It is a reproductive paradox: giving birth to offspring just to eat them. But the behavior is widespread in many groups of animals, from bank voles and hedgehogs to house finches, wolf spiders and a lot of fish species. The paradox is increased by the fact that all these species are also very good parents for the rest of the youngsters. A new research published in the December issue of the journal The American Naturalist has tried to explain the co-existence of such opposite behaviors in the same species.

"If it doesn't confer some sort of benefit, you wouldn't expect it to have evolved," said lead author Hope Klug, a graduate student at the University of Florida.

Other researches stressed the hypothesis that parents eat their own eggs or offspring to achieve an energy benefit when food is scarce. But when filial cannibal fish received supplemented food, the outcome was variable: some stopped displaying the cannibal behavior, others didn't.

"It's not just about getting an easy meal," Klug said.

Klug's team made computer simulations of various scenarios of "virtual organisms", filial cannibalism being attributed to populations of animals caring for their offspring.

Filial cannibalism exerted the same evolutionary pressure on the eggs like a predator: faster developing eggs had higher survival chances. Cannibalism could increase the parent's reproductive rate by probably boosting mate attractiveness. Also a limited energy benefit was found.

"You can't explain filial cannibalism in all of these animals with just one benefit. Filial cannibalism could be a way to root out offspring that take too long to mature and therefore require a little too much parental care-this strategy would conserve the parents' energy for subsequent, faster-developing batches of young. They initially overproduce offspring and then later remove some of the inferior offspring," explained Klug.

It could also be about limited resources due to competition within the species, so that the parents have determined levels of energy and time they can dedicate to raising their young and by eating from the eggs they force them to grow up faster.