In the fish world, traditional roles are typically reversed with the male building the nest, completing nest-keeping tasks, and protecting and caring for the young. Since female fish lay their eggs in an already-built nest before swimming away, the hard work ensures a male fish will pass along his genes.

But not all fish obey this rule.

Some male bluegill sunfish, a freshwater fish living in North American lakes and streams, balk at the fatherly responsibilities of nest-building and parental care. At or before a male reaches two years old, he chooses to become either a father figure staying close to his nest or a promiscuous mate labeled a cuckolder.

Sneaky sex

A cuckolder can still reap the benefits of passing along his genes without having to take care of fatherly tasks, however. Once a female sunfish releases her 50 or so eggs into a nest, often a sneaky male will swoop by and swiftly fertilize the eggs.

"If they decide to go down the cuckolder life history, that is the parasitic one that doesn't provide parental care, they become fixed in that for life, they'll spend life being be a cuckolder," co-researcher Bryan Neff of the University of Western Ontario told LiveScience.

In nests swindled by several cuckolders and often containing eggs from several females, many of the resulting offspring are not related. This fishy lifestyle makes it difficult for nest-mates to identify their real siblings. Now researchers have discovered how the baby bluegill recognizes genuine siblings. 

Smell this

Neff and colleague Tim Hain first had a fish of known origin swim around in a tank of water for a couple of days, "urinating and excreting various molecules from their body that provide an odor," Neff explained. The team repeated this with unrelated bluegill, gathering the scented water separately. Then they poured a kin-scent into one side of a fresh tank and another scent into the other side.

Bluegill born from cuckolder nests swam over to the tank section holding kin-scent, spending most of their time in that area.

"They're essentially saying, ‘does this individual smell a lot like me?' And if they do, they can be pretty sure that the individual is their brother or sister," Neff said.

The reasons for such active recognition are not clear yet to the researchers. Neff said the true siblings may help one another by not fighting in the nest, and once they leave the nest they might cooperate in foraging and defending against predators.

The study is detailed in the current issue of the journal Current Biology.