There was a time when the Laysan albatross might seem a perfect icon for the virtues of marriage. When naturalists visited the bird’s nesting grounds in the Pacific, they’d find males and females bonded in pairs for life. Each breeding season the pairs of birds would nuzzle their heads together and perform other adorable courtship rituals. After they mated, the female would lay an egg. Both the male and female would take turns sitting on the nest to incubate it, taking three week shifts. After the chick hatched they’d rear it together until the end of the breeding season. The birds would then fly out to sea in different directions, but they’d return the following year and start up their partnerships all over again. The albatrosses would repeat this behavior for life–which, in their case, can last for many decades.

But then scientists realized they weren’t seeing the birds correctly. It turned out that sometimes a pair of albatrosses were both females. On the Hawaiian island of Oahu, for example, 31 percent of the pairs are same sex couples.

View Images Two female Laysan albatrosses. Photo by Eric VanderWerf

Lindsay Young, a biologist who has been conducting a study on Laysan albatrosses for a decade, bristles when people try to use her research as a weapon in cultural debates. “‘Lesbian’ is a human term,'” she told the writer Jon Mooalem, who wrote a wonderful feature about same-sex animal couples for the New York Times Magazine in 2010. “The study is about albatross. The study is not about humans.”

I was reminded of Young’s words when I read her latest report on the birds, which appears today in the Proceedings of the Royal Society. Young and her colleague Eric VanderWerf analyzed ten years of field notes for every female Laysan albatross on Oahu–all 145 of them. In that data, they searched for an evolutionary explanation for why so many of the birds are in same-sex pairings. The answer they’ve reached is an intricate one–and it hinges on what it means to be an albatross, not a human.

Over the course of the study–from 2003 to 2012–Young and VanderWerf observed a number of same-sex pairs sticking together for years. Each breeding season, the females would find male albatrosses to mate with. Then they’d return to their own nest to lay their eggs. Like male-female pairs of albatrosses, they would take three-week shifts. But a pair of albatrosses can only incubate a single egg, and so when both females laid one, one of their eggs died. From year to year, it appears, the females alternate between which bird gets to lay an egg.

From an evolutionary point of view, Yong and VanderWerf found, the numbers are bleak for a female albatross with a female partner. At best, she’ll get to lay one egg every two years. That’s half the maximum rate that a female with a male partner can hope for. In the real world, however, all the birds fall far short of their upper limit. For one thing, albatrosses sometimes skip a breeding season. For another, many albatross chicks die in their first year.

But same-sex pairs have worse luck than male-female pairs, Young and VanderWerf’s research shows. An average female with a same-sex partner produces 80 percent fewer chicks a given year than one with a male partner. Over the ten years of the study, a female in a same-sex pair ended up with just one offspring on average. Her counterpart in a male-female pair ended up with 2.14 of them.

View Images Two female albatrosses. Photo by Lindsay Young

There may be many reasons that same-sex albatrosses have fewer offspring. One may be that the females themselves are at a greater risk of dying. Young and VanderWerf point out that males take over the first three weeks of sitting on the nest after a female lays an egg. In same-sex pairs, the female that lays the egg goes straight into an incubation shift. Unable to return to sea to find food, she starves for three weeks and puts her health at risk.

Young and VanderWerf’s study is impressive for having stretched across a decade, but that’s just a small fraction of the fifty years during which a Laysan albatross can lay eggs. The differences that Young and VanderWerf have documented between same-sex and mixed sex pairs probably expand drastically over the entire lifetime of the birds.

The scientists also found that the albatrosses weren’t completely locked into their relationships. At the start of each new breeding season, a few females switched from a male partner to a female one. Likewise, females abandoned female partners for males. These switches illustrate just how much better male-female pairs do than same-sex ones. When a female bird switches from a female to a male partner, her productivity doubles.

What’s also striking about these switches is that they are far from random. Females that fail to produce a chick are three times more likely to end up with another female the next year than females that succeed.

Young and VanderWerf also found that females are much more likely to switch from females to males than from males to females. But they only switched to males if they had a successful breeding season the year before. Not a single female albatross that failed to reproduce one year went on to switch to a male partner the next year. (The full complexity of a female albatrosses’s life is mapped out in the figure at the bottom of this post.)

Young and VanderWerf sketch out an explanation for all of these different patterns–same-sex pairs included–that starts with one of the most important facts about albatrosses: they’re sharing the ocean with us.

Laysan albatrosses soar across the ocean for fish. Female and male albatrosses go their separate ways to go fishing, and the males appear to be more likely to go after the bait set out by long-line fishing boats. The deaths of male albatrosses may account for the fact that, at some albatross colonies, the majority of birds are females. At the site on Oahu where Young and VanderWerf do their research, for example, 60 percent of the birds are females.

In these places, there simply aren’t enough males to go around. A female will have her highest possible reproductive success if she can find a male partner, but there’s a fair chance that she won’t. If she can find a female partner, on the other hand, she may only be able to produce one chick in a decade. That’s pretty paltry compared to females with male partners. But it’s infinitely better than the zero chicks a female albatross will produce if she has no partner at all.

This scenario can also account for the behavior of the males. In many species, males compete intensely with each other for the chance to mate with females. They may also put on an elaborate courtship to gain a female’s favor. The females can then choose which males they will mate with. But this arrangement evolves because the females invest a lot of time rearing their young. At any moment in these species, there are a lot of available males and relatively few females.

Among the Layson albatrosses on Oahu, the situation is flipped. There are many females and relatively few males. The fact that females only switch to males after a successful year may be the result of males choosing among many potential mates. They’re keeping an eye on other albatrosses, and they prefer fertile females. The males also take advantage of the abundance of females by serving as sperm donors. By mating with same-sex female birds, they can have several offspring, while only putting in the effort to raise one chick.

View Images From left to right: soda can, albatross egg, and chicken egg. From Albatrosses at Work: www.wfu.edu/biology/albatross/atwork/atwork.htm

For albatrosses, same-sex partnerships aren’t a genetically hardwired adaptation implanted in the brains of a minority of birds. Instead, they’re just one part of a flexible set of behaviors that the birds can switch between to make the best of the situation they find themselves in.

What does any of this mean for same-sex marriage laws? I’ll only answer that question if you lay an egg the size of a soda can first.