Rogue: When they come out... does it hurt?
Wolverine: Every time.
That snippet from the X-Men movie concerns the metal claws Wolverine can extend through his hands on mental command. Now we know some frogs actually do something similar.
A team led by Harvard University biologist David Blackburn was spurred to investigate African frogs after Blackburn was scratched in the field by animals that didn't seem to have anything to scratch with. It had long been known that several species of the family Arthroleptidae sported small bony nodules near the fingertips, but no one had ever investigated these properly. Now we know they are claws that can be extended through the skin, at the cost of a non-trivial injury to the frog.
COMMENT: It's another reminder of how endlessly inventive nature can be. Still, the evolutionary reasons why the frog would have a hidden defense of this type are puzzling. Why not have the claws out all the time and avoid all that trouble, or develop retractable sheaths like cats? We don't know.