This is the first egg-laying amphibian found to feed its babies ‘milk’

In the middle of the night in a humid coastal rainforest, a litter of pink, hairless babies snuggle with their mother. They stir and squeak for milk, their mother obliges, and they are sated. But these are no puppies or cubs. They are snake-shaped amphibians, far closer to frogs than foxes.

These ringed caecilian moms feed their hatchlings a kind of “milk” brewed in the reproductive tract, researchers report in the March 8 Science. The long, cylindrical creatures are the first egg-laying amphibians known to feed hatchlings this way. The discovery suggests the evolution of parental care across animal life is more diverse than researchers thought. 

For an animal with so few discernable external features, caecilians are a fount of strange biology. Caecilians are elusive, legless, burrowing amphibians that are nearly blind (SN: 6/19/17). Some species, like the ringed caecilian (Siphonops annulatus) in the new study, have poisonous slime, may be venomous and feed their own skin to their young (SN: 7/3/20).

Herpetologist Carlos Jared of the Instituto Butantan in São Paulo and his colleagues have been studying these eccentric animals for years. In previous studies, the team noticed that ringed caecilian hatchlings, which live their first two months out of the egg in their mother’s care, spent much of their time around the end of her body near the shared opening of the reproductive, digestive and urinary systems — an anatomical part called the vent. The female would periodically expel a thick fluid from the vent, which the young would enthusiastically feed on. 

“Some [young] even stuck their heads inside this opening,” Jared says.

In the new work, the team collected 16 females and their newly hatched litters from Bahia state in Brazil, bringing them into the lab for observation. There, the researchers recorded the amphibians’ interactions, accumulating over 240 hours of video footage. The team recorded 36 feedings, which often involved the babies wriggling and nibbling at their mother’s vent while making high-pitched noises. Mom would then raise that end of her body and release the fluid. This happened up to six times per day and appeared to be in response to the babies’ pleas. 

The squeaking and begging is a particularly fascinating observation, says Mark Wilkinson, an evolutionary biologist at the Natural History Museum in London, because the adults are thought to be sensitive only to lower sound frequencies.

The team also examined the internal anatomy of some of the adult female caecilians and analyzed the nutritional and biochemical makeup of the nutritional fluid. It is secreted by glands in the mother’s oviduct that enlarge while raising her hatchlings. It’s also rich in fats, much like mammal milk. This nutritious resource may help explain how the hatchlings grow so fast — bulking up their mass by up to 130 percent, an additional 0.27 grams, in the first week out of the egg — despite not leaving their mother’s side and feeding on her skin only once every few days.

Isabella Capellini, an evolutionary biologist at Queen’s University Belfast in Northern Ireland, wonders if there is conflict between squiggly siblings for milk access and how that competition might play out. She’d also be curious to know more about how milk production affects the mother caecilian.

“In mammals, lactation is the most expensive stage of reproduction for the mother,” she says. “It would be useful to study whether milk production is as expensive in caecilians too. How is the mother impacted in the short- and long-term?”

It’s not known how these amphibians have evolved their version of “milk.” The substance is relatively rare in animals. Outside of mammals and some caecilians, it’s found only in certain spiders, fishes, cockroaches and birds, as well as two amphibians that give live birth.

In these egg-laying caecilians, the oviducts are behaving similarly to those of live-bearing caecilian species, which sometimes feed their babies a milky substance while in the womb, but do not feed them after they’re born.

“That makes it easier to conceive of how [live-bearing] could have evolved from [egg-laying] species that already used their oviducts to produce food,” Wilkinson says. “We really have learned a lot about caecilians in the last few decades, but we are only seeing the tip of the iceberg.”