“I don’t answer that question,” she told me.

A FEMALE LAYSAN albatross is physically capable of laying only one egg per year — that’s just how it’s built. Nevertheless, since as early as 1919, biologists have periodically found nests of albatrosses (and similar species of birds) with two eggs inside them, or with a second egg just outside, as if it had rolled out. (This will inevitably happen; there’s simply not enough room in the nest for two eggs and one Laysan albatross.) Scientists have a term for the phenomenon of extra eggs in a nest: a “supernormal clutch.” But in the case of the albatross, they never had a watertight explanation.

In the early 1960s, one ornithologist tried to put the whole cumbersome mystery to rest by asserting that some of those female birds must simply be able to lay multiple eggs. The claim was apparently based on sketchy data, but supernormal clutches were so rare that it was hard to rack up enough observations to disprove the hypothesis. Real progress was finally made in 1968, when Harvey Fisher, a dean of midcentury albatross science, reported on seven years of daily observations made at 3,440 different nests on the Midway atoll in the middle of the Pacific. Fisher concluded that “two eggs in a nest are an indication that two females used the nest, although at different times.” He was describing “egg dumping,” whereby, for example, an inexperienced female accidentally lays her egg in the wrong nest. From then on, egg dumping was a default explanation for supernormal clutches in albatrosses. After all, Fisher had also declared that “promiscuity, polygamy and polyandry are unknown in this species.” Lesbianism apparently never occurred to anyone — even enough to be cursorily dismissed. As Brenda Zaun recently told me, “It never dawned on anyone to sex the birds.”

Zaun, a biologist with the U.S. Fish and Wildlife Service, was studying a Laysan colony on Kauai 40 years after Fisher’s publication. She realized that certain nests there seemed to wind up with two eggs in them year after year; the distribution of the supernormal clutches wasn’t random, as it would presumably be if it were caused exclusively by egg dumping. On a hunch, Zaun pulled feathers from a sample of the breeding pairs associated with two-egg nests and sent them to Lindsay Young, asking her to draw DNA from the feathers and genetically determine the sexes of those birds in her lab. When the results showed that every bird was female, Young figured she’d messed up. So she did it again — and got the same result. Then she genetically sexed every bird at Kaena Point. “Where it wasn’t totally clear, or I worried that maybe I mixed up the sample, I actually went back into the field and took new blood samples to do it again,” Young told me. In the end, she genetically sexed the birds in her lab four times, just to be sure. She found that 39 of the 125 nests at Kaena Point since 2004 belonged to female-female pairs, including more than 20 nests in which she’d never noticed a supernormal clutch. It seemed that certain females were somehow finding opportunities to quickly copulate with males but incubating their eggs — and doing everything else an albatross does while at the colony — with other females.

Young gave a talk about these findings at an international meeting of Pacific-seabird researchers. “There was a lot of murmuring in the room,” she remembers. “Then, afterward, people were coming up to me and saying: ‘We see supernormal clutches all the time. We assumed it was a male and a female.’ And I’d say: ‘Yeah? Well, you might want to look into that.’ ” Recently, journals have asked her to confidentially peer-review new papers about other species, describing similar discoveries. “I can’t say which species,” she explains, “but my guess is, in the next year, we’re going to see a lot more examples of this.”

It may seem surprising that scientists sometimes don’t know the true sexes of the animals they spend their careers studying — that they can be tripped up in some “Tootsie” -like farce for so long. But it’s easy to underestimate the pandemonium that they’re struggling to interpret in the wild. Often, biologists are forced to assign sexes to animals by watching what they do when they mate. When one albatross or boar or cricket rears up and mounts a second, it would seem to be advertising the genders of both. Unless, of course, that’s not the situation at all.

“There is still an overall presumption of heterosexuality,” the biologist Bruce Bagemihl told me. “Individuals, populations or species are considered to be entirely heterosexual until proven otherwise.” While this may sound like a reasonable starting point, Bagemihl calls it a “heterosexist bias” and has shown it to be a significant roadblock to understanding the diversity of what animals actually do. In 1999, Bagemihl published “Biological Exuberance,” a book that pulled together a colossal amount of previous piecemeal research and showed how biologists’ biases had marginalized animal homosexuality for the last 150 years — sometimes innocently enough, sometimes in an eruption of anthropomorphic disgust. Courtship behaviors between two animals of the same sex were persistently described in the literature as “mock” or “pseudo” courtship — or just “practice.” Homosexual sex between ostriches was interpreted by one scientist as “a nuisance” that “goes on and on.” One man, studying Mazarine Blue butterflies in Morocco in 1987, regretted having to report “the lurid details of declining moral standards and of horrific sexual offenses” which are “all too often packed” into national newspapers. And a bighorn-sheep biologist confessed in his memoir, “I still cringe at the memory of seeing old D-ram mount S-ram repeatedly.” To think, he wrote, “of those magnificent beasts as ‘queers’ — Oh, God!”

“What Bagemihl’s book really did,” the Canadian primatologist and evolutionary psychologist Paul Vasey says, “is raise people’s awareness around the fact that this occurs in quote-unquote nature — in animals. And that it can be studied in a serious, scholarly way.” But studying it seriously means resolving a conundrum. At the heart of evolutionary biology, since Darwin, has been the idea that any genetic traits and behaviors that outfit an animal with an advantage — that help the animal make lots of offspring — will remain in a species, while ones that don’t will vanish. In short, evolution gradually optimizes every animal toward a single goal: passing on its genes. The Yale ornithologist Richard Prum told me: “Our field is a lot like economics: we have a core of theory, like free-market theory, where we have the invisible hand of the market creating order — all commodities attain exactly the price they’re worth. Homosexuality is a tough case, because it appears to violate that central tenet, that all of sexual behavior is about reproduction. The question is, why would anyone invest in sexual behavior that isn’t reproductive?” –— much less a behavior that looks to be starkly counterproductive. Moreover, if animals carrying the genes associated with it are less likely to reproduce, how has that behavior managed to stick around?