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Platypuses Glow Green Under Ultraviolet Light

The web-footed monotremes join a small cast of fluorescent, nocturnal mammals

Platypuses' nocturnal nature made researchers suspect they might glow under ultraviolet light. ( Mammalia 2020; 10.1515/mammalia-2020-0027)
smithsonianmag.com

Disney Channel may not have been far off with when they colored Perry the Platypus bright teal. It turns out real life platypuses are blue-green, too—at least when they’re under an ultraviolet spotlight.

Research published in the journal Mammalia last month shows that if scientists shine light with wavelengths between 200 and 400 nanometers—that’s ultraviolet, just a little too short for humans to see—on a platypus, then the animal’s brown body reflected back bright blue-green light with a wavelength between 500 and 600 nanometers, George Dvorsky reports for Gizmodo. Emitting light of one color after absorbing light of a different color is called fluorescence, and scientists only recognize a couple of known fluorescent mammals. Now platypuses have joined the exclusive club.

The authors of the study specifically looked at two platypus specimens kept at the Field Museum of Natural History in Chicago and one specimen kept at the University of Nebraska State Museum.

"It was a mix of serendipity and curiosity that led us to shine a UV light on the platypuses at the Field Museum," said lead author Professor Paula Spaeth Anich, associate professor of biology and natural resources at Northland College in a statement. "But we were also interested in seeing how deep in the mammalian tree the trait of biofluorescent fur went.”

Platypuses are semi-aquatic, egg-laying mammals that live in eastern Australia. When Europeans first laid eyes on preserved platypus skins, they suspected that they were the Frankenstein-style result of a taxidermy trick, Mindy Weisberger writes for Live Science.

But they’re no hoax—platypuses have bills like ducks, webbed feet like otters, and tails like beavers, making them perfect for swimming along the bottoms of rivers, lakes and streams. Males have venomous spurs on the inner side of each ankle, and females sweat milk from pores on their stomachs. They’re most active at night when they hunt frogs, fish and insects in the water.

Their nocturnal nature is what caught these researchers’ attention. The same team of scientists previously discovered that flying squirrels are fluorescent, Meilan Solly wrote for Smithsonian in 2019. That discovery happened by accident—while the researchers were studying lichen, one pointed a UV flashlight at a flying squirrel that was chowing down at a birdfeeder. Surprisingly, it lit up bright pink.

Before then, the only known fluorescing mammals were Didelphidae marsupials, which include two dozen species of American opossums.

“The lesson is that, from our diurnal primate standpoint, we are overlooking many aspects of animal communication and perception that happen at twilight and night-time,” Northland College biologist Paula Spaeth Anich, co-author on both the flying squirrel and platypus papers, told National Geographic’s Jake Buehler in 2019.

With that in mind, they decided to check whether platypuses glow. They found that all three specimens, which included a male and a female, glow blue-green under UV light.

The last common ancestor of monotremes like platypuses, marsupials like opossums, and placental mammals like flying squirrels lived about 150 million years ago, reports Gizmodo. Which begs the question, did fluorescence evolve in mammals before the three branches split apart? Or did these species all evolve the same solution to common challenges of living in the dark?

There are lots of animals beyond mammals that glow under ultraviolet light. Puffin beaks fluoresce, as do scorpion and millipede shells, tree frog skin, even the fossils of some crustaceans, Julissa Treviño reported for Smithsonian in 2018.

As for how platypuses make use of their fluorescence, the researchers conclude that it probably isn’t involved in mating, because both the male and female specimens glowed equally. So it’s possible that the fluorescence helps the animals see and interact with each other at night, or that it might help the small mammals hide from predators, per Live Science.

“Field-based research will be essential to document platypus biofluorescence and its ecological function in wild animals,” the researchers conclude in their paper.

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