So Cute You Could Just Eat Them Up

Most likely, your mother nurtured you for years and you never worried that when you came home from kindergarten, she'd gobble you up. But many animals do occasionally eat their young, including many that also are attentive parents. New research indicates that, when it comes to deciding whether to chow down on junior, Ma and Pa may be motivated by more than hunger. For example, parents may selectively eat their weaker offspring to favor the stronger ones, an evolutionary model predicts.

When biologists first encountered filial cannibalism decades ago, they thought it was a rare "social pathology"--a quirk of evolution without any benefits. However, researchers realized it was more common than they had supposed--it's been documented in mammals, insects, spiders, and birds--and some researchers came to see it as a way for parents to invest in future reproduction at the expense of their current offspring. By eating their current brood, starving parents might live to breed again and again in the future. Not all studies supported this explanation, and other researchers have come up with a variety of little-tested hypotheses to explain such cannibalism. To sort out the more plausible ideas, Hope Klug, an evolutionary biologist at the University of Florida, Gainesville, and Michael Bonsall, a mathematical ecologist at the University of Oxford in the U.K., created a mathematical model.

Their model introduced an imaginary individual with a mutation for filial cannibalism into a population of generic egg-laying animals. The gene for cannibalism spread throughout the population if it gave parents extra calories, but it also took root when parents could choose to cannibalize lower-quality offspring. The gene also flourished if the model was tuned so that eating the hatchlings made the remaining eggs develop faster (as it does in some animals) or when it increased the parents' reproductive rate--for example, when fewer babies made them more attractive to potential mates.

One key result, according to Klug, is that that overcrowding, or "density dependence," alone is apparently not enough to favor the spread of cannibalism genes. That possibility had been widely discussed in recent years.

Andrea Manica, a population biologist at the University of Cambridge in the U.K., says the model is comprehensive. The paper, published in the December issue of The American Naturalist, is a step forward in fitting filial cannibalism into the broader framework of parental care, he says, and, most importantly, it makes a "set of solid predictions for empiricists to test."