Brandon Keim in Nautilus:
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.
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?
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?