for National Geographic News
A Chinese frog that uses ultrasonic communication can tune its ears like a radio dial to block out lower pitched background noise, a new study finds. This makes the concave-eared torrent frog the only known animal that can physically control which frequencies it hears by opening and closing parts of its ears.
"This was contrary to everything that we knew about [the frog's] auditory system," said study co-author Albert Feng of the University of Illinois at Urbana-Champaign.
Feng's team speculates that the tiny frog—which lives near rushing streams and noisy waterfalls in central China—uses the adaptation to block out background noise when it wants to hear the calls of mates or rivals.
Open and Shut
The concave-eared torrent frog is the only amphibian known to make ultrasonic calls, or communications in frequencies far above the range of human hearing.
Just a few other animals, including bats and dolphins, are thought to have this ability.
Earlier this year, Feng and colleagues reported that male torrent frogs can localize sound with unusual accuracy to find females during ultrasonic mating duets.
Further studies of the amphibian's hearing showed that its eardrums vibrate in response to ultrasonic noises, but only some of the time.
This surprised the team, because in all other frogs eardrums always respond the same way to a sound stimulus.
Further examination revealed that the Chinese frogs were actively opening and closing their eustachian tubes, two narrow channels that connect the mouth cavity to the left and right ear.
Closing the tubes improved the frogs' ability to hear high frequencies and ultrasounds, while opening them increased sensitivity to low-frequency noises.
The finding is detailed in this week's issue of the Proceedings of the National Academy of Sciences. "This mechanism is truly unique in the animal kingdom," commented James Saunders, an auditory expert at the University of Pennsylvania.
Saunders pointed out that humans can also "selectively hear" different sounds. For instance, people can single out the sound of a bassoon over other instruments during an orchestra recital.
But selective auditory attendance in humans is mostly a trick of the mind. It involves neurons in the brain homing in on sounds coming from certain directions.
By contrast, the Chinese frogs have evolved the biological equivalent of earmuffs to block out all sounds of a certain frequency range.
Study co-author Feng speculates that the frogs' tunable ears are an adaptation to their noisy home environments.
For example, shifting to high-frequency hearing could help the frogs pick out mating calls during a storm, when the low-pitched noises of plunking raindrops, booming thunder, and rushing water dominate.
"If you or I were in this situation, we would be trapped," Feng said.
"The background noise is coming from everywhere, so our kind of selective hearing wouldn't do us any good.
"The frogs just say, I'm not hearing this. I'm going to switch to another channel."