International Business Department           Liu Bojia           March 11, 2023

  For mammals, newborn young individuals enjoy special maternal care, and in order to ensure their growth, females secrete nutrient- and lipid-rich milk to provide nourishment, a developmental stage that humans also share. Although this scenario is very familiar to us, breastfeeding is actually very specific to the animal kingdom, with many animals developing embryos that will simply receive nutritional support only from the yolk of a fertilised egg, and will not undergo the process of breastfeeding after birth.

  In recent years, as scientists have looked at a wider range of animal behaviours, some exceptional examples are emerging. Some studies have shown that breastfeeding is not unique to mammals, and that a few other animals, such as insects and fish, are also capable of producing nutrient-rich fluids for their offspring, with a 2022 study in Nature revealing that ants can secrete a nutrient-rich fluid similar to "milk" to feed their larvae.

  A recent paper in the journal Science shows another animal that produces "milk" to nurture its offspring: an amphibian called the earthworm salamander. They resemble earthworms and worms, mostly live underground, and have little or no eyesight. They reproduce by laying eggs, but scientists have found that when their offspring hatch outside the body, female salamanders also secrete nutritious fluids to nourish their young, marking the first time a study has shown that egg-laying amphibians also engage in "lactation" feeding behaviour.

  According to the paper, the researchers looked at a species called Siphonops annulatus, which has a more specialised reproductive behaviour, in which female Siphonops annulatus lay their eggs on land and then incubate them. The newborn pups are armed with teeth, which allow them to nibble on their mother's skin and obtain the nutrients and lipids they need to grow from the skin tissue. But in their observations, the scientists found that the nibbling behaviour only occurs about once every seven days, which doesn't seem to be often enough to support the pups' growth and development.

  To find out more about where else the pups get their nutrition, the researchers collected 16 nesting females and their pups from the forest, then filmed them for a long period of time and analysed their behaviour for more than 200 hours.

  As a result, they saw that the pups would often gather around their mother's cloaca and stimulate her by making physical contact and high-pitched sounds. In response to this signal, the female salamanders would secrete a lipid-rich, carbohydrate-rich fluid from the cloaca, much like mammalian milk, and the pups, sensing that milk was being secreted, would then stick their heads into the cloaca to feed.

  The researchers were very surprised by this result, which shows that amphibians also have a very special parent-child communication pattern, and the pups can wake up their mothers to feed them through special behaviours, which adds a brand-new parenting mechanism for amphibians, and is important for understanding the evolution of reproduction and parent-child parenting behaviours in animals.

  The review article points out that this study is only a phenomenal discovery, and there are many more directions for researchers to explore in the future, such as what are the genetic and endocrine factors that contribute to the production of "milk" in annelid salamanders? When did amphibians first evolve this type of parenting behaviour? Answering these questions will also help us better understand the origin and evolution of milk.