Why these fish moms cannibalize their babies - National Geographic

Raising babies can be exhausting—so much so that some mouthbrooding mothers snack on their young, according to a new study.

A central African cichlid fish, Astatotilapia burtoni—commonly called Burton's mouthbrooder—sometimes consumes more than three-quarters of their own eggs and baby fish.

"It is a surprise," says Peter Dijkstra, a biologist at Central Michigan University and senior author of a study published today in Biology Letters. "It really is a lot."

Mouthbrooding is common among fish, particularly cichlids, colorful freshwater species that are popular as aquarium pets. After their eggs are fertilized, A. burtoni moms will shelter them in their mouths for roughly two weeks to protect them from predators. (Read about other long-suffering animal moms.)

After the eggs hatch into tiny larval fish, the babies venture outside their mother's mouth, but scurry back in when there's danger. It's a great start for the young fish, but it's extremely tiring for the mother cichlid, who can't breathe properly or eat during that period.

The new study shows, however, that the suffering moms can counter that by consuming some of their young, a behavior called "filial cannibalism."

And now, for the first time, scientists have linked the levels of cannibalism to maternal health, says study leader Jake Sawecki, a researcher at Michigan State University.

Moms boosting their health by eating their own babies seems counterintuitive, especially from an evolutionary perspective, he says—after all, "eating your own offspring doesn't pass on your genes." (Learn about other animals that sometimes kill their babies.)

But the research shows the antioxidant boost that cannibalistic moms gain from feeding on their young likely allows them to spawn again a few months later.  

Stressed out

For the study, Sawecki and Dijkstra raised several mixed-sex groups of cichlids in a laboratory at Central Michigan University. Over several weeks, the team identified around 80 females that had recently spawned.

The scientists delicately removed all the eggs from the females' mouths. They then placed about 25 eggs each into half of the females' mouths using a plastic pipette. The fish that didn't get their eggs back were raised as a control group and subject to the same handling.

A mother cichlid fish protects her young by keeping them in her mouth. But a parasite has evolved a way to use her own defenses against her.

After a two-week observation period, the researchers noted that the mouthbrooding mothers had eaten about 40 percent of their offspring on average; and that more than 93 percent of moms—29 out of 31—had snacked on at least some of their young. (See National Geographic's pictures of animal mothers and babies.)

Next, the scientists assessed the mother fish's levels of oxidative stress, which can be measured by biochemical markers in the fish's tissue. For instance, high levels of particular chemicals in the liver can indicate stress, which can lead to cell damage, disease, and infection.

At three stages of the study, a few of the mother fish were killed so their tissues could be tested. The researchers found that mothers with higher levels of oxidative stress consumed more of their offspring, presumably benefiting from the bursts of antioxidants they ingested.

While the latest study looked at only one cichlid species, Sawecki expects that filial cannibalism is "an adaptive strategy that many species use," he says.

Baby-size snack

Karen Maruska is a biologist at Louisiana State University who also studies A. burtoni and wasn't involved in the study.

"It adds an interesting piece to the puzzle of how these mouthbrooding females are able to survive and maintain their own health during the two-week brooding period when they can't eat," says Maruska.

Eating your young isn't going to earn these fish a Parent of the Year award. But mouthbrooding cichlids are still attentive parents, says ichthyologist Prosanta Chakrabarty, curator of fishes at the Museum of Natural Science at Louisiana State University.

"Most people think fish just drop their eggs and ignore them forever, and that's true of some species," says Chakrabarty. But mouthbrooding cichlids "are good parents."

Even if they sometimes need a baby-size snack.