You will never believe which animal makes milk for its babies

Motherhood takes many forms. Most vertebrates, such as birds, reptiles, amphibians and fish, reproduce by laying eggs filled with nourishing yolk that their offspring use as an initial source of nutrition before hatching. Mammals are changing the game by giving birth to live young and feeding them fatty, sugary milk while they’re on their feet.

But nature breaks the rules all the time, and the latest animals to confound the yolk-milk binary are caecilians, the egg-laying, legless amphibians that resemble worms. Research published On Thursday, Science magazine suggested that they also feed their young a milky substance, but through their behinds. This behavior is unknown in amphibians.

It adds to the curiosity of caecilians, who were already known for feeding newborn babies’ skin torn from their mothers’ backs as a nutritious snack after birth.

“It’s like they’re from another planet,” he said Carlos Jared, a caecilian researcher at the Butantan Institute in São Paulo, Brazil, and author of the study. “To me they’re like Martians.”

Caecilians are “one of the least understood” vertebrates, said Dr. Jared. Because they spend most of their lives underground, they are difficult to find and even harder to study.

Since 1987, his team has been musing about caecilians that make milk. After several trips to the cocoa plantations in Brazil’s Atlantic Forest, his team managed to collect sixteen mothers of the Siphonops annulatus species caecilians and their numerous young. Each mother has four to thirteen babies. Back in the lab, they filmed each family for the two months from hatching until the worm-like amphibians became independent.

Each mother never left her nest, not even to nurse, and the young wriggled around on her back and nuzzled up to the end of her body. This is where the offspring excitedly compete to nibble a white, viscous fluid from the mother’s cloaca, nearly poking their heads into it.

The puppies suck on this milk several times a day and grow to more than twice their size in the first week. When pharmacologists examined the substance, which is produced in special glands in the oviduct of the caecilian mother, they found that it was fatty and rich in carbohydrates, like mammalian milk.

Crucially, the videos show the baby caecilians energetically sneaking towards the mother and then making high-pitched clicking sounds as they appear to ask for this milky substance.

“They’re crying, they’re making noises, click click click click, it looks like begging behavior,” said Pedro L. Mailho-Fontana also from the Butantan Institute, which studied hours of video material.

Milk feeding and this type of communication between parents and young have not been found in other amphibians.

“It’s very unique,” ​​said Dr. Mailho-Fontana. Feeding milk could boost the young’s microbiome and immune system, just like it does in humans. Because not all hundreds of caecilian species lay eggs (some give birth to live pups that have already scraped the mother’s skin with their tiny hooked teeth from the womb), his suspicion is that this strange combination of laying eggs and producing milk is an evolutionary step to move from one birthing method to another.

“Evolution happens in different and non-linear ways,” said Dr. Mailho-Fontana.

Or perhaps Cecilian mothers are simply loving parents who use different feeding techniques, according to Marvale Wakeprofessor of integrative biology at the University of California, Berkeley, who was not involved in the study.

But these findings are just a starting point: It’s still unclear whether other caecilian species do this, and how, why, when and where this amphibian milk evolutionarily came from, said Dr. Wake.

There are a variety of “completely strange” reproductive techniques and life histories in the world of amphibians, he said David Blackburn, curator of herpetology at the Florida Museum of Natural History, who was not involved in the study. Sometimes, though, they’re so weird that it takes a long time for science to fully piece them together. The species, he added, was first identified in 1822. “So it only took us 200 years, that’s right, more than 200 years to discover this,” said Dr. Blackburn. “Caecilians continue to surprise.”

He wonders about the two hundred or so other caecilian species out there.

“Okay, now we have skin food and cloacal milk,” said Dr. Blackburn. “What else is there?”