Chinese frog that squeaks like a mouse

A frog that sings like a bird has been found to have another unusual trait - it can communicate in high-pitched squeaks that are inaudible to the human ear.

The frog lives in the fast-moving streams and waterfalls of east-central China and uses ultrasonic calls to make itself heard above the noise of running water.

It is the first time that an animal other than a bat, marine mammal or a rodent has been found to be capable of producing and hearing high-frequency sound.

Scientists found some years ago that the concave-eared torrent frog emits ultrasounds but the latest study, published in the journal Nature, shows the sounds are used as calls rather than being a by-product of other vocalisations.

The human ear cannot hear sounds with frequencies higher than 20 kilohertz but the frog has been recorded emitting sounds higher than 128kH which can carry above the noise of splashing water.

"Nature has a way of evolving mechanisms to facilitate communication in very adverse situations," said Professor Albert Feng of the University of Illinois at Urbana-Champaign.

"One of the ways is to shift the frequencies beyond the spectrum of background noise. Mammals such as bats, whales and dolphins do this, and use ultrasound for their sonar system and communication," he said.

The frog - Amolops tormotus - has an unusually recessed ear canal which protects a thin and delicate eardrum that can detect high-frequency sounds, Professor Feng said. "Thin eardrums are needed for detection of ultrasound. Recessed ears shorten the path between eardrums and the ear, enabling the transmission of ultrasound to the ears," he said.