Tiny Frog Hears With Mouth
September 3, 2013

Tiny Frogs Species Hears With Its Mouth

[ Watch the Video: Gardiner's Frogs Use Their Mouths To Hear ]

Michael Harper for redOrbit.com - Your Universe Online

Scientists had long assumed the Gardiner’s frog was deaf due to its lack of a middle ear. After further investigation and some X-ray images, however, they have now discovered that the frog species from the Seychelles Islands is capable of hearing through tiny bones in their mouths. Unlike other auditory systems common in a broad majority of living organisms, the Gardiner’s frog is able to listen as it croaks, picking up the vibrations of sound in its mouth. The results of their work are now published in the journal PNAS.

According to the scientists, it’s the location of this frog which makes this discovery all the more exciting.

The Seychelles Islands are spread out in the Indian Ocean east of Somalia, Kenya and Tanzania and it is believed they were once a part of the African continent. Because they have been isolated from the rest of the continent - and the rest of the world - for millions of years, Renaud Boistel from the University of Poitiers and the French National Center for Scientific Research (CNRS) says this could help them better understand lifeforms from millions of years ago.

“These small animals, Gardiner’s frogs, have been living isolated in the rainforest of the Seychelles for 47 to 65 million years, since these islands split away from the main continent,” explained Boistel in a statement. "If they can hear, their auditory system must be a survivor of life on the ancient supercontinent Gondwana."

To test their hypothesis and document the hearing ability of the frogs, the scientists from the CNRS, the Nature Protection Trust of Seychelles, and the European Synchrotron Radiation Facility (ESRF) set up loudspeakers in the Seychelles wilderness to attract the small frogs. The played frog calls through these loudspeakers and were surprised as male Gardiner’s frogs hopped towards the speakers.

"If you play the call, they respond," explained Dr. Justin Gerlach from the Nature Protection Trust of Seychelles. "Either they change position - they may move to face where the call is coming from - or quite often they will call in response. It's very funny actually; [the frogs would] even attack the loud speaker."

After confirming that the frogs are not deaf, the team of scientists then set out to understand how they were able to hear without a middle ear. It had previously been suggested the frogs were able to interpret the vibrations of sound waves in their lungs. In this method of hearing, the sound waves are picked up by muscles in the animal’s pectoral girdle and transmitted to the inner ear. After taking some X-ray shots of the frogs, it was determined the amphibians do not hear in this way.

“As these animals are tiny, just one centimeter long, we needed X-ray images of the soft tissue and the bony parts with micrometric resolution to determine which body parts contribute to sound propagation,” explained Peter Cloetens, a scientist at the ESRF.

[ Watch The Video: Frogs That Hear With Their Mouth ]

They then began running some computational models to test the theory that the frog uses its mouth as an amplifier for sound. With thinner and fewer layers of tissue between their mouths and their inner ears, the Gardiner’s frog is able to easily transfer sound picked up from its mouth and direct it to the brain via a mixture of fluids in their skulls.

“This combination of a [resonating] mouth cavity and bone conduction allows Gardiner's frogs to perceive sound effectively without use of a middle ear,” explained Dr. Boistel.

Though they live in an isolated environment, wildfires, invasive species and increased tourism have left these frogs endangered. To lose this animal says Dr. Boistel, would be to lose 65 million years’ worth of evolutionary history.