Cuckoo catfish may be the worst parents ever. Rather than raising their own cannibalistic offspring, they force other fish species to raise them in their own mouths.

But how do they do it? New research shows the fish are more likely to find free daycare in the mouths of fish with bad eyes, passive temperaments, or which are generally naïve to their tactics, according to a study recently published in the journal Environmental Biology of Fishes.

The paper, co-authored by Anna Vinton, a PhD student in ecology and evolutionary biology at Yale University, also suggests that the fish that most often fall prey to their trickery have evolved some tactics to thwart the catfish. But some of those schemes can backfire.

Mating Proclivities

The fish, which take their name from the birds that drop their eggs in the nests of other species—cuckoos—are native to Lake Tanganyika in east central Africa. In these waters, the cuckoo catfish and the cichlids pursue diametrically opposed mating and child-rearing strategies.

The cuckoo’s trickery begins when the female cichlid is ready to lay eggs and attracts the attention of a male. Under normal conditions, the female horei cichlid (Ctenochromis horei) lays her eggs one or two at a time and immediately circles back around to grab them in her mouth.

At this point, the male cichlid entices her to chase him around, by waving an anal fin with egg-like spots as a lure. The female follows him around until she gets a mouth full of his sperm, which fertilizes the eggs. The male and female will repeat this courtship routine again for hours until she has a clutch of anywhere from 10 to 100 or more fertilized eggs in her mouth.

This cichlid hanky-panky attracts mating pairs of cuckoos. When the cichlid lays an egg, the female catfish swoops to lay a clutch of her own, sometimes gobbling up the cichlid egg at the same time. Meanwhile, the male dives in to fertilize her eggs. All of this happens in the few seconds it takes between the cichlid laying her egg and doubling back to eat it.

Martin Reichard, who coauthored a similar study earlier this year but was not involved in Vinton’s research, says the presence of the cuckoos sometimes makes the cichlid mother panic. To prevent the catfish from eating her eggs, she quickly gathers up everything—including the parasite’s brood.

“[The catfish] is basically creating a mess, a very chaotic situation,” he says.

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The Trials of Youth

Once the parasitic cuckoo eggs are safely ensconced in the loving mouths of their adoptive mother, they have the upper hand. Whereas cichlid eggs typically hatch between six and seven days after fertilization, the catfish eggs will hatch between two and four days. After emerging, the interlopers quickly go to work feeding on the unhatched cichlid eggs.

The catfish youth grow much larger than young cichlids, sometimes approaching an inch in length. That’s big, considering the mother is only around four inches herself. Sometimes, if no cichlids are left, the catfish turn to cannibalism.

“If they have nothing else to feed on then it’s really the survival of the nastiest individual,” Reichard says.

Vinton and her coauthors wanted to see whether different cichlid species would react differently to the cuckoo’s intrusive parenting technique, as many cichlids from African lakes exhibit similar mouth brooding. Over a five year period, they kept cichlids and cuckoos in various sized cages with five different species of cichlids: the horei which lives in Lake Tanganyika and is well-acquainted with the antics of the catfish, and three others from Lake Malawi and the greater Lake Victoria system which are cuckoo-free. They also tested an albino cichlid morph which doesn’t exist in the wild. They examined 100 broods from each of the five cichlid species, and counted how many times each was parasitized.

They found that the cichlid species most familiar to the cuckoos had broods parasitized only 17 percent of the time compared to between 25 and 33 percent for fish from cuckoo-free lakes. This may be because the males of these species were more aggressive to the catfish than the other cichlids; or possibly their mating strategies were more difficult to infiltrate.

Worst off were the albino cichlids, which had nearly half of their broods parasitized. Vinton says this may be due to the albinos’ relative lack of aggression, and poorer eyesight. The study was well done and had a large sample size, Reichard adds.

Counter Tactics

According to Reichard, the horei cichlids in Lake Tanganyika have a defensive strategy, though it’s costly. Horei cichlids will sometimes spit out their entire clutch of eggs. This would be a great strategy for ditching cuckoo catfish but for the fact that the cichlids are prone to paranoia, sometimes spitting out their clutch of eggs even when a catfish hasn’t dropped off their vicious little children.

“Very often they make a mistake and they spit out their own eggs,” he says. While it isn’t clear what prompts this paranoia, he speculates that the catfish’s mere presence in the area may be enough.

Even though it seems like the mothers should know better, evolution doesn’t always trump their maternal instincts. Reichard says that these cichlids are like the mothers that adopt the chicks of cuckoo birds and care for them despite obvious differences in appearance.

“They can perhaps tell the eggs apart, but once the egg hatches, the maternal instinct is so strong that they will care for offspring that looks completely different,” he says.