Tiny Gardiner's Frogs Use their Mouths to Hear

Based on what's known about how creatures hear throughout the animal kingdom, the tiny Gardiner's frog of the Seychelles Islands should be deaf -- it has no middle ear with an ear drum like other frogs or other amphibians or even species as distant as apes and humans, which all share biological similarities in the mechanics of hearing.

But after playing recordings of frog songs in the Gardiner's natural habitat and soliciting a response from Gardiners living in the area, researchers concluded that Gardiner's frogs are not deaf, as had been presumed, and further probing has revealed that the frogs use their mouths to do it, something quite uncommon, if not entirely unheard of, in the animal kingdom.

The international team of researchers published their results Monday in the journal Proceedings of the National Academy of Science.

"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, lead researcher from the French National Centre for Scientific Research.

Boistel and his colleagues speculated on a number of biomechanical configurations that would allow the tiny frogs to hear while lacking a middle ear with an eardrum, including an extra-tympanic pathway via the lungs, or a connection to the inner ear forged by a pectoral muscle connection.

"Whether body tissue will transport sound or not depends on its biomechanical properties. With X-ray imaging techniques here at the ESRF [European Synchrotron Radiation Facility], we could establish that neither the pulmonary system nor the muscles of these frogs contribute significantly to the transmission of sound to the inner ears," said Peter Cloetens, a scientist at the ESRF who took part in the study. "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."

After working through a number of calculations, the research team concluded that the Gardiner's frogs receive sound through through bone conduction in their heads. The frogs' mouth acts as a resonator or amplifier for frequencies emitted by other Gardiner's frogs

"Synchrotron X-ray imaging on different species showed that the transmission of the sound from the oral cavity to the inner ear has been optimized 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 researchers wrote in a statement issued by the ESRF.

Boistel suggests that the frogs' unique method of hearing is a product of millions of years of isolation in the forests of the Seychelles.

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