One of the smallest frogs in the world has no middle ear or eardrum but can still hear the croaks of other frogs -- through its mouth, French researchers say. Scientists using X-rays said they\'ve established Gardiner\'s frogs from the Seychelles islands are using their mouth cavity and tissue to transmit sound to their inner ears. Most frogs do not possess an outer ear like humans, but possess a middle ear with an eardrum located directly on the surface of the head. Incoming sound waves cause vibrations that are sent to the inner ear and then to the brain. \"However, we know of frog species that croak like other frogs but do no have tympanic middle ears to listen to each other. This seems to be a contradiction,\" Renaud Boistel of the University of Poitiers said. X-ray images helped the researchers confirm the mouth of Gardiners\'s frogs acts as a resonator, or amplifier, for the frequencies emitted by this species, providing a pathway from the mouth through bones to the frogs\' inner ear. \"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.