One of the world’s tiniest frogs use their mouths to hear. It is almost like they can swallow sound! Scientists have discovered how this tiny frog, which is called Gardiner’s Seychelle frog, can hear without ears.

The frogs can only be found in the Seychelle islands and they are just one centimeter in length, or 0.39370 of an inch. These tiny creatures can hear with the combination of their mouth chamber and tissue to transmit sounds to their inner ears. The findings were reported in the U.S. journal Proceedings of the National Academy of Sciences.

Almost all species of frogs have no outer ears, like other creatures. They have instead a middle ear with their eardrum on the located on the top of the their head. Sound waves cause the eardrum to vibrate and these are then carried to the inner ear via ossicles. Once there, the inner ear uses hair cells to translate the sounds into electric signals that are then sent to the brain.

Until this latest discovery, scientists believed it was impossible to hear sounds without the presence of a middle ear. They believed this because 99.9 percent of sound waves reach an animal via reflection at an animal’s skin.

Renaud Boistel, from the University of Poitiers in France said, “However, we know of frog species that croak like other frogs but do not have tympanic middle ears to listen to each other. This seems to be a contradiction.”

To test their theory, researchers set up a PA system in the frog’s Seychelle habitat and sent out taped frog “songs.” The broadcast of these songs made the male frogs respond. This proved that the frogs could hear the sounds transmitted from the PA system..

The researchers then followed up their theory by attempting to find out how these “deaf” frogs could hear sounds. It was proposed that these tiny amphibians, which have been around for millions of years, used an extra tympanic (meaning like a drumhead) “pathway” through their lungs. An alternative theory suggested the use of muscles that connected the frogs pectoral girdle to the area of its inner ear or even a sort of bone electrical conductor.

Due to the incredibly tiny size of the frogs, researchers X-rayed the soft tissue and bony areas with micrometric resolution to see which parts of the frogs body transmitted the sounds to the inner ear. The end result was that none of the proposed methods of hearing by the frogs was correct.

It was only after repeated trials that they discovered that the sound was captured via the frogs heads. The trials proved that the tiny creature’s mouth acted as an amplifier for the frequencies transmitted, or emitted, by the Gardiner Seychelle frogs. So in essence these tiny creatures appear to almost be capable of swallowing sound.

Synchrotron (a particular type of cyclic particle accelerator) X-ray images of other species proved that the emission of sound from the creatures’ oral cavity had adapted by evolutionary means. This evolution meant that the frogs tissue was thinner between the mouth and the inner ear.

Boistel explained, “The combination of a mouth cavity and bone conduction allows Gardiner’s frogs to perceive sound effectively without use of a tympanic middle ear.” He added that since the frogs had lived in isolation in the Seychelle rainforest for between 47 and 65 million years, their hearing functions must be a “survivor of life forms on the ancient supercontinent Gondwana.”

The discovery that these tiny frogs literally hear with their mouths does make it sound as though they can swallow sound.

By Michael Smith

United Kingdom

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