Platypus Fur Glows Blue-Green Under Ultraviolet Light

At dusk, on a quiet creek in eastern Australia, a platypus slips into the water and paddles off along the bank, its sleek brown coat beaded with silver drops. To our eyes it is about as plain as an animal can be: brown on top, a little paler underneath. Shine an ultraviolet lamp on that same fur in a dark room, though, and it lights up with a soft blue-green glow.

Nobody went looking for this. In 2020 a team led by the mammalogist Paula Spaeth Anich of Northland College in Wisconsin reported it in the journal Mammalia, and the discovery came almost by accident. The researchers had noticed a flying squirrel glowing under ultraviolet light, went to museum collections to check more squirrels, moved on to the marsupials, and found that the drawer next door held platypuses.

What the scientists actually saw

The team examined platypus specimens at the Field Museum in Chicago and the University of Nebraska State Museum, collected long ago in Tasmania and New South Wales. Under ordinary light the fur was evenly brown. Under ultraviolet it glowed green to cyan. The fur takes in invisible ultraviolet light, at wavelengths of roughly 200 to 400 nanometres, and gives back visible light that peaks at about 500 nanometres. That is fluorescence, and it is not the same as the light of a firefly: the fur makes no light of its own, it only changes the colour of the light that falls on it.

• 200–400 nm the ultraviolet light the fur takes in
• ≈ 500 nm where its glow peaked in the 2020 study
• 125 mammal species found to fluoresce in a 2023 museum survey

What people often get wrong

The tempting story is that the platypus has a glowing superpower all its own, one more oddity to add to the duck-like bill and the eggs. The evidence points the other way. In 2023 researchers from the Western Australian Museum and Curtin University examined mammals from the museum's collection with a spectrometer and found fluorescence in all 125 species they looked at, from every living order of mammals. Pale, unpigmented fur glowed most, and the authors concluded that for many species the glow is likely to be simply a consequence of that pale fur.

The two studies do not quite agree on the colour, either. The 2023 team found the platypus glowed brightest on its pale belly fur, with a blue peak near 450 nanometres, while the 2020 team described green to cyan near 500. And a fresh specimen glowed less than ones preserved in the museum for many years, which led the 2023 authors to suggest that the glow plays a minor role, if any, in the animal's life.

Why the glow might matter, or might not

Scientists genuinely disagree about what the glow does. Anich has suggested that fur which soaks up ultraviolet light might help a mostly nocturnal animal stay less visible to predators that can see ultraviolet. An Australian wildlife ecologist quoted by Science News doubted that, and wondered instead whether the glow helps solitary platypuses notice one another. The 2023 study adds a third possibility: that it does nothing at all and is just a side effect of pale fur. For now, nobody knows which is right.

What is certain is that our eyes are not the measure of what an animal looks like. By a creek under the moon, a platypus may look quite different to some other creature than it does to us. So the next time you meet a plain brown animal, remember that it may be keeping a glow that only the right light reveals.