When in danger, certain African frogs have a bizarre defense — claws hidden entirely within their toes can burst through their skin.
With powerful thrashes and kicks, these concealed weapons can readily draw blood, scientists now reveal.
Do these claws bring the X-Men's Wolverine to mind — a comic-book superhero armed with claws that pop out from his limbs?
"But of course!" said researcher David Blackburn, an evolutionary biologist at Harvard University.
The claws of most vertebrates — that is, creatures with backbones — are normally composed of keratin, the same hard substance found in nails and hooves, covering the bones at the very tips of fingers and toes.
But the claws now found on the hind feet of several frogs in Central Africa are unlike others found in any vertebrates alive today. These unique weapons lack any keratin covering — they are naked bone.
Scientists actually first noticed these claws more a century ago.
"The first mention of claws in relation to these frogs was published in 1900 by the Belgian zoologist George Albert Boulenger," Blackburn said.
However, Boulenger and later researchers were unsure of the significance of the claws, and often interpreted their piercing of the skin as abnormal or as inadvertent results of how the frogs were preserved. Despite several brief mentions of these claws in the early 20th century, "there has never been any detailed study of the anatomy or inquiry into the way in which these structures might work," Blackburn said.
Blackburn first became interested in these claws while collecting live frogs in Cameroon.
"I was surprised by several frog species that kicked wildly and seemed to be scratching my skin," Blackburn recalled. "When I asked around as to whether anyone was aware of these unusual claws, few of my colleagues knew much if anything about them."
He and his colleagues investigated the anatomy of museum specimens of 63 species of frogs preserved in a formaldehyde solution. Of these, 11 had these claws. Both males and females possess them.
These sharp claws are held in place by specialized tissues within the toe. Based on the anatomy of the amphibians, in order to defend themselves with these hidden weapons, it seems that muscles in the toes can flex the claws, causing their curved, barb-like tips to break free of their moorings and puncture the skin. They are the only vertebrate claws known that pierce their way out to go into service.
Feared by hunters
The frogs then rake their claws in the hope of tearing into the skin of their foes. They claws are capable of inflicting deep bleeding wounds, the researchers noted — Cameroonian hunters who catch the frogs for food use long heavy spears to kill them at a safe distance.
"Humorously, while professional scientists were mostly or entirely unaware of these unusual claws, people in Cameroon are well aware of them," Blackburn said. "These frogs can also be hunted by catching them off-guard with a machete chop to the head. I have even had hunters relate to me that they shoot them with a gun!"
The claws may eventually retract back inside, and the skin and flesh of the frogs may then regenerate and heal the puncture wounds the claws created, although that "remains to be seen," Blackburn said.
"Interesting and unusual discoveries can be made in a museum just as often as out in the field studying living animals," Blackburn said.
Blackburn and his colleagues James Hanken and Farish A. Jenkins Jr. detailed their findings online May 28 in the journal Biology Letters. Financial support was provided by the National Science Foundation-funded AmphibiaTree project and a Putnam Expeditionary Grant.