Gardiner's frogs hear through their mouths, researchers say
When animals don't have outer ears, they make up for it with inner ears. They transmit sound through an eardrum that is located at the crown of their head. Sound waves make the eardrum vibrate, and the eardrum transmits the waves to the inner ear, where they are translated into electric signals and sent to the brain.
Most frogs have this middle ear and eardrum system. But the world's smallest frogs, Gardiner's frogs from the Seychelles islands, don't even have the middle ear or eardrum. Without the eardrum, it seems impossible that they are able to hear, because skin deflects 99.9 percent of sound waves.
And so, scientists have long been puzzled by the Gardiner's frogs' ability to croak and to hear.
Now, in a study published Sept. 2 in the journal Proceedings of the National Academy of Sciences (PNAS), European researchers say they've solved this evolutionary mystery. Through x-rays, they've determined that Gardiner's frogs use their mouth cavity and tissue to transmit sound to their inner ears.
"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," lead researcher Renaud Boistel of the University of Poitiers in Poitiers, France, said in a press release.
The researchers first tested whether Gardiner's frogs communicate with audio cues by setting out loudspeakers in the Seychelles islands rain forests and playing recordings of frog songs. The male frogs responded, indicating that they could hear.
Once they confirmed that the frogs can hear, they started to determine the biological mechanism. They considered whether muscle tissue could transmit sound, or if the process was happening in the lungs.
"Whether body tissue will transport sound or not depends on its biomechanical properties," explained researcher Peter Cloetens of the European Synchrotron (ESRF) in Grenoble. "With x-ray imaging techniques here at the ESRF, we could establish that neither the pulmonary system nor the muscles of these frogs contribute significantly to the transmission of sound to the inner ears."
"As these animals are tiny, just one centimeter long, we needed x-ray images of the soft tissue and the bony parts with micrometric resolution to determine which body parts contribute to sound propagation."
The x-rays showed that the mouth works like an amplifier. It takes in and transmits sound waves emitted by other Gardiner's frogs so that they can hear one another. Two evolutionary adaptations unique to Gardiner's frogs allow for this hearing mechanism: the layers of tissue between their mouth and inner ear are thinner and fewer than what is found in other frog species.
