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Kids these days: Can't get them to move out of the house, can't eat them.

Unless, that is, you're a sand goby or one of the many species of animals that occasionally snacks on its young.

New research has found a possible reason for parental cannibalism: Some offspring just take too long to grow up.

In research published earlier this month in the journal Biology Letters, scientists studied male sand gobies, a type of fish that are in many ways ideal fathers. They alone care for thousands of eggs in the one to two weeks they take to hatch.

"Over all, Dad does a pretty good job of taking care of the eggs, except for one thing - he tends to eat about a third of them," researcher Hope Klug, an evolutionary biologist at the University of Helsinki, told the website LiveScience.

And the dads aren't just hungry: Even when they have plenty of food, they still cannibalize their young.Normally, animals want to maximize the number of their offspring, so from an evolutionary standpoint such behaviour doesn't seem to make much sense.

But researchers noticed that sand gobies tend to eat the larger eggs, which take longer to hatch, because female sand gobies won't mate with males who are babysitting eggs. Researchers believe the fish fathers are trying to reduce the time spent caring for their young, allowing them to jump back into the mating season faster and thus produce more offspring overall.

Different animals have different reasons for eating their young - and those reasons may be perfectly sound, even if it's hard for humans to accept in the case of cute furry babies. Officials at the Nuremberg Zoo in Germany were criticized after one of its polar bears ate two of her cubs.

"Predators are known to eat their offspring if there is something wrong with the cubs or the surroundings make a successful upbringing unlikely," the zoo said in a written statement, adding that the cubs probably were diseased. Nevertheless, zookeepers decided to separate a third baby polar bear from its mother and hand-raise it.