A Tiny Frog That Should Be Deaf Hears With Its Mouth

GardinersFrogFar 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

This Moth Has Better Hearing Than A Bat (Or Anything Else)

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The greater wax moth (Galleria mellonella) is locked into a high-frequency, evolutionary battle with the bats that prey upon them. Bats emit ultrasonic for the purpose of echolocation, sending out sounds at frequencies as high as 212 kHz. Now scientists have discovered that not only can the great wax moth hear the bats’ echolocation calls, but the moths can hear even higher frequency sounds, up to 300 kHz. The research team from the University of Strathclyde in Glasgow, Scotland report their findings in Biology Letters.

Such auditory sensitivity is “unprecedented,” the scientists write. No known bat produces sounds at that high a frequency, so why the moth evolved to hear such sounds is a bit of a mystery. But the researchers suspect that the super-hearing helps the moths avoid predators or communicate with each other, perhaps in courtship.

“The use of ultrasound in air is extremely difficult as such high frequency signals are quickly weakened in air,” the lead researcher James Windmill said in a statement. “It’s not entirely clear how the moths have developed to be able to hear at such a high frequency, but it is possible that they have had to improve the communication between each other to avoid capture from their natural predator – the bat – which use similar sounds.”

Image courtesy of Sarefo via Wikimedia Commons