Rare Frog in Japan Has Spiked Thumb for Fighting, and Mating Too

A rare breed of Japanese frog, the Otton frog (Babina subaspera) has an evolutionary adaption as weird as it is awesome. This rare breed of frog that is native to Amami islands of southern Japan, sports combat-ready spikes which shoot from a pseudo thumb on the forelimbs.

According to the study led by by Dr Noriko Iwai from the University of Tokyo, the Otton frog uses this protruding spike for both mating and fighting. What is unique about this frog is that it has an extra digit-like structure, a trait it shares with 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," said Dr Iwai. "The digit encases a sharp spine which can project out of the skin, which fieldwork demonstrates is used for combat and mating."

Since the year 2004, Dr Iwai has been studying the rare frogs in order to gain more understanding about the species' distribution, breeding habits and range. All these are the contributing factors to any conservation strategy.

Initially she began exploring how the Ottons use their pseudo thumbs and it was then that she discovered that, though both male and female pose this spike, it is used only by the males. She also noticed that the males were found to have larger pseudo thumbs when compared to females.

Dr Iwai believes that the spikes evolved for anchoring to the female known as amplexus the Latin for embrace, during mating.

"While the pseudo-thumb may have evolved for mating, it is clear that they're now used for combat," said Dr Iwai. "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."

According to the report, "the conditions on the Amami islands make combat, and the need for weaponry, a key factor for the frogs' mating success. Individuals fight over places to build nests, while the chances of a male finding a mate each night are rare, thus the ability to fight off competitors may be crucial."

 This fighting style helps confirm the theory that the spines were original used for embracing mates.

"More research is needed to look at how the pseudo-thumb evolved and how it came to be used for fighting," concluded Dr 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."

This discovery was published in the Journal of zoology.