X-Men fans rejoice: Wolverine has come to life, as a frog. When the comic book warrior faces a fight, metallic blades spring forth from his hand. A new study concludes that certain African frogs are similarly equipped, having sharp, claw-shaped bones that pierce through their own fingertips when the animal is threatened.

More than 100 years ago, scientists observed the mysterious bony appendages in museum specimens of the Arthroleptidae frog family, but they had no idea what to make of them. Some speculated that the protrusions were an artifact of the preservation process. Harvard University biologists David Blackburn decided to solve the mystery once and for all after having the frequent misfortune of being injured by the amphibians while doing field research in Cameroon. "The frogs will start kicking and drag these claws against your skin," he says. "I've gotten bloody scratches from them many a time."

Due to strict government regulations on removing live animals from Cameroon, Blackburn's team had to do their anatomical studies on preserved museum specimens. In addition to the talon-shaped finger bones others had seen, the researchers found a small bony nodule nestled in the tissue just beyond the frog's fingertip. When sheathed, each claw is anchored to the nodule with tough strands of collagen, but, as Blackburn had discovered firsthand, when the frog is grabbed or attacked, the frog breaks the nodule connection and forces its sharpened bones through the skin.

This bizarre skeletal feature is found in only 12 species within the Arthroleptidae family, Blackburn's team reports online this week in Biology Letters. Why some members of this family developed such a dramatic form of defense is still a mystery, though the researchers speculate that because amphibians have a remarkable flair for regeneration, the African frogs may heal up afterward, just like Wolverine.

Amphibian researcher and biologist David Wake of the University of California, Berkeley, says that this type of weaponry appears to be unique in the animal kingdom. But David Cannatella, a herpetologist at the University of Texas, Austin, questions whether the bony protrusions are meant for fighting. They could allow a frog's feet "to get a better grip on whatever rocky habitat they might be in," he says.

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