WASHINGTON (Reuters) - At least 11 species of African frogs carry a built-in concealed weapon -- they can sprout claws on demand to fight off attackers, U.S. researchers reported on Monday.

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When threatened, the frogs can puncture their own skin with sharp bones in their toes that they then use to claw their attackers, David Blackburn and colleagues at Harvard University reported.

“It’s surprising enough to find a frog with claws,” Blackburn, a graduate student, said in a statement.

“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 looked at museum specimens of 63 African frog species. In 11 central African species the bones at the ends of the toes 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 the frog flexes a certain muscle in the foot, the sharp bone separates from the nodule and bursts through the skin.”

While the finding is new to science, it is not news to locals. “Cameroonian hunters will use long spears or machetes to avoid touching these frogs,” Blackburn said. “Some have even reported shooting the frogs.”

For their part, the frogs probably use this defense rarely, Blackburn said.

“We suspect, since the frog does suffer a fairly traumatic wound, that they probably use these claws infrequently, and only when threatened,” he said.

“Most vertebrates do a much better job of keeping their skeletons inside,” he added.