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Frog once believed deaf hears with its mouth

In a counterintuitive find, researchers reported that Gardiner’s frog hears with its mouth.

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Gardiner’s frogs have no middle ear, the region where the process of hearing usually begins. Instead, the frog hears with its mouth.

R. Boistel/French National Centre for Scientific Research

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When a Gardiner’s frog wants to listen, it doesn’t shut its mouth. It opens it.

Scientists have found that this tiny species of frog hears with its mouth, a find that upends the ideas that frogs must have a middle ear to hear and that this particular frog, lacking the critical body part, must be deaf. While researchers believe that other amphibians might also listen with their mouths, the Gardiner's frog is the first such species confirmed to do so.

“It’s a novel mechanism of hearing in frogs and tetrapods in general,” says Renaud Boistel, a researcher at the French National Centre for Scientific Research and a co-author on the paper, in an email. “It remains to be seen if other tetrapods exploit the same mechanism.”

Gardiner’s frog has long been known as an unusual animal. The amphibian – green-brown colored, with a dark brown strip on its side like a serration on a small stone – is one of the smallest frogs in the world, at just 11 millimeters in length. Even smaller than that are the petite, metamorphosed froglets – not tadpoles – to which the frog gives birth. The rare frog also lives on just two of the islands in the Seychelles group and at just the highest altitudes at those islands. It is classified as vulnerable, as human development clears out the trees and brush that furnish its limited habitat.

And the frog was also thought to be deaf. Gardiner’s frogs have no middle ear, the region where the process of hearing begins in about 94 percent of frogs. In most frogs, which don’t have the outer ear that humans have, sound waves first vibrate the middle ear’s eardrum. From there, the waves are transmitted to the inner ear and are then re-routed to the brain as electric signals.

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