Frog Hears with Its Mouth

A new paper details how, despite not having a middle ear fitted with an eardrum, Gardiner’s frogs are still able to hear.

Not to beat about the bush, an international team of researchers has found that, as odd as this may sound, these amphibians hear with their mouth.

More precisely, they use their mouth cavity and layers of tissue to pick up sound waves and transmit them to their inner ear, data collected with the help of X-rays has shown.

Researchers explain that, although most other frog species lack an outer ear similar to that of humans, they do nonetheless possess a middle ear that houses an eardrum and ossicles.

The eardrum is located close to the surface of the head, and vibrates every time it gets hit by incoming sound waves. These vibrations are then picked up by the ossicles, which take care of delivering them to the inner ear.

Here, the vibrations are translated into electric signals that travel to the brain. The latter is responsible for interpreting them, sources say.

In the case of this frog species, the sound waves reach the inner ear via the amphibian's head. Thus, they are first picked up by the oral cavity, which acts as an amplifier.

They are then transmitted from the mouth to the inner ear through several tissue layers that stand between them. These tissues are both less thick and less numerous than the ones found in other amphibian species.

“With X-ray imaging techniques here at the ESRF, we could establish that neither the pulmonary system nor the muscles of these frogs contribute significantly to the transmission of sound to the inner ears.”

“The combination of a mouth cavity and bone conduction allows Gardiner’s frogs to perceive sound effectively without use of a tympanic middle ear,” researcher Renaud Boistel explains.

A detailed account of this investigation and its findings was published in the journal Proceedings of the National Academy of Science yesterday, September 2.