Far away on the islands of Mahé and Silhouette in the Seychelles in the western Indian Ocean can be found one of the world’s smallest frog species, Gardiner’s frog, just 10 to 11 millimeters in length. These fogs should be deaf, because they lack a middle ear (in humans, that’s the bit with the eardrum). That part of the ear is thought to be a necessary adaptation to life on land, helping sound to make the transition from air to tissue, where it can be translated into nerve impulses that reach the brain. But Gardiner’s frog has evolved a different method for hearing — it uses its mouth, reports a team of French researchers in a new study in PNAS.
Scientists had been somewhat perplexed by Gardiner’s frogs because, despite lacking the necessary equipment for hearing, the amphibians appear to have no problems communicating. The French team began by recording calls from frogs on Silhouette and playing them back over loudspeakers to frogs in their natural forest habitat. When a male heard the call from one of its own species, it called back a response. But the frogs didn’t respond when they heard sounds from other species. They were definitely hearing and well enough to make species distinctions.
Dissecting the frogs might be a classic technique in the classroom, but here it just wasn’t an option; these frogs are far too small to see anything useful with dissection. Instead, the researchers used a technique called X-ray synchrotron holotomography to image the frog’s inner anatomy. Then they used that data to create a computer simulation of the amphibian and determine how the sound was traveling through the frog’s head.
Previous studies had suggested that sound passed through the lungs on its way to the inner ear, where it would be translated into nerve impulses. Another theory was that sound was conducted through the animal’s bones. But the computer simulations revealed that the mouth (or oral cavity as the scientists name it) is actually the ideal anatomical structure for amplifying sound. The cavity resonates sound at a frequency that nearly matches the frequency most commonly found in the frogs’ calls. The researchers also found that the tissue that separates the inner ear from the inner mouth in Gardiner’s frogs is very thin, and there are fewer layers of tissue, which helps the sound to pass through.
The frog’s mouth might also play a role in determining the direction of sound, the researchers suggest. And, they note, this discovery shows that the middle ear isn’t quite so necessary for life on land as had been thought.
Image credit: R. Boistel/CNRS