Platypus Hair Shares a Puzzling Feature With Bird Feathers, Adding to the Egg-Laying Mammal’s List of Unusual Characteristics

The species’ melanosomes—tiny, pigment-filled structures inside hair cells—are hollow, a trait never before seen in mammals

Sarah Kuta | Daily Correspondent

March 23, 2026

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Platypuses are true oddballs. They look like a mish-mash of several animals, with duck-like bills, beaver-like tails and otter-like bodies. They’re egg-laying mammals, and they hunt by sensing their prey’s electric signals caused by muscle movement. They glow bluish-green under ultraviolet light and they have five times the number of sex chromosomes as most other mammals. Mature males are venomous, while females produce milk but lack nipples.

Now, a new discovery adds yet another unusual characteristic to this list. Platypus melanosomes—the tiny, pigment-filled structures inside the animals’ hair cells—are hollow, a trait previously thought to be unique to bird feathers, according to a study published March 18 in the journal Biology Letters.

“Excitingly, over 200 years after its description as something in between birds and mammals, we find additional convergence between the platypus and birds,” the researchers write in the paper.

Melanosomes are small structures that help determine the color of feathers, fur, skin, hair and eyes. In mammals, melanosomes are usually solid. But, in birds, these structures can be hollow.

Researchers at Ghent University in Belgium were compiling a database of mammal melanosomes when they noticed something strange: Platypus melanosomes seemed to be empty inside. So they investigated further by sampling hair from the bodies of 10 additional platypuses, then looked at them under a high-powered microscope. They also examined fur from two species of echidna—the platypus’s closest living relative and fellow egg-laying mammals—as well as several species of marsupials, including a wombat and a Tasmanian devil. The platypus hairs were the only ones that turned out to be hollow, the team found.

After searching through the scientific literature, the researchers determined that the platypus is the only known creature with hollow melanosomes out of the 126 mammal species whose melanosomes have been studied. While it’s possible that other mammals also have hollow melanosomes, “I find it very, very unlikely that it wouldn’t have been found already,” lead author Jessica Leigh Dobson, a biologist at Ghent University, tells Science News’ Jude Coleman.

The researchers also discovered that platypus melanosomes have a unique shape. They’re not only hollow, but also spherical—an unusual combination that has never been observed in vertebrates. In birds, hollow melanosomes are typically rod-like or flattened, not round. Some mammals have spherical melanosomes, but they’re solid. 

An analysis of the melanin, or pigment, inside the platypus hair also produced some surprising results. The researchers found mostly eumelanin, which produces browns and blacks, with potential trace amounts of pheomelanin, which creates red and yellow hues. While the finding aligns with platypuses’ dark coloration, it’s perplexing because phaeomelanin, not eumelanin, is usually associated with spherical melanosomes.

“This doesn’t really conform with what we currently know about how melanosome shape correlates with color,” Dobson tells BBC Wildlife Magazine’s Helen Pilcher.

What’s more, in birds, hollow melanosomes contribute to iridescence, the optical phenomenon that makes some animals’ feathers take on shifting, rainbow-like hues from certain angles. But platypus hair is not iridescent—it’s simply drab shades of brown. And mammals that do have iridescent fur—such as golden moles and giant otter shrews—have solid melanosomes.

For now, researchers don’t know why platypuses have hollow melanosomes. They suspect this adaptation might have something to do with the creatures’ aquatic habitat; perhaps the melanosomes help with insulation, for example. “My gut feeling is it’s nothing to do with color, it’s to do with some other lifestyle attribute,” says Tim Caro, an evolutionary ecologist at the University of Bristol in England, who was not involved with the research, to Science News.

This proposal, however, raises the question of why hollow melanosomes haven’t been found in other aquatic mammals. “Further work is definitely needed to find out why they have them,” Dobson tells Bénédicte Salvetat Rey of the Agence France-Presse.

Since platypuses share several traits with birds, the researchers now wonder whether they also have similarities in the genes responsible for melanin production and melanosome formation—an area ripe for future studies.

Sarah Kuta | Read More

Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.