One of the smallest frogs in the world, Gardiner’s frogs from the Seychelles Islands, can croak and hear other frogs even if they do not have a middle ear with an eardrum.
A group of researchers from the Laboratoire de Mécanique et d'Acoustique in Marseilles, Institut Langevin of ESPCI ParisTech, the Institute of Systems and Synthetic Biology at the University of Evry (France), the European Synchrotron ESRF in Grenoble, the Nature Protection Trust of Seychelles led by CNRS and the University of Poitiers’ Renaud Boistel used X-rays in solving the mystery and establishing the fact that these frogs are hearing through the use of their mouth cavity and tissue by transmitting sound to their inner ears.
Even though the auditory systems of many lineages of animals have undergone many changes since the Triassic age, between 200 and 250 million years ago, the middle ear with eardrum and ossicles are always present. Some animals, however, remarkably frogs, do not have an outer ear like humans, but has a middle ear with an eardrum located on the surface of the head.
To ascertain the truth about these frogs using sounds from communication, the researchers arranged loudspeakers in their natural habitat and played pre-recorded frog songs. During the process, frogs responded to the songs which proved that they can hear sounds.
Identifying the mechanism how these frogs hear sounds was the next step. Different ways has been proposed, like extra-tympanic pathway through the lungs, bone conduction, and muscles which in frogs connect the pectoral girdle the region of the inner ear. However, with the X-ray imaging techniques at ESRF have proven that neither the muscles of these frogs not pulmonary system contribute notably to the transmission of sounds to inner ears.
The third theory was that the sound was received through the frog’s head which researchers have supported by numerical simulations. It was confirmed that the frog’s mouth acts as an amplifier or resonator for the frequencies produced by these frogs.
The study was published in the Sept. 2 issue of PNAS.
