Surprise: Egg-laying amphibian provides nutrient-rich “milk” to its young

An egg-laying species of worm-like amphibian feeds a lipid-rich milk-like substance to its hatchlings, according to a new study. The findings report a previously unobserved behavior and offer new insight into the species’ parental care and communication. Among vertebrates, the embryonic yolk is often the only nutritional investment mothers offer to feed offspring. However, some species have developed parental care behaviors involving the production and provisioning of specialized foods, such as the production of lipid-rich milk in mammals. Feeding offspring with nutrient-rich milk was long seen as a trait unique to mammals. However, several non-mammalian species, including spiders, have been documented as producing nutrients to feed offspring in a functionally similar way to mammalian milk. Here, Pedro Mailho-Fontana and colleagues report previously unobserved “milk” provisioning behavior in an egg-laying species of caecilian amphibian. While studying the unique skin-feeding behaviors of Siphonops annulatus, a caecilian native to Brazil, Mailho-Fontana observed that the offspring also consumed a material secreted from the mother’s vent. According to the findings, S. annulatus produces a lipid- and carbohydrate-rich resource in glands within the oviduct walls (which the authors refer to as milk) and feeds it to hatchlings several times throughout the day and seemingly in response to physical touch and sound signals from the offspring. This type of parent-offspring communication is not known to exist for any other amphibian. This provisioning occurred for roughly two months after hatching and contributed to rapid hatchling growth. “The study by Mailho-Fontana et al. opens new areas of research for caecilians and for amphibian biology in general,” writes Marvalee Wake in a related Perspective. “It also provides an expanded approach to investigate the evolution of derived modes of reproduction in the broadest sense, and to better understand key aspects of evolutionary biology.”