Dime-Sized Frog Likely Uses Mouth to Help It Hear

ABC News(PHILADELPHIA) — The Gardiner’s Seychelles frog, so tiny it fits easily on a fingernail, belts out high-pitched peeps every couple of minutes. While its tiny size and mouse-like squeaks are reason enough to raise eyebrows, researchers at the Centre de Neurosciences Paris-Sud found these frogs may be using their mouths to better hear what’s going on in their surroundings.

Herpetologists — those who study amphibians and reptiles — have known that these frogs lack a middle ear, which, consisting of several bones and muscles, is what enables many animals (and humans) to hear. Renaud Boistel, a researcher at CNPS, said the Gardiner’s Seychelles was not the only frog to lack an inner ear.

“There are approximately 6,000 frogs in the world,” Boistel told ABC News. “But about 300 frogs are without middle ears, around 6 percent.”

The middle ear itself isn’t what transforms changes in air pressure into something that the brain interprets as sound. Yale Cohen, an associate professor of head and neck surgery at the Perelman School of Medicine at the University of Pennsylvania, uses the analogy of a swimming pool to explain the role of the middle ear.

“If you’re underwater and you’re yelling to your friends underwater, they can’t hear you,” said Cohen. “Water is more dense than air, so acoustic energy bounces off the water and most of that energy is lost. What the middle ear does is it amplifies the sound to compensate for the difference in density.”

It was a mystery how the Gardiner’s Seychelles frog could hear anything. To figure it out, Boistel used X-ray imaging to see if there was anything unique in the frog’s anatomy that could replace the middle ear’s ability to amplify sound.

Using the X-ray data, Boistel created a model of what sounds could be amplified within the frog’s mouth, given its size and shape. “Only the oral cavity has the ability to resonate at 5,738 hz,” he writes in the Proceedings of the National Academy of Sciences paper.

That particular frequency is relevant to the frog because it closely matches the average frequency of the frog’s calls, at around 5,710 hz. “Thus, the oral cavity appears as the ideal frequency-tuned candidate to amplify the acoustic signal,” writes Boistel. In a sense, it uses its oral capabilities to assist its aural ones.

Cohen says that Boistel’s paper gives really good, strong evidence to the theory that the oral cavity plays a big role in the Gardiner’s Seychelles frog’s hearing, but that it’s not yet proved. “You have to play with the frog’s mouth, to somehow manipulate his oral cavity, to see how this would work,” he said. “It needs to be shown, but it’s still very cool.”

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