Is Wolverine actually an amphibian?

Frogs in Africa are able to protect themselves with bones that break out of their skin

Anyone who has seen the movie X-Men surely remembers the character of Wolverine, played by Hugh Jackman. In the original comics, Wolverine was a wild man in yellow tights who found a solution to every problem using long bone teleps that sprouted from his palms. The telepaths tear their way out of his own skin as well - a particularly unpleasant phenomenon - but due to his enhanced healing powers, Wolverine usually ignores this problem.

When Len Wayne first invented Wolverine, he certainly didn't imagine that there was an animal in nature that mimicked the superhero's abilities. But recently researchers from Harvard University decided that even African frogs are able to make sharp bones emerge from their skin and use them as weapons against predators. The bones tear the skin of the frogs in the process, but the frog comes out alive if it managed to keep the predator away from it.

"It's surprising enough to find a frog with claws," says David K. Blackburn, a PhD student in Harvard's Department of Evolutionary and Organismal Biology. "The fact that these claws work by cutting through the skin of the frogs' legs is even more amazing. These are the only vertebrates known, whose claws have to pierce their way in order to act." Blackburn first became acquainted with clawed frogs while conducting field research in the African country of Cameroon. When he picked up one of the frogs, which was about the size of a fist, it kicked its hind legs hard and scratched him to the point of bleeding.

When Blackburn returned to the United States, he examined 63 specimens of African frogs kept in museums. He observed that in 11 species, all originally from central Africa, the bones at the end of the toes were pointed and curved, with smaller bones at the end of the toes, only loosely attached to the rest of the bone. Blackburn determined that the small bone bodies were attached to the rest of the finger by a collagen-rich sheath.

"These bodies are also connected to the skin around them by dense networks of collagen fibers," says Blackburn. "The webs seem to hold the claw-like bones in place relative to the skin. In this way, when the frog contracts a certain muscle in the leg, the sharp bone separates from the vagina and breaks out of the skin."

"Most vertebrates manage to keep their skeletons inside the body much more successfully," he adds with a smile. The claw-like structure is not a normal nail, but is made of pure bone, devoid of the keratin sheath that surrounds the nails of vertebrates. It is also not similar to the cat's nail, which is able to slide back into a special structure in the cat's foot. The area where the frogs' leg bones emerge is covered with skin that looks completely normal.

Although the scientific literature mentioned these frogs as early as 1900, they did not receive much exposure in the United States, and appeared only in a small number of museum collections. Even the few researchers who wrote about them a century ago were confused and explained their punctured skin as a result of damage that occurred during the preservation of the specimens.

These frogs are used as a food source in Cameroon, after frying. The hunters, well aware of the danger of injury from them, try not to touch them as long as they are alive. "Poachers from Cameroon use long spears or machetes to avoid touching these frogs," says Blackburn. "Some even reported that they were shooting at them." Of the more than 5,000 known species of frogs, Blackburn and his colleagues have located only 11 with claws, and speculate that there may be several more species similarly armed.

Blackburn himself plans to study living African frogs to determine whether the retraction of the leg bones back into the body occurs through an active or passive process. Another direction for research is understanding the regeneration process of damaged skin, after the use of claws. Understanding the process may contribute to our understanding of how wounds heal in human skin as well. Either way, the research provides another glimpse into the wonders of nature around us and the rich inventive power of natural selection. Other researchers who participated in the study, which will be published in the upcoming issue of Biology Letters, are James Hanken and Parrish A. Young Jenkins, all from Harvard.

For news at Harvard University