Tiny Gardiner's Frogs Hear with their Mouths: How to Really Swallow Sound

Frogs may not always hear with their ears. A tiny frog from the Seychelles islands, for example, doesn't even have a middle ear with an eardrum, yet it can still communicate and hear other frogs. Now, scientists have unraveled the mystery of how this frog hears; it turns out that it hears with its mouth.

The way sound is heard is common to many animals. The typical system of hearing noises, in fact, first emerged during the Triassic age around 200 to 250 million years ago. Although this system has undergone many changes since during the course of evolution, most animals still have several things in common when it comes to hearing: a middle ear with eardrum and ossicles.

Most frogs, though, lack the outer ear that many mammals have. Instead, they possess a middle ear with an eardrum located directly on the surface of their heads. Sound waves make this eardrum vibrate and the eardrum delivers this information using the ossicles to the inner ear. There, cells translate them into electrical signals which are then sent to the brain.

Gardiner's frogs, though, don't possess this middle ear. They've been living in isolation in the rainforest of the Seychelles for the past 47 to 65 million years. The fact that they can hear hints that their system is actually a remnant from the life forms on the ancient supercontinent Godwana.

Yet what exactly is the system? The researchers used X-ray imaging techniques to figure out exactly what it might be. They found that neither the pulmonary system nor the muscles of the frogs contributed significantly to the way that the frogs heard. Instead, it turned out that the sound was received through the frogs' heads. More specifically, the mouth acts as a resonator, or amplifier, for the frequencies emitted by this species.

"The combination of a mouth cavity and bone conduction allows Gardiner's frogs to perceive sound effectively without use of a tympanic middle ear," said Renaud Boistel, one of the researchers, in a news release.

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

Want to learn more? Check out the video below, courtesy of YouTube.