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Researchers have found an amphibian that makes milk for its babies

Caecilians are amphibians that look superficially like very large earthworms. New research suggests that at least one species of caecilian also produces "milk" for its hatchlings.
Photo by Carlos Jared
Caecilians are amphibians that look superficially like very large earthworms. New research suggests that at least one species of caecilian also produces "milk" for its hatchlings.

A species of worm-like amphibian has been caught on camera feeding milk to its young.

The creature, known as a caecilian, lives underground. Researchers believe that the animal developed the ability to produce a milk-like substance independently of mammals, who are universally known for feeding milk to their young.

Caecilians are descended from the same lineage as frogs and salamanders. Hundreds of millions of years ago, their ancestors burrowed deep into the ground. They lost their legs, their eyes mostly stopped working, and their bodies became long and segmented. A modern caecilian looks a little like a long shimmering earthworm with a head, which has led some to call them icky.

That's a characterization Marta Antoniazzi totally rejects.

"I really don't agree that they are disgusting," says Marta Antoniazzi, a biologist at the Instituto Butantan, in Sao Paulo, Brazil. Brazil is home to lots of caecilians and Antoniazzi is a fan.

"They are kind of elegant, and they have a shiny body and a very nice face," she says.

But wait, there's more.

The particular caecilian species that Antoniazzi and her colleagues study is called Siphonops annulatus. Mothers of this species give birth to broods of wriggly babies who then proceed to eat their own mother's skin off.

"Once a week, they can eat her skin," says Pedro Luiz Mailho-Fontana, a researcher at the Insituto Butantan who was involved in the study.

This doesn't seem to bother mama, and the babies get lots of nutrition from the skin.

Carlos Jared directs the institute's department of structural biology and leads the team that was studying the caecilians. As he was watching this fascinating process, he couldn't help but notice the wriggly little babies had a ton of energy.

"They are so, so active, it's impossible to eat only once or twice per week," he says.

So the team stuck a camera in the nest and started watching. And pretty soon, they noticed the babies were gathering around one particular spot.

"The babies prefer to go to the tail of the mother," he says.

And that's when they saw it. A secretion coming from the tail: "A kind of substance, like milk."

Upon further study, the team found that the milk contained lipids and sugars similar to mammalian milk. It was essentially providing the same function.

"It's a very unusual form of nutrition" for an egg-laying animal, says Mailho-Fontana.

The team published their results in this week's issue of the Journal Science.

Marvalee Wake, a professor of integrative biology at the University of California at Berkeley, who was not involved with the study, says that this species of caecilian has evolved to deal with a similar problem faced by human babies. Just like humans, the little ones are born long before they can fend for themselves. They're vulnerable. And in order to help them grow, while keeping them close, their mother has developed a milk-ish fluid.

"This is convergent evolution," she says.

Convergent evolution is the process by which very different species can evolve similar traits.

But is it really milk?

The Brazilian team doesn't say whether the milk meets FDA standards, but it does contain lipids and sugars. Wake says she thinks it counts.

"If it has all these basic subunits, it's convergent evolution on a nutritive material, and that's what it's all about," she says.

For Antoniazzi, caecilians are a wonderful reminder that very different animals, like puppies and underground worm-amphibians, can share a lot in common.

"Nature is very creative," she says. "Sometimes it gives the same solution to different groups of animals."

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Geoff Brumfiel works as a senior editor and correspondent on NPR's science desk. His editing duties include science and space, while his reporting focuses on the intersection of science and national security.