Some birds chirp to show happiness, but when a male zebra finch sings to a female finch he's never met before, he's looking to have an affair. If she sings back, she's probably willing to have one, too. But affairs make more sense for males than for females; when males mate outside of their typical monogamous coupling, they spread their genes far and wide, but adulterous females don't spread their genes any further than they would otherwise and additional fathers do not help raise offspring. So why do females do it? A new analysis of courtships from thousands of encounters between finches, as well as genetic analyses of paternity, reveals that the female offspring of more promiscuous males are more likely to mate with multiple males themselves . The reason, researchers report online today in the Proceedings of the National Academy of Sciences, may be that some males harbor a "promiscuity gene" that they pass down to their offspring, both male and female.
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