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.
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