Frog Hears With Its Mouth, Surprises Scientists

We’ve told you about an urchin that can eat with its anus and a tadpole that can see through its tail.  And now, in the latest discovery of a surprising use for body parts, scientists say there’s a frog that can hear through its mouth.

Scientists had thought that the Gardiner’s Seychelles frog—at 11 millimeters among the tiniest in the world—was deaf because it doesn’t have a middle ear, a critical component of hearing that’s found in most mammals.

gardiner's seychelles frog picture
A male Gardiner’s Seychelles frog. Credit R. Boistel/CNRS

The middle ear contains three small bones called ossicles that connect the eardrum to the inner ear. When a sound wave hits the eardrum, it moves back and forth, causing the ossicles to move and send electric signals to the brain, which then registers the signals as sound. (See “World’s Loudest Animals—’Power Saw’ Cricket, More.”)

Scientists knew of other frog species without middle ears that would still croak to each other. So they wondered: Can Gardiner’s frogs somehow hear?

To answer the question, a team set up loudspeakers in the frogs’ rain forest habitat of the Seychelles Islands, in the western Indian Ocean, and broadcasted recordings of frog calls. Surprisingly, the male frogs hopped closer to the loudspeaker and responded, indicating they’d heard the noise, said study co-author Thierry Aubin, a bioacoustics expert at the Université Paris Sud in France.

Hear Hear, Little Frogs

Next, the team had to identify how the frogs were hearing—that is, how the sound waves were getting to their brains. So the scientists x-rayed one of the tiny frogs. The images revealed that the frogs’ pulmonary system is poorly developed, suggesting that the lungs aren’t contributing to hearing, according to the study, published September 2 in the Proceedings of the National Academy of Sciences.

So the scientists refocused their experiments on the frogs’ heads. By making various 3-D simulations of how sound travels through the frogs’ heads, the scientists found that the bones in their mouths act as an amplifier for sound waves. (See National Geographic’s frog and toad videos.)

frog-hearing experiment illustration
Experiments revealed that the frog hears through its mouth. The 99.9 percent figure refers to the fact that nearly all sound waves that reach an animal are reflected at the skin’s surface. Illustration by R. Boistel/CNRS

Further x-rays showed that sound travels from the mouth to the inner ear, thanks to an evolutionary adaptation: The frogs have thinner and fewer tissue layers between the mouth and inner ear.

“It’s interesting because this was never found before,” Aubin said.

The team also suspects that the frogs’ odd hearing strategy may be due to their long isolation. About 47 to 65 million years ago, the Seychelles broke off from the supercontinent Gondwana–one of two land masses comprising the supercontinent Pangaea–splitting the frogs off from other species. So hearing through the mouth may actually be a holdover from ancient life-forms, the authors say. (Also see “Ancient Reptiles Had Advanced Hearing, Fossils Suggest.”)

So the next time you have a frog in your throat, think of these little guys who actually hear with it.

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Meet the Author
Christine Dell'Amore, environment writer/editor for National Geographic News, has reported from six continents, including Antarctica. She has also written for Smithsonian magazine and the Washington Post. Christine holds a masters degree in journalism with a specialty in environmental reporting from the University of Colorado at Boulder. Her book, South Pole, was published in 2012.