The XX Factor

Their Chirpin’ Cheatin’ Hearts

The hoary old evolutionary explanation for gender differences is that males are slutty and females are choosy. Males sleep around in order to fertilize as many eggs as possible, while females guard their virtue until Prince Charming comes along. But with the advent of genetic techniques (and in my opinion, female biologists), scientists have found that nature overflows with wanton females. Figuring out evolutionary reasons for looseness in ladies is harder-since each egg can only be fertilized once, having lots of sex won’t necessarily lead to more babies. A study on a European songbird, published in last month’s Current Biology , reveals one possible reason for female infidelity- bastard chicks are bigger and stronger than their legit half-siblings.

The delighfully named blue tit (scientific name: Cyanistes caeruleus ) is a common European songbird with a common songbird lifestyle-they mate for a season, the male defends his territory with song, and the female lays and incubates the eggs. But over 40 percent of the female blue tits are getting around all over blue tit town, cuckolding their mates right and left. When researchers examined the fate of chicks sired in and out of wedlock, they found that the illegitimate chicks were healthier because they were laid and hatched sooner, therefore getting more food and attention than the later-born chicks. If chicks born of cheating hatch earlier, survive better, and live to pass on mama’s ways, no wonder female blue tits can’t keep it in the nest .

The bastard advantage in blue tits is yet another demonstration that the natural world does not conform to conservative talking points. In fact, all that finger-shaking on premarital sex hurting women’s ability to bond with future partners was based on research on the prairie vole. Thought to be a paragon of monogamy, those naughty, bad rodents were caught cheating on their partners-for-life last year . The free-flying blue tit lifestyle is no more unusual than living in a harem or inside your sweetie’s intestine -it’s monogamy that’s hard to find.

Photograph of Blue Tit by Luc Viatour/Wikipedia