Frog With Retractable Spike In False Thumb Uses It For Fighting, New Research Shows

A species of frog that uses a retractable spike hidden in a false thumb for fighting has been discovered. The ‘Otton frog’ was just discovered in the Amami islands of Southern Japan.


Most frogs only possess four digits, but the Otton possesses an extra structure resembling a thumb. The only other frogs known of that have a similar trait are 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 lead researcher, Dr Noriko Iwai. “The digit encases a sharp spine which can project out of the skin, which fieldwork demonstrates is used for combat and mating.”

The rare frogs have actually been studied since 2004, in an attempt to better understand the distribution of the species and its breeding habits and range. These are all important when designing a conservation strategy. During the research, it was observed that while both sexes had a spike, only the males used it.

“Males were found to have larger pseudo-thumbs than the females and Dr Iwai believes that the spikes evolved for anchoring to the female, known as amplexus, the Latin for embrace, during mating,” a Wiley news release states.

“While the pseudo-thumb may have evolved for mating, it is clear that they’re now used for combat,” said Dr 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.”

The Amami islands are a very competitive place for frogs, apparently leading to the emergence of the spike. Individual frogs regularly fight each other over territorial rights, for the best locations to build nests. “While the chances of a male finding a mate each night are rare, thus the ability to fight off competitors may be crucial.”

The frog uses the spike for grappling, though, not for stabbing or cutting motions. According to the researchers, this “fighting style helps confirm the theory that the spines were original used for embracing mates.” Since grappling is the fighting style of the vast majority of sexually competitive animals, though, it’s debatable.

“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 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.”

The research was just published in the Journal of Zoology.

Source: Wiley

Image Credits: N. Iwai

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