That Ozzie-and-Harriet two-parent family might not be the way to raise perfect children--at least not in bird families. New research shows that zebra finch chicks born to single mothers get better care than those born into two-parent families. Moreover, the sons of single finches are more attractive to young female finches on the prowl, making Mom's investment pay off by handing down more of her genes to the next generation.
As any parent knows, the personal cost of raising children is enormous. Behavioral scientists have predicted that animals face conflicting interests when it comes to propagating their genes: caring for the young they already have versus leaving home to produce more offspring. Ideally, each parent would like the other to take care of the children while he or she goes out cavorting. But no one has looked at how this conflict manifests itself.
Behavioral ecologist Ian Hartley of the University of Lancaster, U.K., and colleagues set up 12 zebra finch nests with breeding couples. After an average of four eggs were laid, the team removed the males from half of the bird families, along with half the eggs to keep the workload constant for the remaining females. Chicks raised by single moms received 25% more food than chicks with two parents, they report in the 18 April issue of Nature. But this extra care came at the mothers' expense: Eggs born to mothers after they'd raised a brood by themselves were 20% smaller than those laid by mothers who had previously benefited from a nestbound male.
Perhaps the biggest surprise, Hartley says, came after the finches grew up: Young female finches preferred males from single-parent homes. Although Hartley couldn't find any obvious qualities that made the latchkey sons more attractive, the hunkier offspring--which are more likely to pass on the family genes--appear to be Mom's reward for spoiling her chicks.
Because birds divide the labor of child-rearing much more evenly than mammals do, Hartley says that extrapolating to humans would be hard. Nevertheless, the results reveal that parents "will trade off their own future success for the success of their offspring," says behavioral ecologist Ben Sheldon of the University of Oxford, U.K. The single females can only see the benefit of their hard work after their young grow up.