age, suggesting that it might serve as “practice” for future reproductive activity.
According to Savolainen, the family history of the monkeys indicated that same-sex mounting had a heritability of 6.4%, suggesting that genetics may play a role along with other factors. The behavior did not seem to have any negative impact on reproductive success, and in fact, males engaging in same-sex mounting tended to have slightly more offspring.
Savolainen argues that the mounting activity of the males is not simply a display of dominance, as they often mounted higher-ranking males. He emphasizes that the monkeys exhibited erections, penetration, and even ejaculation, which he believes qualifies as sexual behavior rather than mere dominance.
While it is impossible to determine the thoughts and intentions of animals as they choose their sexual partners, this study challenges the notion that same-sex behavior is unnatural. Jon Richardson, a researcher at the University of Minnesota who was not involved in the study, believes that we should move away from the outdated idea that homosexuality in animals is an evolutionary paradox or aberration.
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