For the study published in the Proceedings of the National Academy of Sciences, DeWoody and the others captured male darters and sunfish to extract undigested eggs from their stomachs. They then used DNA fingerprinting techniques to compare the DNA of the adults with that of the embryos.
They found that the males were unable to distinguish between eggs that were their kin and those that weren't.
"These findings have implications not only for fish ecologists, but also other areas of biology," DeWoody says. "The question of how well animals are able to recognize their kin is a question that many biologists are asking."
But cannibalism isn't the most bizarre part of the spring spawning season. Even for fish, it turns out that it's not always a father-knows-best kind of world. There are other males on the prowl, and these males use deceptive tactics to ensure that they, too, have small fry.
DeWoody and his colleagues also have studied sunfish and found that as few as 85 percent of the fish were nest-builders, which scientists called bourgeois males. The rest, the researchers found, used their unusual sexual development to trick the bourgeois males and reproduce.
There are two types of non-bourgeois males, which scientists call sneakers and satellites.
Sneakers are adult males disguised as immature males. Besides their smaller size and immature appearance, the sneaker males have another difference: dissections show that despite their size, sneakers' testes are three times as large as those of a bourgeois male. "They're not much more than a swimming sack of sperm," DeWoody says.
As their name implies, sneakers approach a nest guarded by a bourgeois male and, when the moment is right, darts over the nest.