'Wolverine' frogs pop retractable claws from their toes

By Ed Yong | May 27, 2008 7:00 pm





David Blackburn from Harvard University revealed how the claws work by studying 63 specimens housed in museums. Two genera in particular – Astylosternus (the night frogs) and Trichobatrachus (the hairy frog) – have well developed claws on eight of their hind toes. Claws are, of course, very common in the animal kingdom and cats also have retractable claws on their feet. But those of the frogs are completely unique for several reasons. 


For a start, they are made of naked bone and lack the sheath of keratin that covers the claws of all other land-living back-boned animals. The way they work is special too. When the claw is ‘at rest’, it lies completely within the flesh of the toe, held in place by a small, bony nodule, which is in turn anchored to the frogs’ skin. The nodule holds the claw in place, and prevents it from being accidentally extended while the frogs go about their day-to-day business.

When the frogs are threatened, they flex a muscle that connects to the lower half of the claw. This severs the connection to the nodule and pulls the claw downward. Its barb-like tip is made of reinforced and thickened bone, which easily pierces through the skin of the toe.

Among back-boned animals, no other claw needs to pierce its way to use. Only salamanders use a comparable technique; some species jab their bony ribs through special sites in their skin that contain poison glands. And unlike the toes of cats, the frogs have no permanent slots through which the claws extend. Every time they are used, they create new wounds. In the X-Men movie, Wolverine, when asked if it hurts to pops his claws, answers, “Every time.” One can’t help but think that the same is true for the frogs.

Blackburn isn’t sure what happens after the frog has deployed its secret weapon. It may be that the animal cannot actively retract the claws and has to wait for them to passively return to the ‘cocked’ position. Nor do we know if the broken connection between the bony nodule and the claw will regrow.

However, many amphibians have extraordinary healing abilities that can even regenerate severed limbs. It may be that the clawed frogs, like their comic-book counterpart, have a ‘healing factor’ that closes up the wounds that open every time their claws are used.

Reference: Proc Roy Soc B 10.1098/rsbl.2008.0219

Images: Frog by Gustavocarra; Wolverine by John Cassaday; toe images from Royal Society



Comments (10)

  1. Poor frogs, I see a big lawsuit from Marvel Comics in their future.

  2. All I have to say is “Cool!”. Too bad it’s the end of the school year – this stuff is amazing. Maybe I need to send a link to the other bio teachers, kinda just “FYI – It’s Cool”. Thanks for posting this. And people wonder why I think science is great.

  3. maliu

    check this out

  4. mike spear

    I wish I had retractable bone claws….

  5. inkadu

    I almost didn’t recognize the snikt without its clothes.
    Very cool stuff. Now how about evolution of claws for a follow up? How does this fit in an evolutionary scheme, compared to the evolution of my cat’s claws?

  6. wazza

    calling a superhero “Frog” just isn’t as cool.

  7. Jason Failes

    “Poor frogs, I see a big lawsuit from Marvel Comics in their future.”
    Or, perhaps some disreputable biotech firm will sequence the frogs’ DNA, copyright it, and will, since these frogs have presumably had this adaptation for much longer than 45 years, sue Marvel Comics.
    Far fetched? Yes. Ridiculous? Yes. Out of character for biotech firms? Sadly, no.

  8. tincture

    That is totally messed up and awesome. Love the blog.

  9. kevin z

    I was a big wolverine fan as a teenager! I immediately caught that. Excellent post and discovery.
    Or do you mean a MUTANT healing factor? Has anyone looked around Cameroon for nuclear plants, chemical factories, gamma radiation sources or nefarious military experimental bases?


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