[♪ INTRO] Retractable claws are pretty rare, but they aren’t unheard of in nature; cats, obviously, have been using them for centuries to destroy our furniture.

And most animals with these types of claws take them out when they need them, and put them away when they’re done. No drama.

No pain. No personal sacrifice. But there is one animal, though, that pays a horrible price for the use of its retractable claws.

It keeps the claws under its skin, which means using them is a pretty unpleasant proposition. Meet the hairy frog, also known as the wolverine frog, and, I mean, once you look at it, you’ll figure out why. They’re called “hairy frogs” because of the hairy filaments the males sprout during mating season, but that’s not the weirdest thing about them.

Hairy frogs use claws to defend themselves, which is not extraordinary in and of itself. But these “claws” are different. First of all, they aren’t even claws, technically speaking.

Real claws are made out of a protein called keratin, which is also present in other things like horns and beaks and human fingernails. But the hairy frog’s claws are not made of keratin, they’re made entirely from bone. In fact, they are kind of just another toe bone, except for their very unusual purpose.

These pointy, claw-shaped bones are hidden, like the rest of their bones, under the surface of their skin. So when a hairy frog feels threatened, there’s only one way it can access its defense system. The claws have to come out through the skin.

Doesn’t sound great! Researchers at Harvard University looked at some preserved hairy frogs to try and figure out how these unusual claws work. And they learned that the claw is attached with connective tissue to a bony nodule.

So when the frog feels danger, the claws disconnect from the nodules and slice their way through the skin. And researchers think it doesn’t pop out when the frog is just hopping around due to a special flexor muscle, which flexes the claw as it moves around. But when the claw is already out, the frog can inflict pretty severe damage to opponents by kicking at their attackers.

But of course, the claw doesn’t come out without cost. The frog has to injure itself in order to use these claws, and the injuries are not insignificant. Studying how they heal and retract their claws is kind of complicated.

See, researchers didn't study live frogs, so they aren’t sure how exactly the retraction part of the equation works. So far, in live frogs, the claws seem to retract into the toe and come back out again, but no one knows if the frog does this at will or if it is more of an involuntary thing. They also don't know what happens next, like after the claw goes back where it came from, the skin might heal over.

But scientists haven't actually seen this happen. They also don’t know if the claws will eventually reconnect to the bony nodule in the same way again, and if the frogs would have to injure themselves to use them again in the future. The trickier part of all of this is that there’s no other animal that has a structure like this.

The closest is a kind of newt that has bony ribs that pierce its skin when it feels threatened. So there isn’t really another organism they can extrapolate from. Because as far as claws go, this is the only known animal of any kind that keeps its claws beneath its skin.

And if you liked all this talk about frogs, I’m glad to announce that there’s more where that came from because a recent episode of our podcast, Tangents, is all about frogs! In this podcast, fun people involved in SciShow get together for a lightly competitive knowledge showcase. Like in that episode, we geek about what actually a frog is.

I mean, it’s not a fish, it’s not a lizard. What's up with that? So if you love science, laughing, and lighthearted, nerdy competitions, you should check it out!

You can find SciShow Tangents anywhere you get your podcasts. [♪ OUTRO]