The researchers found the Otton frog uses spikes which protrude from a false thumb for both combat and mating.
The study, which is published in the Journal of Zoology, was conducted by Dr Noriko Iwai from the University of Tokyo.
The researchers studied the Otton frog (Babina subaspera), whose habitat is the Amami islands of Southern Japan.
Unlike most other frogs the Otton has an extra digit-like structure, a trait it shares with the five-fingered Hypsiboas rosenbergi frogs of Latin America, the Daily Mail reported.
"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," Iwai said.
"The digit encases a sharp spine which can project out of the skin, which fieldwork demonstrates is used for combat and mating," she said.
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Iwai has studied the rare frogs since 2004 in order to understand the species' distribution, breeding habits and range; all factors which will contribute to any conservation strategy.
Once she began exploring how the Otton's use their pseudo-thumbs, Iwai discovered that while both males and females had the spike, it was only used by males.
Males were found to have larger pseudo-thumbs than the females and 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," Iwai said.
"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.
Spikes which shoot from fingers are a key weapon for Wolverine.
Perhaps unfortunately, the comic book hero image is slightly dented by the frogs' fighting style.
Rather than dueling with thumb spikes the males wrestle each other in an embrace, jabbing at each other with the spines.
This fighting style helps confirm the theory that the spines were original used for embracing mates.