I’m all mouth: how 1cm-long Gardiner’s frog gets by without a middle ear

It sounds like a riddle devised by Rudyard Kipling: how can the tiny Gardiner’s frog of the Seychelles hear sounds without the benefit of a middle ear?

Animals need a middle ear to amplify the vibrations picked up by the eardrum as it detects sound waves, but the Gardiner’s frog, which has lived in isolation in the rainforests of the Seychelles for tens of millions of years, lacks this vital acoustic detail.

About 99 per cent of a sound wave reaching an animal is reflected by the surface of its skin. Middle ears have evolved to get round the problem by amplifying the tiny movements of the eardrum when a sound wave causes it to vibrate.

Some experts suggested that instead of a middle ear, the frog used its lungs or bones to amplify sounds, while other experts even doubted whether the small amphibian could hear anything at all, even though it croaked just like many other frogs.

Scientists have since showed that the Gardiner’s frog is definitely not deaf because it responds to recordings of other Gardiner’s frogs. But how did it manage to hear anything at all?

Now a study by researchers using the European Synchrotron Radiation Facility in Grenoble has demonstrated that the one-centimetre-long Gardiner’s frog uses is mouth as an acoustic chamber which amplifies the sounds it needs to hear, such as the croaking of courting frogs nearby.

Tests by the X-ray facility show that the volume of the frog’s mouth is perfectly matched to resonate at the frequencies of the sound waves made by a flirtation frog. The amplified sounds are then shunted to the frog’s inner ear and auditory nerves with the help of its bony skeleton.

“The combination of a mouth cavity and bone conduction allows Gardiners’ frogs to perceive sound effectively without the use of a tympanic middle ear,” said Renaud Boistel of the University of Poitiers, who led the study published in the journal Proceedings of the National Academy of Sciences.

“These small animals have been living isolated in the rainforest of the Seychelles for 47 to 65 million years, since these island split away from the main continent. Their auditory system must be a survivor of life forms on the ancient supercontinent of Gondwana,” Dr Boistel said.