New research, led by Dr. Norikio Iwai, from the University of Toky reports on a unique and odd finding. The Otton frog, found only on a group of islands in southern Japan, has an extra thumb, adapted as a sharp, spike-like weapon. Dr. Iwai also found that the weapon was once used for sex. Originally, the male frog embraced the female with its extra thumb. Iwai's study appears in the October issue of the Journal of Zoology. 

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

The study illustrates how the thumb (pseudo or real) may evolve from four-fingered animals, and may provide insight into how human ancestors developed their thumbs, which are crucial for conducting more sophisticated tasks like carrying objects or fine-motor manipulation. While nobody suggests that the human thumb was used as a weapon, the study demonstrates one of many ways thumbs can evolve.

While both male and female Otton frogs have the spiny thumb, only the males use it for combat, Dr. Iwai found. Otton frogs are very sensitive to outsiders, human, frog and otherwise, and will jab their thumbs at an intruder. Most of the time, however, they fight with other males, embracing the interloper and jabbing it into the frog's side.This embrace-jabbing motion also made Dr. Iwai suspect that the weapon evolved from a more sexual function. The embrace is very similar to the mating stances taken by the frogs, so the thumb could easily be adapted to this wrestling style of combat.

On the Amami islands, successful mating depends on successful combat. The frogs are quite rare, and finding a female to mate with is very challenging. In addition, frogs fight over where to build their nests. Nesting and finding a mate are important for reproduction for these frogs, because they only mate with one mate and fertilize eggs in a single nest.
Amazingly, while scars are seen on many male Otton frogs, none of the frogs in Dr. Iwai's study were killed in combat. "It seems that the intensity of combat in Otton frogs is finely balanced so as not to result in critical or mortal injuries," wrote Dr. Iwai. As long as he gets the girl.