Ultrasonic Frogs Discovered in China Make "Silent" Calls

Amitabh Avasthi
for National Geographic News
March 16, 2006
Noisy waterfalls and claps of thunder can drown out even the most vocal
frog. But some persistent croakers in China have a clever fix: They
switch to ultrasound.

The feat, researchers say, makes the frogs the first amphibians to be placed alongside an exclusive group of mammals, such as whales and dolphins, that have ultrasonic ability.

"It shows a new example of independent evolutionary adaptation in the frogs for life in habitats filled with loud background noise," said Albert Feng, a professor of neuroscience at the University of Illinois, Urbana-Champaign.

Researchers first heard of the unusual animals in 2000 from Kraig Adler, a biologist at Cornell University.

He tipped the scientists off to Huangshan Hot Springs, a popular tourist destination near Shanghai, where he had found frogs with recessed ears.

"I noticed the frogs' sunken ears and thought they must have an odd system of communication. We had no idea they used ultrasonic sounds," Adler said.

That year a research team led by Feng reported that these frogs, called concave-eared torrent frogs (Amolops tormotus), sang like birds.

Two years later, using wide-band recording equipment, Feng and his colleagues discovered that the frogs were also croaking in ultrasound—sound vibrations beyond the limit of human hearing.

The team's findings appear in the current issue of the journal Nature.

Ultrasonic Hearing

To test the frogs' hearing prowess, Feng went to the site in China (map) last summer with devices that could play back sound in both audible and ultrasonic ranges.

The researchers played recorded sounds to eight male frogs.

When the recordings were in the audible and ultrasonic ranges, five of the frogs gave significantly more replies than when no sound was played at all.

In response to one particular ultrasonic playback, a frog shot back with 18 rapid-fire calls.

"Males not only responded to ultrasound but also approached our loudspeaker as if to confront the frog making that sound," Feng said.

In a second test, researchers observed the brain activity of one frog during playback. The frog showed a strong response to ultrasound but no response when both its ears were blocked with modeling clay.

"This confirmed that the hearing was indeed taking place through the ear and not through sound that penetrated the skull to directly stimulate the neurons," Feng said.

Energy-Saving Croak

Researchers theorize that there could be an evolutionary explanation for the frog's ultrasonic hearing.

Males trying to catch a female's attention need to have their calls heard over the din from other suitors, as well as over background noise, Feng noted.

Shifting the call to a much higher frequency not only ensures it is heard, he said, but it also requires less energy than generating very loud audible croaks.

The secret to the frogs' ultrasonic hearing appears to be in their unique ear structure. Their eardrums are superthin and sunken below the body surface to protect the membranes.

But the exact part of the ear that facilitates ultrasonic hearing is not known, Feng said.

"This is an interesting study for several reasons," said Michael Ryan, a zoologist at the University of Texas at Austin.

"It shows the potential for some frogs to hear in a frequency range never suspected and an unusual way to adapt to background noise."

But Feng's study begs the question as to why only males and not females have this capability, he added.

Adler, the Cornell biologist, said, "This is a fabulous paper and a carefully conducted study on the anatomy of an odd frog."

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