“Milk” discovered in an amphibian species

Cubs of a female Siphonops annulatus gather in front of their mother’s “milk source.” © Photo by Carlos Jared

Mammals – but also some representatives of other animal groups – provide their offspring with a type of milk. Researchers have now also discovered an example of this concept in amphibians: a representative of the worm-like creeping amphibian also feeds its newly hatched offspring with a nutrient-rich secretion. Interestingly, milk production is also stimulated by the “begging sounds” of the little ones. This discovery once again demonstrates the astonishing diversity of brood care concepts in the animal kingdom, say the scientists.

Taking care of them can be worth it: Many animals don't just leave their offspring to fend for themselves, but rather actively ensure the well-being of the little ones after birth or hatching. Some feed the young animals with food that is brought in or regurgitated. But as is well known, there are also animal species that supply their offspring with nutrients through specially developed secretions. These special “body juices” are called milk. In mammals, this concept became almost an identity feature. But there are also examples in other animal groups. For example, representatives of the pigeon birds provide their young with so-called crop milk, and representatives of fish, insects and arachnids are even known to produce nutrient secretions to feed their young.

Amphibians in sight

When it comes to amphibians, however, only concepts that are not directly comparable to milk supply were previously known. However, this group of animals also has highly developed forms of brood care. There are frog species that carry their tadpoles around on their backs and provide unfertilized eggs for their nutrition. Intensive forms of care were also known from a bizarre-looking group of amphibians: the creeping amphibians (Gymnophiona), also known as caecilians, are amphibians with a limbless, worm-shaped body shape that occur in tropical regions of the world.

There are both egg-laying and viviparous species among the creeping amphibians. These amphibians were also known to have a special system of providing food for their offspring: in some creeping amphibians, the females produce special skin structures while caring for the brood, which are eaten by the young animals. But now the scientists led by Pedro Mailho-Fontana have discovered an additional system in the South American ringworm Siphonops annulatus that is comparable to the milk supply behavior of other animals.

Hatchlings drink a secretion

When observing females with their newly hatched offspring, the researchers became aware of a special behavior: the young animals seemed to ingest a secretion from the rear body opening of the mother. The team then followed up on this lead in more detail using various investigation methods. As it turned out, during the two-month period of brood care, the female actually produces a nutrient fluid for the young animals, which the team calls milk. Analyzes of this substance showed that it is a secretion rich in carbohydrates and fat, which is similar to the mother's milk of mammals.

Tissue studies have shown that milk is produced by special glands inside the mother's body. Previous studies have already shown that such milk-like nutrients are also produced by some viviparous amphibian species to feed the young during the gestation period. However, they only serve to supply the young animals with nutrients inside the body before birth. But in the case of the egg-laying Siphonops annulatus, the secretion is released after the young are born. It is directed to the back of the body and then offered to the hatchlings.

Begging sounds stimulate production

Interestingly, the animal only releases the rich substance when needed: the researchers found that the mother produces milk in response to the offspring's begging behavior. They not only move in a special way, but also make high-frequency sounds to signal hunger to the mother. After drinking their fill, the little ones no longer show this behavior and the mother stops producing milk, according to the observations.

According to the researchers, the nutrition of the young animals through the provision of skin fragments also plays a role in Siphonops annulatus. But as it became apparent, the milk supply is probably more important for the nutrient supply. After two months of care, the mother animals also appear extremely emaciated: they lose around 30 percent of their body weight, the researchers report.

They now see the discovery as another example of the diversity of brood care concepts in the animal kingdom. “The discovery that an egg-laying member of the creeping amphibian provides milk to its hatchlings is a phenomenon that illustrates the complexity of the evolution of reproductive systems in vertebrates and contributes to our understanding of the emergence of parallel developments,” write the authors.

Source: Science, 10.1126/science.adi5379

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