Duck-billed platypus (Ornithorhynchus anatinus), Scottsdale, TAS, Australia. Credit: Charles J. Sharp. Creative Commons Attribution-Share Alike 4.0 via Wikimedia Commons

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— Digging Deeper with K.J. Britt (@kjbritt.bsky.social) April 4, 2026 at 5:50 AM


It’s no understatement to say that the platypus (Ornithorhynchus anatinus) is a weird animal. It’s a mammal. But it lays eggs and has a bill like a duck. And uses electrolocation like a shark. And it’s also venomous. Weird.

A new study in the Royal Society Publishing’s Biology Letters shows that the platypus is even weirder than that.

The study, published on March 18, A unique hollow melanosome morphology in the hairs of the platypus Ornithorhynchus anatinus, describes a new organelle never seen before in mammals. The paper’s lead author, Jessica L. Dobson is a research explorer with Ghent University in Belgium.

The unique organelle is a hollow, spherical, melanosome found in the platypus’ fur. Melanosomes contain melanin pigments which give the platypus (and many other animals) their dark coloration. Prior to this study, Dobson explained, scientists believed that hollow melanosomes “occurred only in birds”. Different types and shapes of melanosomes can produce different different color patterns in birds and mammals, the the melanosomes in other mammals are “always solid”, making the platypus’ hollow melanosomes so unusual.

The study examined the melanosomes from hairs of 126 different species of mammals, including three monotremes: the platypus, the western long-beaked echidna and the short-beaked echidna.

The monotremes are a group of egg-laying mammals that include the platypus and four species of echidna. Yet, even the platypus’ close relatives lacked the unique melanosome. “We found no evidence of hollow melanosomes in the other two monotreme genera or in any other mammal so far examined,” Dobson wrote.

The origins of mammalian coloration

Dobson was also a co-author on a paper published last year, Mesozoic mammaliaforms illuminate the origins of pelage coloration.

The study examined melanosomes from 116 species of living mammals to “reliably reconstruct the coloration of six Mesozoic mammaliaforms”. Mammaliaforms are a clade of related animals that include all living mammals and some close, extinct, relatives.

The study found that the hairs of Mesozoic mammaliaforms would have been dark or camoflauged, suggesting they were nocturnal and probably hiding out. The authors wrote that

“results suggest that the melanosome variation and color expansion seen in extant mammals may have occurred during their rapid radiation and diversification after the Cretaceous-Paleogene extinction.”

While many mammals today are brown like the platypus - you know, moose, marmots, muskox, so on - the varied color patterns on other mammals (seen today in animals like tigers, leopards and zebras) would have evolved in the Tertiary, after the death of the non-avian dinosaurs.