Maybe her cheatin' heart came from Daddy.
What's the News: Infidelity among monogamous bird couples has always been something of a stumper for biologists. It's easy enough to understand from the male's perspective---the more he plays the field, the more offspring he has. But why do females cheat? True, she might get more genetic variety among her offspring, which could help them survive. But the benefits aren't as clear, especially since she risks losing her mate. To shed some light on the subject, scientists have charted the coupling of zebra finches, which are usually monogamous, and found that there's a genetic reason some females engage in extracurricular activity: they get it from their fathers. And this supports a new take on the evolution of infidelity. Maybe it doesn't have to be in the female's interest to cheat---maybe it just has to be in her dad's interest. How the Heck:
The scientists studied a population of 1,500 captive zebra finches over the course of five generations, making note of each male's come-ons to females and the female's response. They also kept track of how many offspring the finches had and tracked their lineages.
They found that, as suspected, promiscuous females didn't have more offspring than chaste females, while there was certainly a payoff for cheating males. But most intriguingly, they found that female finches who slept around were often the daughters of the cheating males.
This suggests a seeming paradox at work: This behavior, which can be risky for the female, is nevertheless selected for in the population because it's such a strong strategy for males. In other words, the success of the female finch's father and her male offspring make up for any negatives she experiences in terms of evolutionary fitness.
What's the Context:
The idea that the overall benefit of infidelity might make it a useful trait, even though it might not be favorable for one of the sexes, isn't new, but this is the first time it's been experimentally explored with respect to infidelity in a monogamous species.
This reinforces the idea that we don't have to look for benefit at the level of the individual to understand why a seemingly bizarre behavior has been conserved. Trying to explain such behaviors in terms of individual benefits can sometimes lead to fallacies that evolutionary biologists call "just-so stories"---explanations that are tailored to make a trait make sense rather than built on experimental evidence.
The Future Holds: The genes that lead to infidelity in finches haven't been identified yet, so that's another step to be taken. And what about human infidelity? The researchers note that although biochemical aspects of human attraction, like sensitivity to dopamine, appear to be heritable and linked to promiscuity
, the similarities between men and women in terms of attachment and arousal aren't well enough understood for us to tell whether we, too, have behaviors self-destructive to one sex that evolutionarily even out. Reference: Wolfgang Forstmeier, Katrin Martin, Elisabeth Bolund, Holger Schielzeth, and Bart Kempenaers. Female extrapair mating behavior can evolve via indirect selection on males. Proceedings of National Academy of Sciences. Published online before print June 13, 2011, doi: 10.1073/pnas.1103195108
Image credit: wwarby/flickr