Shoot-out superhero claws

Those superhero claws that snick out of some tough guy’s hands when the fighting starts — they’re real all right. Just a little small. And on frogs.

And maybe the strangest part is that these pop-out claws don’t slide through little slits. The claws in two genera of African frogs have to break through the frog’s own skin before tearing into an attacker, says David C. Blackburn of HarvardUniversity.

At least 10 species of frogs in those two African genera can flex out the last bone on four toe tips of each hind foot, Blackburn and his colleagues report in an upcoming Biology Letters. The paper gives the first detailed analysis of the claw mechanism, Blackburn says.

Early descriptions of several frogs in Central Africa mention sharp claws, but Blackburn says he wasn’t thinking about claws when he picked up live frogs in the field while working in Cameroon and was given some bloody scratches. “I was totally befuddled,” he says.

The experience inspired him and his colleagues to analyze the claw structure in museum specimens of these species. Blackburn reports that the claw is an extra spike attached to the front of the bones of the toe. The hairy frog, Trichobatrachus robustus, can extend the little spikes, as can nine species of Astylosternus that he examined. The hairy frogs look novel to begin with, with a fringe of little projections of skin along their thighs that gives them a thick fringe. The others look more conventional as dark residents of rivers in forested mountains.

“Most vertebrates do a good job of keeping their bones in their fingers,” Blackburn says. When disturbed, the frog frees the spur – its tip snapping loose from an internal structure and slicing down through the smooth skin. Whether the spur retracts actively and the skin heals in living frogs isn’t clear yet, Blackburn says.

Some of the residents of the region hunt the larger frogs for food, Blackburn says. Hunters showed him the spiked sticks they carry to avoid getting clawed during a frog hunt.