Dark-eyed junco birds are one of many species that practices monogamy...to a point. While a male and female are pair-bonded - the junco equivalent of marriage - both partners will take on additional partners in the name of better genetics.
Figuring out just why juncos so frequently break their pair-bonding has been tricky. Previous studies have only looked at the birds' offspring during the first year or two of their life, when there's no apparent difference between those who were born to pair-bonded parents and those who were the product of so-called extra-pair couplings.
But Indiana University biologist Nicole Gerlach has discovered the truth after 18 years of extensive research: the real advantages kick in once the junco birds grow up. She explains:
"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. 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."
It's not just the quantity of children and grandchildren - it appears to be a matter of quality as well. The products of extra-pairs are generally more fertile and show a more favorable genetic makeup than their pair-bonded counterparts. Gerlach continues:
"Because we could look at these patterns over many years and many generations, we were able to find strong support for the idea that extra-pair mating by females does produce better offspring… but that they need to grow up before that higher quality starts to show. This is the first case that we know of in which extra-pair paternity has been shown to increase lifetime reproductive success of adult offspring in a free-living songbird."
It's an open question just why the juncos don't simply pair up with the best possible mate in the first place instead of bothering with all these sexual shenanigans. One possibility is that there's a social component to pair-bonding that isn't directly tied to reproductive success - the juncos choose the best life partner, if you will, and they worry about finding the best reproductive partner later. Or it could be that certain juncos are just objectively the "best" partners, and they can't be pair-bonded with everybody, so the juncos put up with being paired off with some loser while waiting to make babies with the local bird-Adonis. Admittedly, the first sounds way less depressing than the latter.
Via Royal Society B: Biological Sciences. Image by Indiana University.