[Photo] Supposedly childless cichlid nannies keep a sharper eye when they sneakily father kids in their group. CREDIT: Todd Anderson, Stanford University News Service

Childless male cichlid nannies are more productive when they "sneak" their own kind into the bunch. Researchers had previously thought that such male helper fish didn't have their own offspring, but new research indicates that they are able to secretly fertilize a large number of the eggs they defend.

"This is the first evidence in a cooperatively breeding fish species that the helping effort of male subordinates may depend on obtained paternity, which stresses the need to consider direct fitness benefits in evolutionary studies of helping behavior," study researcher Rick Bruintjes, of Bristol University, said in a statement.

Residing primarily in Africa's Lake Tanganyika, cichlids are small — they grow to a maximum of 4 inches (10.16 centimeters) — but they are aggressive. They live in social groups of up to 17 fish. Only two of the fish breed, though, the dominant male and dominant female. The rest of the fish, both male and female, serve as nannies, caring for the dominant pair's young, and defending and maintaining the group's territory.

Such altruism is usually explained by the theory of "kin selection" in which the non-breeding animal helps out his relatives because they contain some of his genes, as well. If his "boss" goes forth and multiplies, then he indirectly benefits from his genes also being spread. What's interesting about the cichilds is that sometimes, these subordinate fish are either unrelated or distantly related to the breeding pair.

Researchers had been puzzled why the unrelated cichlids would be so helpful, since it seems to defy the nature of the kin selection theory. Helping around the nest does provide a protective shelter from predators, but this indirect benefit didn't seem like enough, especially if they aren't passing along their genes.

When they studied groups of cichlids in Zambia, they found that the dominant females had mothered 99.7 percent of the offspring, while the dominant males sired only 88.8 percent. This leaves 11.1 percent of all offspring without fathers. Those eggs must have been fertilized by the subordinate nanny fish, researchers say, and they found that happened in 27.8 percent of all clutches (groups of eggs).

These fathers seem to know when they've succeeded. Those nannies that had kids defended their territories more vigorously against predators trying to steal eggs and they didn't stray far, even when they were off guard duty.

The study was published in yesterday's (Oct. 13) issue of the journal PLoS ONE.