Same-sex sexual behavior is very prevalent and linked to evolutionary advantages in monkeys

A new study from Imperial College London has revealed that same-sex sexual behavior (SSB) in macaque monkeys is not just prevalent, but may actually lead to evolutionary advantages. 

Over three years, the researchers observed a wild colony of 1,700 rhesus macaques on the tropical island of Cayo Santiago, Puerto Rico. The experts gained fascinating insights that challenge some long-standing beliefs about non-human animal behavior.

“We found most males were behaviorally bisexual, and that variation in same-sex activity was heritable,” said study first author Jackson Clive from the Georgina Mace Centre for the Living Planet at Imperial. He noted that such behavior has potential evolutionary underpinnings, suggesting that it is not solely the product of unusual environmental conditions or a rarity in the animal kingdom.

Social benefits 

Clive pointed to the possibility of social benefits linked to same-sex activity. “We also found that males that mounted each other were also more likely to back each other up in conflicts,” he said. “Our research therefore shows that same-sex sexual behaviors can be common amongst animals and can evolve. I hope our results encourage further discoveries in this area.”

The researchers meticulously recorded all social “mountings” among 236 males, both same-sex and different-sex interactions. The results revealed that same-sex mountings were more common than different-sex mountings, with 72% of males engaging in the former and only 46% in the latter.

Ongoing debate

Despite the frequency of same-sex sexual behavior being noted in various animal species, ranging from insects to penguins, there is still debate and speculation over its purpose. 

Theories include its role in establishing dominance, the effect of a shortage of different-sex partners, or its use as a method to defuse tension post-aggression. However, empirical data has often been insufficient to lend credence to these hypotheses.

In this regard, the macaque study made significant strides. The researchers discovered a strong correlation between same-sex sexual behavior and “coalitionary bonds” within the macaque colony, suggesting that male pairs often engaging in SSB were more likely to support each other during conflicts. This, in turn, implies that SSB can enhance an individual’s social standing and survival prospects within the group.

Reproductive success 

The study, published in the journal Nature Ecology and Evolution, unearthed new evidence regarding the heritability of same-sex sexual behavior. 

The team found no correlation between SSB and a decrease in offspring. Rather, it appeared that males engaging in same-sex sexual behavior may have higher reproductive success, possibly due to the advantages of stronger coalitionary bonds. 

Genetic data from the study showed SSB to be 6.4% heritable, a rate comparable to other heritable behaviors like grooming and sociality.

The results challenge the idea that same-sex sexual behavior is counter to nature and evolution, also known as the “Darwinian Paradox.” 

Genetic basis 

Furthermore, the researchers discovered a genetic correlation between the tendency of certain males to be “mounters” or “mountees” during same-sex sexual behavior. This suggested that these behaviors might have a common genetic basis and were not related to an individual’s social position or a method of asserting dominance within the group.

While the researchers caution against drawing direct comparisons between macaque behavior and human sexual orientation, they believe their study provides substantial evidence to challenge the belief that SSB is unusual or rare in non-human animals.

Natural behavior 

Professor Vincent Savolainen, the lead researcher and Director of the Georgina Mace Centre for the Living Planet at Imperial, pointed to the implications of these findings. “Unfortunately there is still a belief amongst some people that same-sex behavior is ‘unnatural,’ and some countries sadly still enforce the death penalty for homosexuality. Our research shows that same-sex behavior is in fact widespread amongst non-human animals.”

Savolainen emphasized the team’s mission to deepen our understanding of same-sex sexual behavior, including its potential benefits in the animal kingdom. “Amongst the macaques we looked at in this study, more than two-thirds displayed same-sex behaviour and this behavior strengthened the bonds within the community.”

While more research is needed, this study highlights a poorly understood aspect of animal behavior. By highlighting the prevalence and potential benefits of same-sex sexual behavior in non-human primates, it challenges widely held beliefs and opens the door for further investigation.

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