Rare frog has flick-knife thumbs

Posted by Emma Woollacott

A rare breed of frog has been discovered to have spikes which shoot from its thumbs for fighting and mating, a bit like X-Men's Spyke.

The Otton frog, found in the Amami islands of Southern Japan, has an extra digit-like structure, like those of the five-fingered Hypsiboas rosenbergi frogs of Latin America.

"Why these ‘fifth fingers’ exist in some species remains an evolutionary mystery, but the extra digit of the Otton is in fact a pseudo-thumb," says Dr Noriko Iwai from the University of Tokyo. "The digit encases a sharp spine which can project out of the skin, which fieldwork demonstrates is used for combat and mating."

While both males and females have the spike, it's only used by males, which have larger pseudo-thumbs than the females. Rather unpleasantly, Iwai believes that the spikes evolved as a way for the males to hang on to the females during mating.

"While the pseudo-thumb may have evolved for mating, it is clear that they’re now used for combat," she says. "The males demonstrated a jabbing response with the thumb when they were picked up, and the many scars on the male spines provided evidence of fighting."

The little frogs really need their weaponry. They have to fight, not only to compete for a mate, but also over places to build nests. Unlike Spyke, though, their fighting technique is somewhat cumbersome, consisting of a prolonged wrestling match interspersed with a bit of jabbing - and they have to be careful not to stab themselves in the process.

"More research is needed to look at how the pseudo-thumb evolved and how it came to be used for fighting," says Iwai. "The thumbs' use as a weapon, and the danger of the frogs harming themselves with it, makes the Otton pseudo-thumb an intriguing contribution to the study of hand morphology."