Egg-Laying Amphibians Found to Feed “Milk” to Hatchlings

March 12, 2024

In a groundbreaking discovery, researchers have found that ringed caecilians (Siphonops annulatus), a species of legless, burrowing amphibians, provide a milk-like substance to their hatchlings. The study, published in the March 8 issue of Science, sheds new light on the diversity of parental care in the animal kingdom and challenges our understanding of the evolution of nurturing behaviors.

The Elusive Ringed Caecilians

Ringed caecilians are enigmatic creatures that live underground in the coastal rainforests of Brazil. These limbless amphibians are nearly blind and have a host of unusual characteristics, including poisonous slime, potential venom, and the practice of feeding their own skin to their young. The recent discovery of their milk-like secretions adds another fascinating aspect to their biology.

Observing Maternal Care in the Lab

Herpetologist Carlos Jared and his colleagues at the Instituto Butantan in São Paulo collected 16 female ringed caecilians and their newly hatched litters from Bahia state, Brazil. In the lab, they recorded over 240 hours of video footage, documenting 36 feeding instances. The researchers observed that the hatchlings spent a significant amount of time near their mother’s vent, the shared opening of the reproductive, digestive, and urinary systems. The young would wriggle, nibble, and make high-pitched noises, seemingly pleading for food. In response, the mother would raise her rear end and release a thick, nutritious fluid, which the babies eagerly consumed.

The Composition and Benefits of Caecilian “Milk”

Analysis of the fluid secreted by the female caecilians revealed that it is rich in fats, similar to mammalian milk. The glands in the mother’s oviduct, which enlarge during the period of raising hatchlings, produce this nutritious substance. The milk-like fluid appears to play a crucial role in the rapid growth of the young, with hatchlings increasing their mass by up to 130 percent in the first week after emerging from the egg.

Evolutionary Implications and Future Research

The discovery of milk production in egg-laying caecilians raises intriguing questions about the evolution of parental care. While milk is relatively rare in the animal kingdom, it is found in certain spiders, fishes, cockroaches, birds, and a few live-bearing amphibians. The oviducts of egg-laying caecilians seem to behave similarly to those of live-bearing species, suggesting a possible evolutionary pathway from egg-laying to live-bearing.
Researchers are eager to explore further aspects of this unique parental care system, such as potential sibling competition for milk access and the impact of milk production on the mother caecilian’s energy expenditure. Additionally, the apparent acoustic communication between hatchlings and their mother warrants further investigation, as it challenges the current understanding of caecilian sensory capabilities.

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