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Few creatures can boast of devotions so deep as greylag geese. Most are monogamous; many spend their decade-long adult lives with the same goose, side-by-side in constant communication, taking another partner only if the first should die. It’s a remarkable degree of fidelity, and it includes relationships of a sort that some humans consider unnatural.

A greylag goose coupleBiodiversity Heritage Library via Flickr
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Quite a few greylags, you see, are gay. As many as 20 percent by some accounts. That number might be high: It includes those males who first take a male partner but later pair with a female, or whose first bond is with a female, but after she dies, takes up with a gander. That said, plenty more are exclusively homosexual from beginning to end.

Which raises the question: Why?

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That’s puzzled quite a few scientists—those who study greylag geese and also the hundreds of other animal species in which homosexuality is, confoundingly, found. After all, evolution is driven by reproduction. In animals, that requires—self-cloning reptiles not withstanding—the union of opposite sexes. Through a reproductive-success lens, homosexuality would appear counterproductive, if not downright aberrant. It’s certainly not aberrant, though, considering its ubiquity.

So to frame the question a bit more scientifically: Is homosexuality, in the words of Kurt Kotrschal, a behavioral biologist at the University of Vienna, “preserved because there was some stabilizing selection, or is it an unavoidable product of brain development?” Was homosexuality useful in evolution’s grand pageant—or just something that popped up and stuck around?

Researchers don’t have a simple answer. Not even Kotrschal, who has studied greylag geese for decades, working at a research station named for the late, great zoologist Konrad Lorenz, whose most famous studies involved the same bird.

Lorenz himself considered homosexuality useful. “We can be sure that every one of these instincts has a very special survival value,” he wrote in 1963, describing how pairs of partnered males frequently attained social superiority in goose colonies. Their superiority in turn attracted lone females with whom one gander might briefly copulate, before returning his attention to the true object of his affections, wrote Lorenz. By that light, homosexuality serves to promote reproduction. That is one possible explanation; there are plenty more.

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Other scientists have suggested that homosexual couples might perform some important social duty, such as helping to raise other couples’ goslings or guarding colonies from predators. That would help same-sex couples’ relatives rather than themselves, a well-known evolutionary strategy called kin selection, illustrated most dramatically by honeybee workers who forgo reproduction and sacrifice themselves for their hive’s greater good.

Kotrschal himself doesn’t think this likely—there isn’t much evidence for obvious, give-the-nephews-a-wing-up helpfulness in greylag geese, though it could manifest in other, subtler ways. Perhaps homosexuality is the inevitable byproduct of emotional systems that fuel mate pairing: You can’t have heterosexual love without some overflow.

As many as 30 percent of Canada geese, one of the most ubiquitous birds in North America, may in fact be so disposed—something we’d probably notice more if the sexes didn’t look so very much alike.

Or, as homosexuality seems to occur more frequently in species where parenting duties are concentrated in one sex, maybe homosexuality arises when one sex has more free time. A harmless indulgence, then, which might also explain why homosexuality in greylag geese seems to correlate with sex ratios. If there’s lots more males than females, well, some of those males will turn to each other for companionship—and for a species in which social status is paramount, with unpaired individuals bullied and relegated to low-grade foraging grounds, being in a same-sex couple beats being alone. Homosexuality would thus be a byproduct of sociality and competition. It could also be influenced, noted Kotrschal, by what different sexes have evolved to consider most important in mates. For some, the quality of a relationship might trump the niggling particulars of sex.

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Or all of the above, or some. These speculations are not exclusive of one another. Their applicability also varies: Homosexuality exists in so many far-flung corners of the animal kingdom that it likely didn’t originate in a single common ancestor, but evolved again and again. “I don’t think there’s one overarching or unifying explanation,” says Paul Vasey, director of the Laboratory of Comparative Sexuality at Canada’s University of Lethbridge. “Different explanations are required due to the unique evolutionary histories of each species.”

Homosexuality’s dynamics indeed differ. Sometimes, as with greylag geese, homosexuality is only found in one sex; in other species, including Canada geese, both males and females form same-sex unions. (As many as 30 percent of Canada geese, one of the most ubiquitous birds in North America, may in fact be so disposed—something we’d probably notice more if the sexes didn’t look so much alike that only people who make a habit of studying them can tell the difference.)

Claudia Wascher, a zoologist at Anglia Ruskin University, adds another nuance: If homosexuality is often adaptive, as she thinks, it’s also not going to be a straightforward trait inherited by some fixed percentage of a population, with frequencies changing in the simple manner of color patterns or height. Rather, the potential for becoming homosexual will vary from individual to individual, like curiosity or boldness or any other personality trait, and be shaped by the complex interaction of biology with social and environmental circumstance.

The sexual proclivities of a species, then, are constantly under evolutionary pressure. It might even be inappropriate to discuss homosexuality in species-wide generalities. It’s likely evolved on different trajectories in different communities, and that evolution continues now, in countless different ways.

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As an example of seemingly hard-wired behavioral traits changing rapidly, Wascher points to carrion crows, who breed in fiercely territory-defending pairs throughout Europe. But in northern Spain, where there’s lots of food yet few nesting sites, they instead live in large, cooperatively breeding groups. Take eggs from Switzerland to Spain, or vice versa, and the hatchlings adopt the habits of crows in their new ecology. Different dynamics, different tendencies.

“It could be very similar with homosexuality,” Wascher says. One could even imagine that certain environments cultivate homosexuality more or less than others. Which could make for some intriguing research: How do various urban and rural conditions, for example, influence goose sexuality?

Of course, at the individual level, those dynamics are academic. Biologists can talk of kin selection and evolutionary strategies, but the birds themselves aren’t likely aware of those dynamics any more than we are in choosing our partners.

What matters for each goose isn’t evolution but attraction—all the emotions and experiences that feed a special bond are themselves rooted in biological systems shared by heterosexual and homosexual individuals alike. “Love,” says Wascher, “is love.” 

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Brandon Keim is a freelance journalist who writes about science, technology, and nature. His work has appeared in Wired, Aeon, Scientific American Mind, and other publications.

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