September 24, 2008

Strange BiologyAfrican Frogs Hide A Secret Weapon

Claws that break out of the skin—that’s something that only occurs in the comic books, right? Well, move over, Wolverine, as you've got some real-world competition. Scientists working in the African country of Cameroon recently found a frog that breaks its own bones in order to produce claws when the frog feels threatened. These claws, found on the toes of their back feet, punch right through the skin for use in self defense.

David C. Blackburn, an evolutionary biologist, discovered the frogs' unusual form of protection after being scratched by one of the frogs he picked up in the Cameroonian forest. Blackburn was surprised (quite painfully) by the presence of the frog's claws. Upon return to Harvard University, where he was a PhD student, Blackburn searched through the literature to see if such a frog had been described before. In their research, he and his colleagues found that similar clawed frogs had been discovered in 1900 in the French Congo (now called the Republic of Congo) by the Belgian zoologist George Boulenger.

While examining specimens at Harvard's Museum of Comparative Zoology, Blackburn discovered that the Cameroonian frog had quite an unusual anatomy. The frogs' claws were actually normally a part of the toes, and only popped through the skin when absolutely necessary.

In these frogs, the claws are contained within a mass of connective tissue in the toe. The sharp end of the claw is attached to a small piece of bone at the top of the frog's toe. The other end of the claw is attached to a muscle. When the frog is provoked, the muscle pulls the claw downward, breaking the tip of the claw from the bone and cutting through the skin of the toe pad. When extended through the skin, the frog's claws look like small barbs on the underside of the frog's back feet.

A specimen from a frog in which the bones (claw and nodule) have been stained with red.

These frogs are the only vertebrates known to have claws that break through the skin in such a dramatic manner. And, unlike the claws of other vertebrates, which are made of keratin, the claws of these frogs are made of bone.

Because Blackburn and his colleagues were only able to study museum specimens, they are unsure what happens to the frog after its claws are extended. Forcing the claws through their toes creates a traumatic wound to the frog's feet. The researchers hypothesize that the claws slide back into the toes after the muscle relaxes, allowing the wound to heal.

Currently, the only known predators of the frog are Cameroonian people. They hunt the frogs using specialized, elongated spears that let them catch the frogs without getting scratched. The rediscovery of these unusual frogs illustrates how much remains to be discovered about amphibians.

More to Explore

• Concealed weapons: erectile claws in African frogs
• Frog Keeps Its Claws Hidden Until Needed
• 'Horror Frog' Breaks Own Bones to Produce Claws

Last updated September 24, 2008 | Comments (18) | Permalink | Strange Biology