They were long thought to be easy prey for predators. But at least 11 species of African frogs carry a built-in concealed weapon: claws that can sprout out on demand to fight off attackers.

US researchers on Monday reported that when threatened these frogs can puncture their own skin with sharp bones in their toes, which they then use to claw their attackers.

David Blackburn, graduate student, and his colleagues at Harvard University said in a statement: "It's surprising enough to find frogs with claws."

"But the fact that those claws work by cutting through the skin of the frogs' feet is even more astonishing. These are the only vertebrate claws known to pierce their way to functionality."

Blackburn became aware of the frogs when one scratched him in Cameroon.

He studied museum specimens of 63 African frog species, and found that the bones at the ends of 11 central African species were pointed and hooked, with smaller, free-floating bones at their tips.

"These nodules are also closely connected to the surrounding skin by dense networks of collagen," Blackburn said. "It appears they hold the skin in place relative to these claw-like bones, such that when they flex a certain muscle in the foot, the sharp bone separates from the nodule and bursts through the skin."

The finding is new to science but not new to locals. "Cameroonian hunters use long spears or machetes to avoid touching these frogs," Blackburn said. "Some have even reported shooting the frogs."