• Evolution

  Like the X-men character Wolverine, the frog Trichobatrachus robustus has concealed bone claws. Unlike Wolverine, T. robustus has to break its own bones to extend them.
  
  This incredible defence mechanism is made possible by a number of adaptations. At rest, the claws are in a mass of connective tissue. One end of the bone is connected by a chunk of collagen to another small bone at the tip of the

  frog's toe, while the other end is connected to a muscle. David Blackburn (Harvard University) and his team believe that when the frog is attacked, this muscle contracts and the claw pierces the toe pad. Unlike other retractable claws, these claws are made of bone (rather than keratin, as is usually the case) and come straight through the skin.
  
  Blackburn had found out about this ability firsthand while on an expedition in Cameroon in 2008. As he picked up a frog it waved its hind legs around wildly, drawing blood when they connected with him. Once back in the U.S., he examined museum specimens of Central African frogs and found 11 species that had this ability, all in the genera Trichobatrachus, Astylosternus and Scotobleps. It's thought that once the threat has passed, the muscle relaxes - withdrawing the bones back into the foot - and the tissue heals. Savage defenses like this have been seen before in amphibians (the Spanish newt forces its ribs through its skin) but as Blackburn says, "These are the only vertebrate claws known to pierce their way to functionality."
  
  Photo credit: Gustavocarra / Creative Commons License.
  
  For more on the Spanish newt, see: http://tinyurl.com/alnqvka
  
  Sources:
  http://www.newscientist.com/article/dn13991-horror-frog-breaks-own-bones-to-produce-claws.html
  
  http://www.sciencedaily.com/releases/2008/06/080623125003.htm
  
  For Blackburn's paper, see: http://rsbl.royalsocietypublishing.org/content/4/4/355.full