By Helen Briggs
BBC News science reporter
A rare Chinese frog has entered the record books as the first amphibian known to communicate using ultrasound.
Noisy surroundings led to the creative croak, scientists say (Image: Albert Feng)
Until now, only a few mammals - such as bats, whales and dolphins - have been found to use the very high frequency sound to contact each other.
The frog may have evolved the mechanism to be heard above the babble of running water, scientists tell this week's edition of the journal Nature.
The frog lives alongside fast-flowing streams in Anhui Province, China.
During the rainy season, the water level rises dramatically, creating a noise that drowns out the calls of many small animals.
Amolops tormotus can be heard above the din by producing ultrasonic (greater than 20 kilohertz) calls beyond the spectrum of the background hubbub.
The Chinese frog's vocalisation is very unusual. Most frog calls go either up or down only; but A. tormotus uses multiple upward and downward sweeps of notes more in keeping with the sounds made by birds, whales or primates.
A team led by Albert Feng, a US professor, visited the bank of China's Tao Hua Creek in search of the frog.
They heard the warbling melody of what they thought was a bird coming from some undergrowth.
It turned out to be the song of a male frog of the very species they wanted to study.
Dr Feng wondered whether other members of the species were able to hear and respond to the calls or if it was just for show.
He devised an experiment to find out. The team recorded a frog's calls, split them into their constituent frequencies and tested other frogs' responses to them in the wild.
They found that most of the frogs responded to ultrasonic and audible sound ranges, half of them sending back their own ultrasonic and audible calls in response.
Only male frogs were tested and it is not yet known whether female frogs, which have a different ear structure, use ultrasound.
"Nature has a way of evolving mechanisms to facilitate communication in very adverse situations," said Dr Feng.
"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. Frogs were never taken into consideration for being able to do this."
Dr Feng believes the capacity to respond to ultrasound may well be found in other amphibians and birds.
The fact that it has been found in amphibians, which are on a different evolutionary pathway from mammals, suggests it has evolved several times independently in the animal kingdom.
"Humans have always been fascinated by how some animals can discern their world through a sensing system vastly different from our own," said Feng, a professor of molecular and integrative physiology at the University of Illinois at Urbana-Champaign.
"The electromagnetic sense in fishes and homing pigeons, polarised light vision in ants, chemical sensing of pheromones in insects and rodents, echolocation by ultrasound in bats and dolphins, are just a few examples.
"Frogs that can communicate with ultrasound adds to that list and represents a novel finding, because we normally think such ability is limited to animals equipped with a sophisticated sonar system.
"This suggests that there are likely many other examples of unexpected forms of communication out there."
Professor Tim Halliday of the Department of Biological Sciences at The Open University, UK, said other types of frog had also been able to solve the problem of living near noisy streams, where they cannot make themselves heard.
"At least two other frog species, one from Borneo, the other from South America, have resolved this dilemma in a quite different way," he told the BBC News website.
"Rather than calling to attract females and deter rival males, they use visual displays, waving their brightly coloured feet."
Dr Feng's lab at the University of California at Los Angeles worked with colleagues at the Shanghai Institutes of Biological Sciences; and the Institute of Biophysics in Beijing.