These amphibians feed their offspring with “milk” from their behinds and allow them to eat their skin

Parental care in the animal kingdom takes many forms. From those who lovingly protect their eggs, to families who stay together for their entire lives. Although parental care and the provision of milk to offspring are usually seen as a trait of mammals, it appears for the first time that amphibians also participate.

While more well-known amphibians are frogs and toads, which belong to the order Anura, and the order Caudata, which includes salamanders, there is a third group of amphibians that is less well studied: Gymnophiona, also known as the caecilians.

Caecilians are snake- or worm-like amphibians with approximately 222 species. They have no limbs and live mainly in tropical areas. Within these species are species that lay eggs and species that produce live young. Both groups of mothers provide the offspring with nutrition after birth. In some species this is known as skin feeding, where the offspring actively consume the mother’s skin after hatching. While this sounds gross, it’s about to get a lot worse.

Siphonops annulatus with eggs on a black ground background

Siphonops annulatus with her eggs before they hatch.

Image courtesy of Carlos Jared

The researchers studied the ringed caecilian (Siphonops annulatus), which is known to be an egg-laying or oviparous species. Siphonops annulatus It is known to nourish offspring through skin feeding, but the researchers also observed the young consuming a substance from the mother’s vent. By closely monitoring the hatchlings’ progress in captivity, the team saw that the hatchlings always had a stomach full of fluid, indicating that the substance they consumed through the vent had important nutritional value.

The team studied 16 adult females with offspring to better understand the process. The animals were all collected during fieldwork in Ilhéus, Bahia state, Brazil. Each mother had between four and thirteen young. During the entire study period, the young and their mothers remained curled up against each other most of the time. Caecilin mothers do not feed while caring for their young and their skin changes color to reflect the production of a lipid-based skin nourishment product.

In addition, the team observed that the young were very interested in the mother’s vent and the tip of her body, often touching them or even nibbling them. The offspring also produced a high-pitched sound when interacting with the anus, causing the mother to lift the end of her body and expose the anus, creating a thick sticky substance of transparent milk, which was then consumed by the young. This type of communication between the mother and her offspring has not been seen before in any other amphibian species.

The team found that some pups occasionally blocked the mother’s air opening for other offspring by pushing their snouts in, suggesting that there is competition between siblings for this mother’s milk.

The team saw 36 cases of milk feeding and although they also observed skin feeding, this was much less common. This whole process is extremely costly for the mothers who do not leave their young for the entire two months of feeding and lose about 30 percent of their body weight by the end of the parental care period.

“Mailho-Fontana et al.’s study opens new areas of research for caecilians and for amphibian biology in general,” writes Marvalee Wake in a related perspective. “It also provides a comprehensive approach to investigate the evolution of derived reproductive modes in the broadest sense of the term, and to better understand key aspects of evolutionary biology.”

The article was published in Science.

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