Some of the tiniest frogs on Earth have no middle ears or eardrums but can hear by using their mouths, scientists said Monday.

Gardiner's frogs live in the rainforests of the Seychelles, a series of 115 small islands in the Indian Ocean, north of Madagascar.

Most frogs have eardrums on the outsides of their heads. The eardrums vibrate when incoming sound waves hit, sending the vibrations to the inner ear, then the brain.

But not the wee Gardiner's frogs, which measure about a centimeter long or the size of a thumbtack.

Researchers thought that the creatures might be deaf, until they tested them by playing pre-recorded sounds of other frogs croaking. They found that male Gardiner's frogs croaked back, as if in conversation, proving they could hear.

Advanced X-ray images showed that neither the lungs or the muscles of the frogs were helping transmit sound to their inner ears.

Instead, scientists realized that the frog's mouth is what acts as an amplifier for the sound frequencies the frog emits.

The system is boosted by very small amount of thin tissue between the mouth and 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 Renaud Boistel of the University of Poitiers and French National Center for Scientific Research (CNRS).

The findings in the Proceedings of the National Academy of Sciences, a US journal, add to what is known about how some creatures, including frogs and turtles, evolved the ability to hear.

"We show that the presence of a middle ear is not a necessary condition for terrestrial hearing, despite being the most versatile solution for life on land," said the study.

[Image via Agence France-Presse]