Love and War: Japanese Frog Sports Spiked Finger for Combat and Mating

A rare breed of Japanese frog (Photo) has been found sporting a spiked pseudo-thumb for combat and mating, researchers said.

A team of researchers led by Noriko Iwai from the University of Tokyo, Japan, noticed that both males and females of the rare Otton frog (Babina subaspera) have a fifth finger.

Unlike frogs that have four toes, the Otton frogs sporting a fifth digit-like structure was puzzling the researchers.  They found that the fifth finger (in fact a pseudo-thumb) has a sharp spine projecting out of the skin. It is used as key weapon to fight males for their territory and mate with females.

Since 2004, Iwai and her research team have been observing the Otton frogs living only on the Amami islands of Southern Japan to understand their distribution, breeding habits and range. While both male and female Ottons have spikes in their pseudo-thumb, it was used only by the males.

They noticed that the males sported larger pseudo-thumbs than the females. Iwai believes that the spiked pseudo-thumb evolved to allow males to anchor to the females for mating. It is known as amplexus (the Latin for embrace) - a form of pseudocopulation in which the males fertilize the eggs released by the females.

But the researchers noticed that the frogs use their extra digit-like structure to fight other males over places to build their nests.

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

However, the fights did not cause any grave injury to the males. Rather they were noticed jabbing each other with their spiked pseudo-thumb. This fighting style suggests that pseudo-thumbs were initially used to embrace females.

Further research needs to be done on how the pseudo-thumb evolved and how the frogs started using it for fighting, Iwai said.

"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," she said.

The findings of the study are published in the Journal of Zoology.