The tiny Gardiner's frog - one of the smallest frogs in the world - once believed to be deaf, actually hears sounds through its mouth, scientists have found.
It has been a mystery how the one-centimetre-long Gardiner's frogs from the Seychelles islands, which do not possess a middle ear with an eardrum, can croak themselves and hear other frogs.
An international team of scientists using X-rays has now established that these frogs are using their mouth cavity and tissue to transmit sound to their inner ears.
Some animals notably most 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.
It is not possible to detect sound in the brain without a middle ear because 99.9 per cent of a sound wave reaching an animal is reflected at the surface of its skin.
"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," said Renaud Boistel from the University of Poitiers and French National Centre for Scientific Research (CNRS).
"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. If they can hear, their auditory system must be a survivor of life forms on the ancient supercontinent Gondwana," Boistel said.
To establish whether these frogs actually use sound for communicate with each other, 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.
Researchers conducted numerical simulations that confirmed that 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.
"The combination of a mouth cavity and bone conduction allows Gardiner's frogs to perceive sound effectively without use of a tympanic middle ear," said Boistel.
