The 20-year study, which is reported in The American Naturalist, found that offspring conceived outside sparrows' social pairs go on to have lower reproductive success than within-pair offspring. The findings throw a monkey wrench into theories about why ostensibly monogamous animals might be inclined to cheat.

Most bird species display some form of monogamy. Bonded pairs stay together for a breeding season, a few seasons, or sometimes for life. But beneath this veneer of monogamy, there's plenty of hanky-panky in most species. Why promiscuity exists in monogamous species is "one of the biggest remaining enigmas in evolutionary ecology," said Jane Reid, a research fellow at the University of Aberdeen and one of the study's authors.

One hypothesis for this is that when a female strays she makes it count by mating with a male of higher genetic quality than her social mate. The result is higher-quality offspring that have a better chance of carrying a female's genes into future generations. This study, however, turns that explanation on its head.