Almost a third of Laysan albatross nests in the Oahu colony have two mommies.
That’s the surprise in Lindsay Young’s genetic analysis of breeding pairs on Oahu. Like many other birds, two Laysan albatross court and pair up to form a nest and share the work of feeding a chick. But the males and females look so much alike in this species that the sex of the nest tenders only became clear after genetic tests, Young and her colleagues say in an upcoming Biology Letters.
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About a third of the two-female nests managed to raise chicks, says Young of the University of Hawaii at Manoa. The genetic analysis also identified some of the fathers, males elsewhere in the colony. In the female-female pairs the egg-laying moms had had brief encounters with these males at some point even though the males may have had their own nests. The female-female pairs have shown up in other bird species, but Young says the previous record for female-female pairing in an animal was only 14 percent (in western gulls).
What makes this finding really unusual, says Young, is that even in a good year a Laysan albatross pair can raise only one chick. Even that single beak takes two parents a lot of work to fill. It’s not unusual for an adult from Oahu to fly as far as Alaska on a single fishing trip to collect baby food. In a two-female nest, therefore, only one female gets to be the genetic mom. The other is just busting her tail feathers to spread somebody else’s genes.
This one-mommy-at-a-time household looks tricky in view of reciprocation. “It doesn’t pay to be the helper first, especially if there’s a long delay between when you give the help and receive it, because you’re never sure the other individual is going to reciprocate,” says Young.
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On Oahu, the two-female nests do include pairs that have been together for multiple years, Young and her colleagues report. And during those years, evidence shows that at least some of the pairs ended up raising a chick for each mom over the course of the partnership.
Oahu’s female albatross nesting could reflect the skewed 59 to 41 female to male ratio that turned up in the analysis, the researchers say. The birds started to breed on the island only in the early 1980s, and more females have arrived than males, Young says. Among animals, one sex often tends to move away from home more readily than the other. The female-female pairs have only half the success in raising chicks that other pairs do, but that’s better than failing without any kind of mate, Young says.
“Yes, sex ratio is the likely driver” of the female-female pairs, says Jeremy Hatch of the University of Massachusetts in Boston. He’s studied female-female pairing in a population of roseate terns.
Two-egg clutches do show up in low percentages in colonies of albatross species other than the Laysan, reports Peter Ryan of the University of Cape Town in Rondebosch, South Africa. “I guess this paper will require us to spend a bit more money on sexing birds to confirm we don’t have same-sex pairs,” Ryan says.