The study shows that same-sex sexual behavior is widespread and heritable in macaque monkeys

Observations of a feral macaque colony over a three-year period show that same-sex sexual behavior among males is widespread and may be beneficial.

The results, published today in Nature Ecology and Evolutionsuggest that same-sex sexual behavior (SSB) has evolved and may be a common feature of primate reproduction.

Carried out by researchers at Imperial College London, the observations and genetic data constitute the first long-term study of SSB in males within a species. Their study challenges the beliefs of some that SSB is a rare behavior in non-human animals or just the product of unusual environmental conditions.

First author Jackson Clive, of the Georgina Mace Center for the Living Planet at Imperial, said: ‘We found that the majority of males were behaviorally bisexual and that variation in same-sex activity was heritable. This means that behavior may have an evolutionary basis; for example, we also found that males who mounted each other were also more likely to support each other in conflict – perhaps this could be one of the many social benefits of same-sex sexual activity.”

“Our research therefore shows that same-sex sexual behaviors may be common among animals and may evolve. I hope our findings encourage further discoveries in this area.”

Origins of homosexual behavior

The team studied 236 males in a colony of 1,700 free-living rhesus macaques on the tropical island of Cayo Santiago, Puerto Rico. In addition to observing their behavior and conducting genetic analyses, the team had access to pedigree records, which detail each individual’s parentage back to 1956.

The researchers recorded all social montages for the 236 males, both males on males (homosexual behavior, SSB) and males on females (different-sex behavior, DSB). They found that same-sex male editing was widespread: 72 percent of males in the sample engaged in same-sex editing, compared with 46 percent for different-sex editing.

SSB has been observed in thousands of different animals, from insects to penguins, leading many to theorize why it happens. These theories include ideas about establishing dominance in groups, the shortage of partners of different sexes, and the reduction of tension following the assault, but little data is available to support any theory.

The team investigated several of these theories with their data, finding that, for this colony of macaques, SSB in males was strongly correlated with ‘coalition bonding’. This means that male couples who regularly engage in SHW were more likely to support each other in conflicts, providing them with an edge in the group.

heritable behaviors

The researchers also investigated whether SSB led to any fitness costs — a reduction in the amount of offspring they have. In fact, they found the opposite: Males who practiced SHW may be more successful at reproducing, potentially due to the benefits provided by more coalition ties.

In line with this, the team also investigated whether SSB was heritable. Using pedigree data, they found that SSB in males was 6.4% heritable, which provides the first evidence of a genetic link to primate SSB outside of humans. This figure is similar to other heritable behaviors in primates, such as grooming and sociability.

These results support arguments against the idea that SBB “challenges nature and evolution” (the so-called “Darwinian paradox”).

The team also found some genetic correlations between males who were more often ‘mounters’ or ‘mountees’ when practicing SHW, suggesting that these sub-behaviours may have a common basis. Furthermore, whether individuals were more likely to be mounters or mountees was unrelated to their social standing, suggesting that asserting their place in the hierarchy is not a major factor for Sai Baba in this species.

Macaques and humans

Although the researchers caution against direct comparisons with humans, they say their study challenges the beliefs of some that SSB is a rare behavior in non-human animals or just the product of unusual environmental conditions.

Instead, the results suggest that some degree of SSB can evolve adaptively, depending on context, and thus could be a common feature of primate reproductive ecology. There are many examples of other primate groups engaging in different forms of SSB, so further extensive genetic studies could strengthen this conclusion.

Lead researcher Professor Vincent Savolainen, director of the Georgina Mace Center for the Living Planet at Imperial, said: ‘Unfortunately there is still a belief among some people that same-sex behavior is ‘unnatural’ and some Unfortunately, countries still apply the death penalty for homosexuality. Our research shows that homosexual behavior is indeed widespread among non-human animals.

“Our mission is to advance the scientific understanding of homosexual behavior, including the exploration of the benefits it brings to nature and within animal societies. Among the macaques we examined in this study, more than two-thirds exhibited homosexual behavior, and this behavior strengthened bonds within the community.”

The research was funded by the Natural Environment Research Council (NERC), the American Institute of Bisexuality and the Evolution Education Trust.