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"Amphibian horror" isn't a movie genre, but on this evidence perhaps it should be. Harvard biologists have described a bizarre, hairy frog with cat-like extendable claws.Trichobatrachus robustus actively breaks its own bones to produce claws that puncture their way out of the frog's toe pads, probably when it is threatened.David Blackburn and colleagues at Harvard University's Museum of Comparative Zoology, think the gruesome behaviour is a defence mechanism.The researchers say there are salamanders that force their ribs through their skin to produce protective barbs on demand, but nothing quite like this mechanism has been seen before.The feature is also found in nine of the 11 frogs belonging to the Astylosternus genus, most of which live in Cameroon.Instant weapon"Some other frogs have bony spines that project from their wrist, but in those species it appears that the bones grow through the skin rather than pierce it when needed for defence," says Blackburn.At rest, the claws of T. robustus, found on the hind feet only, are nestled inside a mass of connective tissue. A chunk of collagen forms a bond between the claw's sharp point and a small piece of bone at the tip of the frog's toe.The other end of the claw is connected to a muscle. Blackburn and his colleagues believe that when the animal is attacked, it contracts this muscle, which pulls the claw downwards. The sharp point then breaks away from the bony tip and cuts through the toe pad, emerging on the underside.