A rare frog species is armed with combat-ready spikes which shoot straight from the fingers, like that of a comic book hero, says a Japanese scientist. The discovery by Noriko Iwai, from the University of Tokyo, reveals how the Otton frog uses spikes which protrude from a false thumb for both combat and mating.

The study focused on the Otton frog 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 Journal of Zoology reports.

"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 Iwai. "The digit encases a sharp spine which can project out of the skin, which fieldwork demonstrates is used for combat and mating."

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, according to a Tokyo statement.

Once she began exploring how the Ottons 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.