Frog chorus goes ultrasonic

16 March 2006

Thought the frog song was a load of old 'ribbit'? Think again. By tuning into a Chinese frog's frequency, scientists have revealed that amphibians can communicate with ultrasound like bats and dolphins.

Antenna puts an ear to the ground...

This story was published in Nature on 16 March 2006.

This frog's other talents have already hit the headlines. In 2002 surprised scientists discovered it can sing like a bird.

Image: Albert Feng

The rare frog, called Amolops tormotus, is the first amphibian to join a select elite of mammals, including bats, whales and dolphins, with the ability to communicate at sound frequencies that humans can't hear.
'We discovered that these frogs produce calls that extend into the ultrasonic range, and demonstrated that other frogs can hear and communicate with these ultrasounds.'
Albert Feng, sound communication expert, University of Illinois

Albert Feng, sound communication expert, University of Illinois

The frogs are from Huangshan Hot Springs, in the Yellow Mountains area of China. Alive with noisy waterfalls and gurgling streams, Huangshan is home to a wide variety of wildlife.
Using an ultrasonic recording device, the scientists recorded a frog call and separated the audible part of the call (that humans can hear) from the ultrasonic part (the sounds that only bats, dolphins and some rodents hear).

Huangshan Hot Springs

Image: iStockphoto.com/Chee Woon Peng

With loudspeakers positioned near the frogs, the scientists played the ultrasonic part of the call to the frogs.
'When male frogs hear another male's call they generally respond by producing their own calls - this response is what causes large frog choruses.'
Albert Feng

Two scientists hunt for frogs under leaves.

Image: Albert Feng


Listen to the audible part of a frog's call. Their unusual vocal variety means males can warble like birds - a first for frogs.

Audio: Albert Feng

The team recorded the sounds made by the frogs in response to the ultrasonic call played via a loudspeaker.

'We showed that male Amolops frogs produce croaks when they hear the audible or ultrasonic components of another male's call.'
Albert Feng
Scientists think the frogs developed this ability in order to get heard above the forest racket - a bit like having their own communication channel.
'The extraordinary shift in the frequency range of hearing in these frogs allows them to communicate in the presence of intense background noise of nearby water streams.'
Albert Feng

Huangshan is alive with noisy waterfalls.

The frogs' calls are recorded using this kit which picks up ultrasonic sound.

Image: Albert Feng

Ultrasound refers to very-high-frequency vibrations above the range audible to the human ear, i.e. above 20 kiloHertz - these frog calls can hit frequencies of over 128 kiloHertz.
The frogs have specially adapted ears for picking up ultrasound. Their eardrums are very thin, and they're tucked inside the ear canal.
'Putting the eardrum at the end of the ear canal provides protection for the thin eardrum, and shortens the distance between the eardrum and the inner ear. This transmits high-frequency sound more efficiently.'
Albert Feng

The frog's ear canal protects its ear drums.

Image: Albert Feng

But it's only the male frogs that have their eardrums positioned inside. Females don't have this adaptation. Experts think this is because, in the frog world, males do the calling and females do the choosing.
'Male frogs are competing with other males when they call for females. The ability to hear each other in ultrasound lets the males monitor what other males are doing.'
Albert Feng

So while the females are hearing the audible call of the male, the males are also listening in to the ultrasonic part of other males' calls.
'Frogs communicating with ultrasound is surprising, because we normally think such ability is limited to animals equipped with a sophisticated sonar system.
'Humans have always been fascinated by how some animals can detect their world through a sensing system vastly different from our own. This research suggests that there are likely many other examples of unexpected forms of communication out there.'
Albert Feng

Are there other animals out there that communicate in ways humans don't expect?

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