Some mammalian same-sex sexuality linked to social adaptation, researchers say

(CN) — Same-sex sexual behavior occurs in over 1,500 animal species, including insects, fish, reptiles, birds and mammals. In the latter's case, a new study suggests the behavior may have evolved multiple times and may play an adaptive role in maintaining social relationships and mitigating conflict.

The study published Tuesday in Nature Communications comes from Spanish researchers José Gómez, Adel Gónzalez-Megías and Miguel Verdú, who investigated the evolution of same-sex sexual behavior in mammals by compiling existing data on 5,747 extant and recently extinct species and testing for evolutionary relationships with other behaviors.

What Gómez and his colleagues found was that same-sex sexual behavior is not just common across animal species and genders, it also appears to have multiple origins, while occurring more frequently in social species.

The findings led them to suspect that the behavior helps establish and maintain positive social relationships, particularly among bonobos, chimpanzees, bighorn sheep, lions, wolves and many goats. However, they also found that same-sex sexuality in males is more likely to evolve in species displaying adulticide — a phenomenon where adults of a species kill other adults — suggesting the behavior may be an adaptation to quell violent male interactions.

It's notable that, until now, research has considered the prevalence of same-sex sexual behavior as a “Darwinian paradox,” the authors write. This is because the behavior has similar mating costs as opposite-sex sexuality, requiring energy, time, injuries and disease, but without a direct contribution to reproduction.

To understand the behavior’s origins, previous studies have proposed non-adaptive hypotheses ranging from limited mating opportunities and sexual frustration to mistaken identity — all of which occur in certain animal species. There’s also a hypothesis involving indiscriminate sexuality, meaning an animal may engage in sexual behaviors with other same-sex or opposite-sex individuals. However, this hypothesis suggests the behavior is the ancestral condition for sexually reproducing animals, thus explaining its widespread occurrence in animals.

In contrast, there are adaptive explanations — all central to the findings of the new study — which suggest that same-sex sexual behavior can be favored by natural selection. One hypothesis indicates that same-sex sexual behavior contributes to establishing and maintaining positive social relationships, while the other suggests it diminishes intrasexual aggression and conflict.

Both occurrences have been described in some, though not all, prior research, where same-sex sexuality appears to facilitate reconciliation between female bonobos and Japanese macaques. Likewise, the behavior seems to reinforce alliances between small groups of male bottlenose dolphins and strengthen dominance hierarchies in herds of American bison.

But a deeper understanding of how the behavior evolved requires testing in a “broader phylogenetic context,” the researchers write. By compiling all existing information on mammalian same-sex sexuality, the researchers did just that, producing a litany of observations on the topic.

For starters, the researchers report how same-sex sexual behavior has been documented in 261 mammalian species (4% of species) from 62 families (50% of families) and 12 orders (63% of orders), where such behavior included mounting and genital contact (87%), courtship (27%) and pair bonding (24%).

“Same-sex sexual behavior is particularly prevalent in nonhuman primates, where it has been observed in at least 51 species from lemurs to apes,” the researchers write, adding that the behavior occurs for both females and males whether they’re in captivity or the wild.

While some species with same-sex sexuality indicate that the behavior is incidental or occurring under specific situations, about 40% of species displayed moderate or frequent same-sex sexual behaviors during mating season, as well.

The researchers also found that same-sex sexual behavior is not randomly distributed across mammalian phylogeny but, instead, “frequent in some clades and rare in others.” For instance, while same-sex sexuality is common for even-toed ungulates, carnivores, kangaroos, wallabies, rodents and primates, the data indicated that closely related species do not necessarily share the sexual behavior.

Through 1,000 iterations of stochastic character mappings, the researchers also estimated that same-sex sexual behavior had been gained and lost multiple times during mammalian evolution.

The researchers say that they found sociality is correlated with male and female same-sex sexuality and that the behavior, overall, was significantly more common in social species than non-social ones.

The occurrence of adulticide is significantly correlated with the behavior of males, the researchers say, where male same-sex sexual behavior is more prevalent in species that are adulticidal regardless of their social status. The same could not be said of female same-sex sexual behavior, as “the frequency of species displaying female same-sex sexual behavior was similar between female adulticidal and non-adulticidal species both in social and non-social species.”

The researchers write that same-sex sexual behavior seems to have been present in the ancestor Hominidae, suggesting that its evolutionary origin in humans can be traced to the ancestor we share with other apes. They note how evidence suggests Hominidae were a social species, as well, exhibiting mostly male adulticide.

However, since the study defines same-sex sexual behavior as any temporary sexual contact between members of the same sex, the researchers say it should be distinguished from homosexuality, the “more permanent same-sex preference” found in humans.

“For this reason, our findings cannot be used to infer the evolution of sexual orientation, identity and preference or the prevalence of homosexuality as categories of sexual beings,” the study states.

Nonetheless, the researchers say that their phylogenetic study may provide a potential evolutionary history of the occurrence of human same-sex sexual behavior. Their findings, they say, may change as more information surfaces, and that it does not preclude factors like genetics, mistaken identity or excitement — all of which resemble same-sex sexual behavior in some species.

“Briefly, our findings are consistent with the idea that, rather than a maladaptive or aberrant behavior, same-sex sexual behavior in mammals is a convergent adaption facilitating the maintenance of social relationships and the diminishing of intrasexual conflicts,” the study states.

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