Why The Duck-billed Platypus Sweats Milk

By D.C. Demetre •  Updated: 08/02/22 •  6 min read

When Europeans first came upon it in the 1700s, naturalists believed Australia’s duck billed platypus must be a hoax, stitched together by some prankster taxidermist. It had the beak of a duck, the tail of a beaver, webbed feet on four stumpy legs, and the furry body of an otter or mole, perhaps.

Now that it is recognized as a real species, Ornithorhynchus anatinus still confuses scientists. It lays eggs instead of giving birth to live babies like other mammals; it sweats milk; has venomous spurs, and is the only animal with 10 sex chromosomes.

duck billed platypus
In 2021, an international team of scientists created a genome map of the platypus, along with one for the only other egg-laying mammal, the echidna (Tachyglossus aculeatus). They aimed to investigate how these became two of the world’s oddest mammals.

“The complete genome has provided us with the answers to how a few of the platypus’ bizarre features emerged. At the same time, decoding the genome for platypus is important for improving our understanding of how other mammals evolved—including us humans. It holds the key as to why we and other Eutheria mammals evolved to become animals that give birth to live young instead of egg-laying animals,”

said biology professor Guojie Zhang, University of Copenhagen.

What Is A Monotreme

Monotremes are one of the three groups of living mammals, the others being placenta-bearing animals and marsupials. The only monotreme animals are found in Australia and New Guinea. Still, there is fossil evidence that they were once more widespread.

In fact, monotremes existed millions of years before the emergence of any modern-day mammal. In this ancient group, the 2 mammals that lay eggs are the platypus and the echidna.

“Indeed, the platypus belongs to the Mammalia class. But genetically, it is a mixture of mammals, birds, and reptiles. It has preserved many of its ancestors’ original features — which probably contribute to its success in adapting to the environment they live in,”

Zhang said.

Do Platypus Sweat Milk?

platypus do sweat milk

Credit: hobvias sudoneighm CC-BY

Just why are platypuses so weird? One of the duck billed platypus’ strangest traits is that, although it lays eggs, it also has mammary glands used to feed its babies, not through nipples, but from milk that it sweats from its body.

The research team was interested in the sex chromosomes of the monotremes, but the previous mapping of the platypus had only sequenced 25% of its chromosomes.

Vitellogenin, an egg yolk precursor that transports protein from the liver to become part of the yolk, is usually only found in the blood or lymph of egg-laying females. During our own evolution, humans dropped all three so-called vitellogenin genes. Chickens, on the other hand, continue to have all three.

Zhang et al. showed that platypuses still carry one of these three vitellogenin genes, despite having lost the other two roughly 130 million years ago, which allows them to continue to lay eggs. The platypus isn’t as dependent on creating yolk proteins as birds and reptiles since it produces milk for its young.

Jurassic Milk

In all other mammals, casein genes take the place of vitellogenin genes. Casein genes are responsible for the ability to produce casein protein, a significant component in mammalian milk.

The 2021 research found that platypuses also carry casein genes and that the composition of their milk is similar to that of humans, cows, and other mammals.

“It informs us that milk production in all extant mammal species has been developed through the same set of genes derived from a common ancestor which lived more than 170 million years ago — alongside the early dinosaurs in the Jurassic period,”

Zhang explained.

Do Platypus Have Teeth

Young platypuses have 6 teeth but they fall out around the time they leave the breeding burrow where they hatched. The adult platypus does not have teeth. Instead of teeth, platypuses have two horn plates they use to grind their food, which consists of worms, larvae of insects, shrimp and crayfish.

The genome mapping showed that the platypus lost its teeth roughly 120 million years ago when four of the eight genes dedicated to tooth development vanished.
Monotremes boast 10 sex chromosomes, with five Y and five X chromosomes. That compares to humans, and every other mammal on Earth, with two, the X and Y chromosome system in which XX is female and XY is male.

This is another reason why platypus genomes are weird ones. The study suggested that these 10 sex chromosomes in the ancestors of the monotremes organized in a ring form, later broken into many small pieces of X and Y chromosomes.

As a result, we know that most monotreme sex chromosomes have more in common with chickens than humans. It also shows an evolutionary link between mammals and birds.

Are Platypuses Venomous?

Platypus venomous spur

The platypus venomous spur. Credit: 1995 E. Lonnon CC-BY

The male platypus has a hollow spur on its hind foot connected to venom-producing glands. The platypus is not lethally poisonous to humans, although its effects are described as highly painful and long-lasting.

One early account of the effects of platypus venom was recorded by C. J. Martin and F. Tidswell in an 1895 article:

“…the platypus stuck its spurs into the palm and back of his right hand with such force and retained them in with such strength that they could not be withdrawn until it was killed. The hand instantly swelled to a prodigious bulkThe pain from the first was insupportable[he] did not recover the perfect use of his hand for nine weeks.”

It is currently unknown if the pain caused is a result of the swelling around the wound or if the venom has some chemical that acts directly on pain receptors.
A deeper understanding of how platypus venom produces its strong and lasting pain effects could help science understand mechanisms behind similar chronic pain states for which no effective medications are available, such as certain cancer-associated pain syndromes.

Sources:

Martin, C. J. and Tidswell, F. (1895) Observations on the femoral gland of Ornithorhynchus and its secretion; together with an experimental enquiry concerning its supposed toxic action. Proc. Linn. Soc. NSW. 9: 471-500.

Flannery, T. F.; Rich, T. H.; Vickers-Rich, P.; Ziegler, T.; Veatch, E. G.; Helgen, K. M. (2022). A review of monotreme (Monotremata) evolution. Alcheringa: An Australasian Journal of Palaeontology. 46: 3–20

Gerritsen, Vivienne Baillie (December 2002). Platypus poison. Protein Spotlight (29)

Zhou, Y., Shearwin-Whyatt, L., Li, J. et al. Platypus and echidna genomes reveal mammalian biology and evolution. Nature 592, 756–762 (2021).