Researchers: Same‑Sex Behavior in Animals Was Evolution’s Starting Point, Not an Anomaly


Nov 28, 2019


Same-sex behavior has been observed in more than 1,500 species, from bottlenose dolphins to Japanese macaques, to red flour beetles, to Laysan albatrosses — one-third of whose couples are female pairs mated for life. Yet, to date, science has viewed this as an aberration from the normal progress of evolution.

Far from being an anomaly, argue a group of researchers in a paper for the journal Nature Ecology & Evolution, same-sex behavior is a baseline from which animal species began their evolution.

In other words, instead of asking “Why do animals engage in same-sex behavior,” the right question is, “Why not?”

“The expectation has been that same-sex sexual behavior evolved in different species independently, against this default background of heterosexual sex,” Ambika Kamath, a researcher at the University of California, Berkeley, and a co-author on the paper, told the New York Times. “And what we’re saying is that baseline isn’t necessarily the right baseline.”

Most animal research has been rooted in the theory that sexual behavior is aimed at maximizing the continuation of the species. This theory has long jarred with the widespread presence of same-sex behavior (SSB). Same-sex behavior doesn’t contribute to the continuation of a species; in fact, the traditional scientific view goes, it takes time, energy and resources away from species’ survival. Over the years, scientists have offered a variety of explanations that sought to wedge in same-sex behavior into this framework, in essence justifying what didn’t appear justifiable.

To Julia Monk, a Ph.D. candidate at Yale University and the paper’s lead author, this suggested the problem lay in the underlying theory.

“The idea that same-sex sexual behavior had to be justified at all seemed like a perspective of dominant cultural norms rather than a more holistic view of the actual biology,” she told the New York Times. “I really disagreed with some of the ways I saw that discussion framed.”

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Monk and her fellow authors suggest same-sex behavior — along with what they call different-sex behavior — was present from the moment the first primordial animal sent out the first come-hither glance. This non-discrimination in sexual practice is found today in hermaphroditic species, like snails, and sexless species, like sea urchins, reports the New York Times. Such simultaneous practice of same-sex and different-sex behaviors from the very beginning makes more sense, as an evolutionary explanation — it would have, and continues to, maximize the chances of reproduction and species’ survival, given that for many species, obvious sex-specific traits were not inherent from the start. Sex differences in size, coloring, anatomy, and behavior evolved over a long period of time, Monk told the New York Times. Even now, for some species, the differences between the sexes are so minute it can be difficult for an individual animal to tell one from the other. Which means the more they mate — with whatever sex — the higher the chances of reproduction.

“So, if you’re too picky in targeting what you think is the opposite sex, you just mate with fewer individuals. On the other hand, if you’re less picky and engage in both SSB and DSB, you can mate with more individuals in general, including individuals of a different sex,” co-author Max Lambert PhD, a postdoctoral fellow at the University of California-Berkeley’s departmental of environmental science, policy, and management, said in a statement.

Research supports this new view: Same-sex behavior increases among male burying beetles when the cost of missing mating opportunities with females is high, suggesting that limiting oneself to different-sex behaviors only reduces opportunities to showcase mating prowess in a mating-rare environment. But this is a rare example of research that isn’t underpinned by earlier, Darwinian assumptions about the centrality of different-sex behavior. It speaks to how cultural biases creep into scientific research, twisting theories to fit the preferred narrative of the scientist behind the work. Evolutionary science, long dominated by white, heterosexual men, has evolved along a certain pattern of blind spots and prejudices.

“So far, most biologists have considered SSB as extremely costly [in evolutionary terms] and, consequently, something that is aberrant,” Lambert said in the statement. “This strong assumption has stopped us as a community from actively studying how often and under what conditions SSB is happening.”

“We’re missing so many observations of sexual behaviors because the people looking at them thought that it must have been an abnormality, based on a preconceived notion of how the world should work,” he told the New York Times.

The authors behind the paper are trying to be careful not to make the same mistake; for this reason, they characterize animal sexual practices in clinical terms as same- or different-sex behavior, as opposed to homosexuality and heterosexuality, which they say are human sexual-cultural identities.

That said, research involving human subjects could benefit from a similar flip in assumptions. Asking “Why not?” instead of “Why?” isn’t just more inclusive — it could open up a host of new findings and understandings about our world.


Written By Liesl Goecker

Liesl Goecker is The Swaddle’s managing editor.


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