Endangered earless frogs use their mouths to hear

The Gardner's frog of Seychelles have no ears, but can hear with mouth.
The islands of Seychelles, off the west coast of Africa, are home to a tiny species of frog that has perplexed scientists for years because, although they appear to be able to hear, they don't have the parts of the ear necessary to hear sounds. Now, scientists have been able to figure out how they manage feat, and it's by using their mouths.

Most animals have an outer ear (that helps collect sound waves and funnel them to the eardrum), a middle ear (the eardrum and tiny bones that amplify the sound waves and pass them on to the inner ear) and an inner ear (that translates the sound waves into electrical impulses to our brain). Some animals can get by with just the middle and inner ear. However, with only an inner ear, an animal would only be able to hear about one tenth of one percent of sounds, because the rest of the sound waves would never reach the inner ear.

The Gardner's frog of Seychelles seems to do just fine, though, so a team of scientists led by Renaud Boistel, of France's National Center for Scientific Research (CNRS), decided to find out how.

A 3D simulation of the inside of the frog gives a look at the structure of its mouth and inner ear.By using careful X-ray imaging of these tiny, 1 cm-long frogs, and even computer simulations, the scientists discovered that the frog's mouth acts like a substitute for both the outer ear and middle ear. It amplifies the sound waves these frogs produce when they croak, kind of like a resonance chamber tuned specifically to its species, and funnels them through thin layers of tissue at the back of the mouth, directly to the inner ear.

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The Gardner's frogs of Seychelles may be the smallest species of frogs on Earth. Their numbers have been on the decline, and since 2004, they were listed as Vulnerable on the International Union for Conservation of Nature (IUCN) Red List, mainly due to habitat loss as the land where they live is cleared for farming, by logging companies, or for human settlements. As of the latest edition of the Red List, the frog's status was changed to Endangered, due to the continued threat of habitat loss.

(Images courtesy: R. Boistel/CNRS)

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