 A small frog native to the Republic of Seychelles lacks a conventional middle ear and eardrum to hear sounds made by other frogs, but new research suggests these peculiar croakers are not deaf, and can instead use their mouth cavities to pick up on noise. The mouth acts as a resonator, or amplifier for the frequencies emitted by Gardiner's frogs 
 
 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. Dr Boistel said: 'The combination of a mouth cavity and bone conduction allows Gardiner’s frogs to perceive sound effectively without use of a tympanic middle ear' 
 
 The species of frog is one of the smallest in the world and does not possess a middle ear with an eardrum. Most four-legged animals have middle ears that contain small, bony ossicles that take vibrations from the eardrum and transmit the sound waves from the air to the fluid-filled cochlea. Top left: the skin of the animal reflects 99.9 percent of an incoming sound wave, hitting the body close to the inner ear. Without a middle ear, sound waves cannot be transported to the inner ear but the mouth acts as a resonating cavity for the frequencies of the frogs' song, amplifying the sound in the mouth. The body tissue between the buccal cavity and the inner ear is adapted to transport these sound waves to the inner ear 
 
 However, frogs do not possess an outer ear like humans but a middle ear with an eardrum located directly on the surface of the head. To determine that Gardiner's frogs do, in fact, use sound to communicate with one another, the scientists set up loudspeakers in the natural rain forests of Seychelles and played pre-recorded frog songs. Dr Boistel said:'The combination of a mouth cavity and bone conduction allows Gardiner's frogs to perceive sound effectively without a middle ear 
 
 Scientists had proposed the frogs might use an extra-tympanic pathway through the lungs or bone conduction. "Whether body tissue will transport sound or not depends on its biomechanical properties," study co-author Peter Cloetens, a scientist at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, said in a statement. "With X-ray imaging techniques here at ESRF, 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." Credit: R. Boistel/CNRS 
 
 By studying X-ray images and numerical simulations, the researchers discovered that Gardiner's frogs receive sound through their heads. 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. Dr Boistel said:'The combination of a mouth cavity and bone conduction allows Gardiner’s frogs to perceive sound effectively without use of a tympanic middle ear'.
– They've got no middle ears or eardrums, yet one of the world's smallest frog species still responds to sound—and now, scientists have figured out how. The Gardiner's frog, found in the Republic of Seychelles, picks up noises through its mouth; from there, the sounds head to its inner ear, LiveScience reports. Researchers investigating the species' hearing capacity blared frog sounds in Seychelles rain forests, and male Gardiner's frogs replied. Scientists used X-rays to figure out what was going on: Gardiner's frogs have less tissue between their mouths and inner ears than do most frogs. The animals' mouths were amplifying the sounds, whose frequencies then traveled through their skulls to reach the inner ear. "The combination of a mouth cavity and bone conduction allows Gardiner's frogs to perceive sound effectively," says a researcher. By contrast, notes the Daily Mail, most frogs have middle ears and eardrums on the surfaces of their heads. The creatures "have been living isolated in the rainforest of the Seychelles for 47 (million) to 65 million years," says the researcher, and their hearing system is likely a remnant of ancient species.