Platypus is even rarer than previously thought, scientists say

This small animal, endemic to the east coast of Australia and Tasmania, is one of the five representatives in the world of the order of monotremes, the only mammals that lay eggs. The other four are species of echidnas, also typical Australians.

It is also one of the few venomous mammals since the male carries a kind of stinger on its hind legs capable of inoculating poison.

Now one more rarity is added to this extraordinary list, this time hidden in its melanin, as shown in a study published Wednesday in the journal Biology Letters of the British Royal Society.

In vertebrates, these pigments protect against ultraviolet rays, contribute to thermal regulation and are responsible for the color of skin, hair and feathers.

They are contained in small structures within cells called melanosomes, whose shape is closely related to color.

Eumelanin, which gives black, gray and dark brown hues, is usually found in elongated melanosomes. Pheomelanin, which generates red, red and certain orange/yellow tones, is contained in spherical melanosomes.

In mammals these melanosomes are always full.

In birds, the opposite occurs: some elongated melanosomes may be hollow or flat, covered only by a thin layer of melanin, which improves their ability to produce different ranges of colors. Arranged in nanostructures, they sometimes produce iridescent colors that interact with light, as in peacock feathers.

The researchers, by creating a database on these melanosomes in mammals, made an “extremely surprising and exciting” discovery, Jessica Leigh Dobson, a biologist at Ghent University (Belgium) and lead author of the study, told AFP.

The platypus’s melanosomes are mostly spherical, which should logically give the animal a red or orange coat.. But this animal is dark brown.

And, most surprising, some of its melanosomes are hollowlike those of birds.

“We also see it in many other mammals […]: echidnas, marsupials, rodents, carnivores, primates and many more. To our knowledge, it is the only example of hollow melanosomes in mammals,” says Dobson.

These melanosomes are dispersed “randomly” in the hair cortex and do not produce any iridescence, adds the scientist. “More research is undoubtedly necessary to understand the reason for its presence.”

The origin of these structures is also unknown.

The ancestors of the platypus and echidnas were probably burrowing aquatic animals, and hollow melanosomes could be an adaptation to an aquatic way of life, as they improve thermal insulation.

“Given that modern echidnas are today terrestrial, this transition could have caused the loss of the hollow melanosomes in them, while they would have been preserved in the platypus, which remains semi-aquatic,” says Dobson.

This hypothesis raises another question, the study concludes: if so, “why is this trait not more widespread among aquatic mammals?”