Image: ANDREW DEWOODY, Purdue University

Male freshwater fish will fervently guard a nest of eggs they've fertilizedbut that doesn't stop them from eating a few when they get hungry. Scientists from Purdue University recently discovered this form of cannibalismwhich they describe in today's issue of the Proceedings of the National Academy of Scienceusing genetic fingerprinting techniques.

In fact, most freshwater fish procreate by way of external fertilization: the male builds a nest, the female lays eggs in it and the male then drops sperm on top of them. Until his offspring hatch, the male diligently tends to the nest (see image), protecting it from predators and fanning his tail to aerate the eggs. And during this period, the male can't leave his nest unguarded. Thus, he resorts to eating some of its contents to survive.

Andrew DeWoody and his colleagues wondered whether these male fish could distinguish between eggs they fertilized and those seeded by other fish, preferring the latter for food. "Occasionally a second male will join the spawning pair," DeWoody explains, "and attempt to 'steal' fertilizations from the primary male." To find out, the team caught several male darters and sunfish and extracted undigested eggs from their stomachs. Then they compared the DNA of the adults with that of the ingested embryos. It turned out that the males had eaten their own progeny