Frogs that hear with their mouths

Scientists have solved the mystery of why one of the world's smallest frogs -- Gardiner's Seychelles frog -- can hear without an ear.

The frogs from the Seychelles islands in the Indian Ocean, which are just one cm long, can hear sounds because they use their mouth cavity and tissue to transmit sound to their inner ears, Xinhua cited researchers as saying in the US journal Proceedings of the National Academy of Sciences.

Most frogs do not possess an outer ear like humans, but a middle ear with an eardrum located directly on the surface of the head.

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

Scientists previously thought that it was impossible to detect sound in the brain without a middle ear because 99.9 percent of a sound wave reaching an animal is reflected at the surface of its skin.

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

To establish whether Gardiner's frogs actually use sound to communicate with each other, the researchers set up loudspeakers in their natural habitat and broadcast pre-recorded frog songs.

This caused male frogs present in the rainforest to answer, proving that they were able to hear the sound from the loudspeakers, the researchers said.

The experts then identified the mechanism by which these seemingly deaf frogs were able to hear sound.

Various mechanisms have been proposed -- an extra-tympanic pathway through the lungs, muscles which in frogs connect the pectoral girdle to the region of the inner ear, or bone conduction.

As these animals are tiny, just one cm long, the researchers used X-ray images of the soft tissue and the bony parts with micrometric resolution to determine which body parts contribute to sound propagation.

They found that neither the pulmonary system nor the muscles of these frogs "contribute significantly" to the transmission of sound to the inner ears.

Numerical simulations helped investigate the third hypothesis, that the sound was received through the frogs' heads.

These simulations confirmed that the mouth acts as a resonator, or amplifier, for the frequencies emitted by this species.

Synchrotron X-ray imaging on different species showed that the transmission of the sound from the oral cavity to the inner ear has been optimised by two evolutionary adaptations -- a reduced thickness of the tissue between the mouth and the inner ear and a smaller number of tissue layers between the mouth and the inner 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.

As Gardiner's frogs have been living isolated in the rainforest of the Seychelles for 47 million to 65 million years, since these islands split away from the main continent, the findings indicated that "their auditory system must be a survivor of life forms on the ancient supercontinent Gondwana", the expert said.