When last we checked on the platypus, it was confounding our expectations of mammals with its webbed feet, duck-like bill and laying of eggs. More than that, it was producing venom.

Now it turns out that even its coat has been hiding a secret — when you turn on the blacklights, it starts to glow.

As noted in the journal Mammalia, shining an ultraviolet light on a platypus makes the animal’s fur fluoresce with a greenish-blue tint. They’re one of the few mammals known to exhibit this trait. And we’re still in the dark about why they do it — if there is a reason at all.

For most humans, ultraviolet light exists outside of the visible spectrum. But certain pigments can absorb it, drain off some of its energy, and re-emit what remains as a color that people can see. Many man-made things contain such pigments, including white T-shirts, Froot Loops and petroleum jelly.

A lot of living things do, too. Scorpions, lichens and puffin beaks all pop under UV light.

Mammals, though, have generally gotten the short end of this paintbrush: So far, not many have been found to fluoresce. There are exceptions, all among nocturnal creatures.

Then one night a few years ago, Jonathan Martin, an associate professor of forestry at Northland College in Ashland, Wis., was exploring the woods with a UV flashlight when he saw that a flying squirrel appeared bright pink. It turns out, he discovered, that all three North American flying squirrel species give off a glow under UV light.

Erik Olson, an associate professor of natural resources, wondered how broadly distributed this trait might be. “Like, what about platypuses? That’s kind of as far from flying squirrels as you can get.”

The team went to the museum’s platypus cabinet “and sure enough,” Olson said.

So why would a platypus fluoresce? “We really don’t know,” Olson said.

Other instances of life-form Lite Brite serve a clear purpose. Bioluminescence helps ocean creatures lure prey and find each other in the depths.

Fluorescence, though, is a bit more opaque. Because it’s a natural property of certain materials, “just finding fluorescence doesn’t mean it has any particular purpose,” said Sönke Johnsen, a sensory biologist at Duke University. The glow could be “just something that’s there because it’s there.”