The frog that hears with its MOUTH: Amphibian once thought to be deaf uses cavity as an amplifier 
 
 
 
 Gardiner’s frogs from the Seychelles islands are just 1cm long and can croak and hear each other without possessing an eardrum 
 
 Using x-rays, French scientists discovered the frogs’ mouths act as a resonator or amplifier for the frequencies ‘spoken’ by the species 
 
 Dr Boistel believes the species' auditory system must be a survivor of life forms on the ancient supercontinent Gondwana 
 
 French scientists have solved the mystery of how one of the world’s smallest frogs can croak and hear other amphibians without possessing an eardrum. 
 
 
 
 Gardiner’s frogs from the Seychelles islands, which are just one centimetre long, were thought to be deaf, but the research revealed they actually hear sounds through their mouths. 
 
 
 
 Using X-rays, the scientists discovered the frogs’ mouths act as a resonator or amplifier for the frequencies ‘spoken’ by the species. 
 
 Gardiner's frogs (pictured) from the Seychelles islands, which are just one centimetre long, were thought to be deaf, but the research revealed they actually hear sounds through their mouths 
 
 HOW DO THE FROGS HEAR? 
 
 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. 
 
 Scientists have been puzzled as to how the frogs are able to use sound to communicate but have found the amphibians use their mouth cavity and tissue to transmit sound to their inner ears. 
 
 The team, led by Renaud Boistel from CNRS and the University of Poitiers as well as scientists from a number of other French institutions and the Nature Protection trust of Seychelles, published the findings in PNAS. 
 
 The way sound is heard is common to many lineages of animals and appeared during the Triassic age, some 200-250 million years ago. 
 
 Although the auditory systems of the four-legged animals have evolved, they have in common the middle ear with eardrum and ossicles, which emerged independently in the major lineages. 
 
 The skin of the animal reflects 99.9 per cent of an incoming sound wave hiting 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. 
 
 Incoming sound waves make the eardrum vibrate, and the eardrum delivers these vibrations using the ossicles to the inner ear where hair cells translate them into electric signals sent to the brain. 
 
 
 
 However, Gardiner's frogs do not have a middle ear and many people doubted that it was possible for the species to detect sound in the brain as a consequence, as almost all of a sound wave reaching an animal is reflected at the surface of its skin. 
 
 Dr Boistel said: 'These small animals, 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. 
 
 A 3D reconstruction of a frog's head with a slice through the eye showing the ouside of the inner ear. 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 adaptation 
 
 'If they can hear, their auditory system must be a survivor of life forms on the ancient supercontinent Gondwana.' 
 
 In order to test if the frogs really do use sound to communicate, the scientists set up loudspeakers in their natural habitat and broadcast pre-recorded frog songs. 
 
 This caused males present in the rainforest to answer, proving that they were able to hear the sound from the loudspeakers. 
 
 Here you can see the 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 a middle ear 
 
 Scientists had proposed the frogs might use an extra-tympanic pathway through the lungs or bone conduction. 
 
 Peter Cloetens, a scientist at the ESRF who took part in the study, said whether body tissue will transport sound or not depends on its biomechanical properties. 
 
 He used X-ray imaging techniques to establish neither the the pulmonary system nor the muscles of these frogs contribute significantly to the transmission of sound to the inner ears. 
 
 Numerical simulations helped to investigate whether the sound was received through the frog’s heads. 
 
 In order to test if the frogs really do use sound to communicate, the scientists set up loudspeakers in their natural habitat and broadcast pre-recorded frog songs. This caused males present in the rainforest to answer, proving that they were able to hear the sound from the loudspeakers 
 
 These simulations confirmed 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. 
 
 
 
 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'. 
 
 
 
 ||||| 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. 
 
 Gardiner's frogs from the Seychelles islands are one of the smallest known types of frogs in the world. These amphibians are seemingly deaf — having no middle ear or eardrum to help process sound waves — but can mysteriously still make their own croaking sounds, and hear the calls of other frogs. In a new study, researchers used X-ray imaging to peer inside the frogs' heads, finding they use their mouth cavities to amplify sounds that travel to the inner ear through connective tissue. 
 
 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. [Image gallery: Cute and Colorful Frogs] 
 
 "However, we know of frog species that croak like other frogs but do not have tympanic middle ears to listen to each other. This seems to be a contradiction," lead study researcher Renaud Boistel, of the French National Center for Scientific Research (Centre National de la Recherche Scientifiqueor CNRS) in Paris, France, said in a statement. 
 
 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. Males in the rain forests promptly answered the songs, signifying they could hear the recording, the researchers said. 
 
 Several ideas had been suggested for how the Gardiner's frogs could hear sounds, including extra auditory pathways through the lungs or special muscles in the frogs that connect to the inner ear. 
 
 "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." 
 
 This illustration shows how a Gardiner's frog can hear with its mouth. 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. Bottom left: the mouth acts as a resonating cavity for the frequencies of the frogs' song, amplifying the amplitude of 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. Credit: R. Boistel/CNRS 
 
 By studying X-ray images and numerical simulations, the researchers discovered that Gardiner's frogs receive sound through their heads. The mouth amplifies the frequencies and the sound is transmitted through tissue and bones in the skull to the inner ear. 
 
 This image shows a segment of a frog's head, displaying the two inner ears (light grey), the brain in between, and the buccal cavity beneath. Credit: R. Boistel/CNRS 
 
 The X-ray images showed reduced thickness and fewer layers of tissue between the frogs' mouths and their inner ears, compared with other frog species, which suggests these auditory adaptations were likely the result of evolutionary forces in Gardiner's frogs, the researchers 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," Boistel said. 
 
 The detailed findings of the study were published online Monday (Sept. 2) in the journal Proceedings of the National Academy of Sciences. 
 
 Follow Denise Chow on Twitter @denisechow. Follow LiveScience @livescience, Facebook & Google+. Original article on LiveScience.
– 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.