Proceedings of the National Academy of Sciences

A Tiny Frog That Hears Without an Eardrum

By Yishane Lee

An intriguing discovery in one of the world’s smallest frogs points to the power of hearing through bone conduction.

Gardiner’s Seychelles frogs, named for islands off the coast of East Africa where they are found, evolved without a middle ear or an eardrum, both of which transmit sound waves to our inner ear. Scientists had always assumed that the tiny vertebrates (which measure just 11 millimeters) could not hear at all. But the frogs croak, and animals usually create sound only in response to hearing sound.

So they must be hearing—but how?

French researchers recently discovered the answer. In an article published in the Proceedings of the National Academy of Sciences in September, they say that the frogs hear with their mouths.

The scientists first used recordings of frog sounds to verify that the frog could hear. It did: Male frogs hopped closer to the sound and croaked in response only to calls from their own species.

Then, they used X-rays to examine the frog’s anatomy. The theory that the frogs were using their lungs to capture sound vibrations was discounted when it was discovered that their lungs are too small to catch the right wavelength. (Other frogs and many fish do use their lungs to hear.) So they focused attention on the frog’s head.

After creating 3-D simulations of sound entering a frog’s skull, the researchers found that the frog’s oral cavity resonates at nearly the same frequency as its call. The frog, they found, has unusually thin layers of tissue between the mouth and inner ear meaning that, in essence, the frog’s mouth amplifies sound waves.

“Our models show how bone conduction enhanced by the resonating role of the mouth allows these seemingly deaf frogs to communicate effectively without a middle ear,” the authors wrote. “The presence of a middle ear is not a necessary condition for terrestrial hearing, despite being the most versatile solution for life on land.”

Since the Seychelles islands, located in the western Indian Ocean, are relatively isolated, hearing through the mouth may be an ancient way that animals heard, before the middle ear and eardrum evolved, they added.

Read the abstract here.

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