By Clare Watson
Sexual behavior between members of the same sex might have evolved multiple times in mammals, according to a new study, adding to numerous examples found across the tree of life.
More than 1,500 species have been known to engage in same-sex sexual behaviors, including bats, beetles, sea stars, snakes, penguins, cows, fish, and worms.
Among mammals, primates are particularly notable, with sexual activity within sexes observed in at least 51 species, from lemurs to apes to, of course, humans.
Once viewed as peculiar outliers, mounting data shows that same-sex behaviors that include courting, mounting, cooing, or copulating are widespread in animals, both male and female, wild or captive.
It’s this data, specifically what’s been published on mammals, that University of Granada ecologist José Gómez and colleagues compiled to test several theories scientists have recently proposed to explain how same-sex behaviors evolved.
“Since it does not contribute directly to reproduction, same-sex sexual behavior is considered an evolutionary conundrum,” Gómez and colleagues write in their published paper. If it doesn’t result in any offspring, why else might it be advantageous?
Most studies have only looked at individual species, though. So Gómez and colleagues used a phylogenetic approach to compare the emergence and prevalence of same-sex sexual behavior among mammals.
If same-sex behaviors evolved to help maintain social relationships, facilitating reconciliation after conflict like what has been observed in female bonobos, or strengthening alliances as seen in male bottlenose dolphins, then those behaviors should be more frequent in social mammal species, Gómez and colleagues reasoned.
Indeed, their analysis (which adjusted for how often a particular species had been the focus of research) found that same-sex behaviors were more prevalent in highly social mammals.
The researchers also found same-sex behaviors to be more common in species that exhibit aggressive and sometimes lethal behaviors. This supports the idea that same-sex interactions may communicate or reinforce social hierarchies, helping to mitigate the risks of violent conflict.
Tracing same-sex behaviors along ancestral lines, Gómez and colleagues’ analysis suggested that same-sex behavior has been “gained and lost multiple times during mammalian evolution”, though it appears to be a recent phenomenon in most mammalian lineages.
Same-sex behaviors aren’t randomly scattered across mammals either; they are more common in some clades and rare in others.
“We fully recognize that these results may change in the future if same-sex sexual behavior is studied more intensively and comes to be detected in many more species,” Gómez and colleagues write.
Before this latest study, researchers had taken issue with similar efforts to explain how same-sex behavior evolved. By presenting same-sex sexual behavior as an ‘evolutionary conundrum’, they say it implies that different-sex sexual behavior is the baseline condition from which same-sex behavior arose.
Rather, in 2019, Ambika Kamath and colleagues suggested a different starting point, one of indiscriminate sexual behavior where ancestral animals mated with individuals of all sexes, perhaps before they evolved recognizable sex-specific traits now used to attract mates.
While Gómez and colleagues’ analysis counters that view for mammals, in that same-sex behaviors don’t appear to be a shared ancestral trait in this group, both groups of researchers caution against transposing theories of animal sexual behavior onto humans, and vice versa.
Same-sex behavior here includes even brief interactions observed between animals, which says nothing of human preferences.
And though we may be related to other mammals, viewing animal behavior through the lens of our own societal norms has long precluded scientists from appreciating the diversity of animal sex.
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