It comes in handy to be heard above the pounding waterfalls and streams in the mountainous region of east-central China where Amolops tormotus, which is known as the concave-eared torrent frog, lives.

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

But until now it was not known that some frogs were able to use ultrasound.

Kraig Adler, a biologist at Cornell University in New York, first noticed the frog with no external eardrums while surveying amphibians in China. He told Feng, an auditory neuroscientist who studies frogs and bats, about his find.

Feng and his colleagues conducted tests on the frogs to determine whether they could hear and respond to ultrasounds.

"Now we are getting a better understanding of why their ear drums are recessed," said Feng, who reported his findings in the journal Nature.

"Thin eardrums are needed for detection of ultrasound. Recessed ears shorten the path between eardrums and the ear, enabling the transmission to the ears," he added in a statement.