Frog once believed deaf hears with its mouth

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.

But when scientists played recordings of the middle earless-frog's calls in the animal's forest, the frogs responded, hopping toward the speakers and chirping back replies, high-pitched peeps. It seemed that the frogs could hear – but how?

Next, the team X-ray imaged the frogs at the European Synchrontron Radiation Facility in Grenoble, France. Did the frogs have a drum-like region in their lungs that ferried sound waves to their inner ear, as other amphibians wanting in the middle ear department do? Did the frogs’ muscles relay sound, or, perhaps, did their bones do so?

As unusual as all those possibilities were, the answer was even more counterintuitive: this bantam frog hears with its mouth. The X-ray results showed that the frogs have unusually thin layers of tissue between their mouth cavities and inner ears. That means that sound waves are taken in through the mouth, amplified, and then passed into the inner ear and to the brain.

“Ultimately the structure allowing these earless frogs to hear (the mouth) is already present and serves other functions such as feeding and sound production,” says Dr. Boistel. “It is a quite unexpected result, but in the end a simple and elegant solution.”

The findings are published in the journal Proceedings of the National Academy of Sciences.

The scientists believe that the frogs’ odd hearing system is an evolutionary holdover, the result of having been isolated in the Seychelles for 47 to 65 million years, after the islands were siphoned off from the ancient supercontinent Gondwana. As the frogs’ landlocked relatives went on to acquire middle ears and hear with them, Gardiner’s frogs, little Luddites, kept on listening with their mouths.