Why promiscuous female birds mate outside their social pair

A team led by an Indiana University biologist has learned about promiscuous female birds and why they mate outside their social pair.

IU postdoctoral research associate Nicole Gerlach and colleagues have uncovered one of the benefits of this promiscuity: more grandkids!

In dark-eyed juncos (Junco hyemalis), individuals that were sired by a male other than their mother”s pair-bonded partner grew up to have higher reproductive success than did individuals whose mother stayed faithful to her partner.

This study on a population of wild songbirds represents the first time that an individual’s paternity has been shown to affect its reproductive success as an adult.

“There are a lot of species that form monogamous social pairs but are decidedly promiscuous when it comes to mating and having offspring, and the question of what females gain from these extra-pair matings has puzzled scientists for a long time,” said Gerlach, of the work with IU Distinguished Professor of Biology Ellen Ketterson, former doctoral student of Ketterson’s Joel McGlothlin, now at the University of Virginia, and Patricia Parker of the University of Missouri, St. Louis.

“What we’ve found is that, at least in juncos, these females are doing it for their kids, and for their kids’ kids.

“In the long run, females are likely to have twice as many grandchildren if they mate with an extra-pair male than if they remain truly monogamous,” Gerlach said.

Sons of extra-pair fathers are also more likely to become extra-pair fathers themselves, suggesting that females may be straying from their social mates based on whether the new male will produce attractive or otherwise high-quality offspring.

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“However, it’s not just sons that are reaping the benefits of having an extra-pair dad. The daughters benefit just as much,” Gerlach added.

The study has been detailed online in Proceedings of the Royal Society B.

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