Clawed Frog Carries a Concealed Weapon

In Cameroon no license is required, is the long and short of it.

Henry Fountain's June 17, 2008 New York Times Science section story featured the recent discovery of a heretofore little-remarked upon quirk of nature first noted over a century ago; the piece follows.

Frog Keeps Its Claws Hidden Until Needed

David C. Blackburn, an evolutionary biologist at Harvard who studies frogs, knew there was something different about the specimen he encountered one day while doing fieldwork in Cameroon. Not surprisingly, it kicked its hind legs wildly when he picked it up. But then Dr. Blackburn noticed that his arm had been clawed. “I got a real nasty scratch,” he said.

A frog with claws? Back at Harvard, Dr. Blackburn and colleagues consulted the literature, examined museum specimens and realized something even more unusual: the claws on the Cameroonian frog, and some related frogs from the same region, are normally contained inside the toes, but pop out through the skin when needed.

The odd bit of anatomy was mentioned in a paper more than a century earlier, but had been little commented on since. “We realized that this was something that was really strange and completely unappreciated,” Dr. Blackburn said.

Through dissection, the researchers discovered that the claw is the last bone of the toe — sharp, small and curved, and attached to an even smaller bony nodule that in turn is attached to a sheath of collagen. When the frog flexes a certain tendon, the bone pulls away from the nodule and pierces the skin. The anatomy is described in a paper in Biology Letters.



The researchers think that at some point the claw settles back inside the foot and the skin heals. “It really is a traumatic wound,” Dr. Blackburn said, and for that reason he thinks the frog doesn’t extend its claws frequently — probably only when threatened.

But so little is known about these frogs that the researchers aren’t even sure what threatens them — other than Cameroonians, who eat them and who know enough about the claws to have devised a special long spear to catch them without being scratched.

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The abstract of the Biology Letters paper follows.

Concealed weapons: erectile claws in African frogs

Vertebrate claws are used in a variety of important behaviours and are typically composed of a keratinous sheath overlying the terminal phalanx of a digit. Keratinous claws, however, are rare in living amphibians; their microstructure and other features indicate that they probably originated independently from those in amniotes. Here we show that certain African frogs have a different type of claw, used in defence, that is unique in design among living vertebrates and lacks a keratinous covering. These frogs have sectorial terminal phalanges on their hind feet that become functional by cutting through the skin. In the resting state, the phalanx is subdermal and attached to a distal bony nodule, a neomorphic skeletal element, via collagen-rich connective tissue. When erected, the claw breaks free from the nodule and pierces the ventral skin. The nodule, suspended by a sheath attached to the terminal phalanx and supported by collagenous connections to the dermis, remains fixed in place. While superficially resembling the shape of claws in other tetrapods, these are the only vertebrate claws known to pierce their way to functionality.

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Said Blackburn in a Harvard Science story about the frog (below), "It's surprising enough to find a frog with claws.... 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."



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