Though the Gardiner's frogs of the Seychelles islands do not have an ear drum, they can still manage to detect other frog's sounds. By using X-rays, French scientists discovered that the amphibians use their mouth cavity and tissue to transmit sounds to their inner ears.

The team, led by Renaud Boistel from CNRS and the University of Poitiers and scientists from a number of other French institutions and the Nature Protection trust of Seychelles, published the findings in PNAS.

Most of the frogs do not have an outer ear like humans but a middle ear with an eardrum is found on the surface of the head.

"Incoming sound waves make the eardrum vibrate, and the eardrum delivers these vibrations using the ossicles to the inner ear where hair cells translate them into electric signals sent to the brain."

One of the world's smallest frogs, measuring just one centimetre long, Gardiner's frogs do not have a middle ear with an ear drum.

"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. If they can hear, their auditory system must be a survivor of life forms on the ancient supercontinent Gondwana," Boistel said.

To determine whether these frogs can detect sounds, researchers placed loudspeakers in their natural habitat and transmitted pre-recorded frog songs. Male frogs present in the rainforest responded, thus proving that they were able to hear the sound from the loudspeakers.

In order to find out how these visually deaf frogs were able to hear songs, the researchers used Synchrotron X-ray imaging techniques and found that the frog's mouth "acted as a resonator or amplifier, for the frequencies emitted by this species."

Two evolutionary adaptations in the inside layer of mouth and the inner ear helped the frog transmit sound from the oral cavity to the ear.

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