Like humans, zebra finches form monogamous partnerships in order to rear children. And, very much like us, these birds are also known to indulge in "extramarital" sex, even when these affairs serve no obvious evolutionary purpose.
Both male and female zebra finches have shown signs of promiscuity, although only one of those is easy to explain from a strict evolutionary perspective. For males, sex with multiple partners can be useful because it gives them a chance to sire many more offspring than they could with just one mate. But for females, that isn't really a possibility, and sex outside their committed partnership brings with it the risk of disease and the breakup of their main relationship.
Wolfgang Forstmeier of Germany's Max Planck Institute for Ornithology set out to figure out the root causes of female finch adultery. To that end, he worked with a captive population of some 1,500 birds over five generations. The researchers used genetic tests to establish clean lines of paternity, and they also videotaped the zebra finch sex...you know, for science.
The research team found that some finches were far more promiscuous than others, with some males seemingly "obsessed" with sex outside their main partnership, while others avoided it almost completely. As you might expect, the more promiscuous males sired the most offspring, and here's where things get interesting.
Forstmeier found that the female offspring of promiscuous males were far, far more likely to be promiscuous themselves than any other females. This suggests these females inherit a genetic predisposition towards promiscuity from their fathers. That explains why female zebra finches are promiscuous even when, if anything, they are at a slight evolutionary disadvantage to their non-promiscuous counterparts - they don't raise any more eggs than their peers, and their young tend to be a bit lighter and weaker.
In and of itself, female promiscuity would probably be selected against in the zebra finch population, since it's most likely a slight net negative for the birds that possess those genes. But that's overwhelmed by the fact that male promiscuity is a huge advantage for these females' fathers and brothers, which means enough of these genes stick around to get passed along to females. It's sort of the evolutionary equivalent of collateral damage. At least the females affected get to have a ton of finch sex, I guess.
Sidney Griffith of Australia's Macquarie University explains what this study means:
"This is an excellent and very important study that I am very excited about. Their study is the most intensive study to date to investigate the behaviour associated with extra-pair paternity. What they have demonstrated really nicely is a genetic correlation between promiscuous behaviour in males and females. It is important to realise that any mating behaviour in sexually reproducing animals may be shaped by the same kind of genetic architecture – genes that are carried by both sexes and may be expressed in both males and females but where strong selection in one sex drives behaviour in the other sex that at first does not appear to make sense."