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.
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