Thanks to Brilliant for supporting this episode of SciShow.

Go to Brilliant.org/SciShow to learn how you can take your STEM skills to the next level this year. {♫Intro♫}. We’re not going to sit here and pretend that platypuses aren’t weird.

They’re mammals but they have a duck face and they lay eggs and they also have venom. Of all the weird animals that exist in the world, they’re up there with the weirdest. And by the way... they’re also fluorescent.

That thick, waterproof fur that reassures us that platypuses are, in fact, mammals also glows in the dark. Before you dismiss all of platypus-kind as just trying to mess with our understanding of reality, though, the fact is this ability isn’t all that weird -- and might be way more common than we realize. Fluorescence refers to the ability of a material to absorb light and then re-emit it in another color.

This is different from bioluminescence, which is when an animal can produce light through a chemical reaction. And actually… lots of animals are fluorescent. Scorpions have fluorescent bodies, for example, and puffins have fluorescent markings on their beaks.

But up until recently, there weren’t too many mammals known to fluoresce — mostly just opossums and their close marsupial relatives. Then New World flying squirrels, which are placental mammals, were found to have a similar talent — they fluoresce pink. And in a 2020 study, researchers shone a UV flashlight at a couple of preserved platypus specimens in a museum -- and found them to glow a lovely shade of cyan.

Scientists aren’t yet sure why any animals, including mammals, would want to fluoresce. So far, all the fluorescent mammals we’ve discovered are either nocturnal or crepuscular -- meaning active during sunrise and sunset. The diurnal, or daytime-active, species we’ve examined don’t fluoresce -- meaning we day dwellers lost out on looking awesome at raves.

Crepuscular animals, like the flying squirrels, are active when UV light is more intense but visibility is otherwise poor, so the ability to absorb UV and emit it as visible light might help them see each other. For platypuses, we think fluorescence might help them hide. Some of their predators use UV light to see, and fur that absorbs UV light might be harder to spot.

But here’s the thing: we can all agree that platypuses are mammalian oddballs, but fluorescence might not really be that weird. It’s actually even possible it’s common. Most of us don’t go around shining black lights at raccoons or coyotes or whatever, so there could be a bunch of fluorescent mammals right under our noses.

And we’ve got reason to think fluorescence could be widespread. Flying squirrels are placental mammals. Opossums are marsupials.

And platypuses are monotremes. That’s all three major groups of mammals. In evolutionary biology, when all the members of a group have a trait, we usually infer they inherited it from their common ancestor.

That means mammalian fluorescence could be an ancestral trait. Though these animals could also have evolved it separately, as an example of convergent evolution. More research is needed, but we should be able to figure this out by looking at whatever genetic mechanism makes these mammals glow.

If it’s the same in all mammals, that hints at it being ancestral. Convergent evolution tends to co-opt different genes to do similar jobs. But it might make sense for all mammals to have been fluorescent back in the day.

You see, early mammals were nocturnal by necessity — keeping to the shadows to avoid becoming dinosaur chow. And fluorescence could have given those early night dwellers advantages just as it does now. So platypuses may have a lot of weird qualities, but the whole glow in the dark thing probably isn’t one of them.

And that in itself is pretty weird. Fluorescence is just one place where biology meets physics. And if you want to know more about how these platypuses come by their bizarre glow, you might enjoy Brilliant’s course Waves and Light.

It’ll teach what waves are and how they travel, from glowing critters to earthquakes. And if you want to learn even more, you’re in luck, because Brilliant has loads of courses designed by leading educators and lifelong learners, to help you understand the world around you. The first 200 people to sign up at Brilliant.org/SciShow will get 20% off an annual premium subscription to Brilliant.

So if you’d like to start off your year right, head on over and see if Brilliant is right for you. {♫Outro♫}.