| 248 shares 10 View 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 Boistelsaid:'The combination of a mouth cavity and bone conduction allows Gardiner's frogs to perceive sound effectively without use of a tympanic middle ear'. Share or comment on this article
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