wolverine frog

June 24, 2008—Eleven species of African frogs—including Trichobatracus robustus (top) and Astylosternus perreti (bottom)—sport a Wolverine-like defense mechanism, scientists have announced. When threatened, the amphibians pierce their skin with toe bones, sprouting makeshift claws with which to attack predators.
David C. Blackburn, a biologist at Harvard University, came across the frogs while conducting fieldwork in Cameroon. When he picked up one of the fist-size amphibians, it kicked its hind legs violently.
"I was surprised to come across frogs that can give you such a nasty scratch when you pick them up," Blackburn said. "When I got back to the U.S., I used preserved museum specimens to study the anatomy of these claws, because it was obviously pretty unusual."
After going through 63 species of African frogs, Blackburn found that in at least 11, the bones at the ends of the toes are connected to smaller and sharper free-floating bones. These smaller end bones are part of structures called nodules that are connected to the rest of the foot by a collagen-rich sheath. (Related: video: "See-Through Frogs Bred By Japanese Researchers" [October 1, 2007].)
By flexing a certain foot muscle, the frog causes the bone to retreat from the nodule and pierce the skin, revealing a clawlike structure. Unlike ordinary claws, such as those of a cat's, the frog bones do not possess a protective coating of a protein known as keratin, nor do they emerge from a specialized structure in the foot.
The frogs, all in the genera Astylosternus, Trichobatracus, or Scotobleps, appear to employ this mechanism only when threatened, as revealing the claws causes traumatic damage to the frogs' skin.
Blackburn's study is described in a forthcoming issue of the journal Biology Letters.

http://news.nationalgeographic.com/news/2008/06/080624-frog-claws.html