One of the world’s smallest frogs, the one-centimeter-long Gardiner’s Seychelle frog, lacks a middle ear with an eardrum yet it can hear. Scientists now find it can use its mouth and bones to hear, findings detailed this week in the Proceedings of the National Academy of Sciences.
The way many lineages of animals hear dates back 200 million to 250 million years ago to the Triassic. Hearing depends on a middle ear that possesses an tympanum, which vibrates in response to sound waves, and the ossicles or ear bones, which conduct these vibrations into the inner ear, which translates them into electrical signals delivered to the brain. Without the middle ear, many tetrapods — four-limbed animals such as reptiles, birds, and mammals — would likely not be able to hear, since 99.9 percent of sound energy reaching them is reflected off the surface of their skins.
Scientists found Gardiner’s Seychelle frog (Sechellophryne gardineri) lacks a middle ear, yet croaks and can listen and respond to other frogs, something they proved by setting up loudspeakers at two locations in the Seychelles in the Indian Ocean and broadcasting pre-recorded frog songs. In fact, many species of frogs lack a middle ear, yet they are still capable of hearing sound waves in the air.
To solve the puzzle of how these seemingly deaf amphibians can hear, bioacoustician Renaud Boistel at the French National Center for Scientific Research and his colleagues employed the most brilliant beams of X-rays in the world at the European Synchrotron Radiation Facility in Grenoble, France. They scanned the soft tissue and the bony parts of Gardiner’s Seychelle frog and seven other frog species with micron-level resolution.
Past research had suggested a number of mechanisms by which Gardiner’s Seychelle frog might hear. For instance, the lungs might help conduct sound, or might muscles, which in frogs connect the pectoral girdle to the region of the inner ear. However, X-ray analysis revealed neither the frog’s lungs nor muscles has the proper biomechanical properties to serve as conduits for sound.
Computer simulations helped reveal that the mouth of the Gardiner’s Seychelle frog behaves as a resonant cavity, amplifying the dominant frequency of the advertisement croaks of the species. Scans of this amphibian and six other species revealed earless frogs probably rely on bone to conduct sound from the mouth to their inner ears — they have minimized the thickness of tissue and the number of layers of tissue between the mouth and inner ear.
“It is a quite unexpected result, but in the end a simple and elegant solution,” Boistel says. “It remains to be seen if other tetrapods exploit the same mechanism.”