Do Some Birds Cheat to Avoid Inbreeding?

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That assumption fell by the wayside as DNA testing showed that female promiscuity is far more common than monogamy.

"Since [1988] there's been a gradual recognition that females must be getting some benefit from seeking multiple partners," said Judson. "In the last five years there's been a quite vigorous search to explain the reasons for cheating on part of either partner."

One explanation is the "good gene" theory; females might want to pair up with a guy who will be a good provider, but if she can also mate with the guy with the brightest feathers, the longest tail, the biggest horns—and get away with it—she'll go for the good genes, too.

The shorebirds study suggests that avoiding genetic incompatibility is another.

Extra Pairing in Shorebirds

Researchers from six countries studied three shorebird species—western sandpipers, Kentish plovers, and Common sandpipers—that are both socially and genetically monogamous. Partners in the three species share incubation duties, and males provide parental care after hatching.

"Social monogamy is about who's tending the nest; genetic monogamy is what's going on in the clutch," said Sandercock.

Chicks with mixed paternity—not the offspring of both partners tending the nest—were found in less than 8 percent of western sandpiper nests, 5 percent of Kentish plover nests, and 20 percent of Common sandpiper nests.

"This is quite different from some socially monogamous songbirds like tree swallows, where you might find up to 40 percent of the chicks in a nest are the result of extra pair matings," said Sandercock.

What would drive a normally monogamous bird to seek additional mating opportunities?

"Incestuous matings bring on infidelity," said Sandercock, in summing up the research. "What's amazing is that we found this to be consistent across all three species."

Figuring out the different reasons that mates cheat will no doubt occupy scientists for years to come.

"Of all of the different hypotheses to explain what benefits females are getting from cheating on their mates, I think avoiding genetic incompatibility will prove to be the most widespread," said Judson, whose book, Dr. Tatiana's Sex Advice To All Creation documents the numerous strategies different species have adapted to achieve reproductive success.

"Incest is one form of incompatibility," she said. "But I think that we'll find many instances in which neither partner in a couple is sterile, but the partners together are genetically incompatible."

The shorebird study suggests several avenues for future research, said Sandercock. Identifying the consequences of inbreeding, the social constraints that cause genetically similar individuals to become partners, and how the birds identify a closely related mate are all missing pieces to the puzzle, he said.

Judson has her own question.

"Why is it that these birds are usually monogamous? They're living in colonies, and have the chance to cheat; what are the possible disadvantages to cheating?"

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