The Deadly Thumbs of Japanese Flick Knife Frogs

The innocent looking “Wolverine” frog conceals its hidden claws (Image: N. Iwai)

The Japanese Otton frog (Babina subaspera) may look harmless, but don’t be fooled by its ordinary green, warty appearance. This frog carries concealed weapons.

A new study has discovered that the Otton frog has sharp retractable claws that shoot out of its thumbs. The rare frog, native to the Amami islands of Southern Japan, uses these “switchblades” to fight and to mate. Conducted by Dr. Noriko Iwai from the University of Tokyo, the new study is published in the Journal of Zoology.

Frog Fingers Five

Since 2004, Dr. Iwai has been studying rare frogs to understand the species’ distribution, breeding habits, and range in order to craft sound conservation strategies. Frogs with five digits on their front limbs are uncommon (most have four); the Otton frog’s shares this rare trait 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.”

Love and War

Both male and female Otton frogs have these secret stilettos, and the males’ spikes are typically larger than the females’. Despite the trait’s being common to both sexes, only the males appear to use their daggers, a finding that leads Dr. Iwai to believe that the spikes evolved to anchor the male to the female during mating.

One may imagine the frogs’ fighting style like Wolverine’s slashing claws or the Three Musketeers’ precise rapiers. But in reality, the frogs’ methods are less cinematic and more down and dirty.

When facing off, male frogs first wrestle each other in an embrace and then jab their opponent with the exposed thumb spikes. This fighting style helps confirm the theory that the spines were original used for embracing mates.

And what are they fighting for? Mates and territory.  Female frogs and ideal nesting places are hard to come by, so the ability to fight off the competition gives the male frog an edge.

“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 thumb’s 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.”


  1. Christina
    Tallahassee. Fl
    October 26, 2012, 10:46 am

    Wow! that is incredible. how weird and neat! man this world is full of surprises 🙂

  2. Mick Welch
    North Queensland Australia
    October 24, 2012, 5:02 am

    Yea ! This is very entertaining and educational. Life is great.