More than 90 species of birds, including cuckoos, are “brood parasites.” They don’t build their own nests; instead, they rely on other birds to care for their young. But the cuckoo catfish (Synodontis multipunctatus) seems to be the only other vertebrate with this strategy. In East Africa’s Lake Tanganyika, the catfish has a very particular target: thick-lipped cichlid fish that use their mouths as nurseries to raise their young. When the cichlid lays her own eggs, which she then scoops into her mouth, riotous catfish couples sneak under her to lay and fertilize their own eggs at the same spot. In the chaos created, the cichlid scoops up both her eggs and theirs.

Last year, researchers showed that some cichlid moms are smart enough to avoid picking up the catfish eggs. Now, those same researchers—along with a second team—have shown how the catfish are fighting back. In their laboratory at the University of Colorado in Boulder, evolutionary ecologist Marcus Cohen and his colleagues compared the development of the catfish and the cichlid eggs. Catfish eggs developed faster, hatched sooner, and were bigger than cichlid eggs laid at the same time, which puts the catfish young at an evolutionary advantage, Cohen’s team will report next month in the Philosophical Transactions of the Royal Society B.