The noise of a waterfall can easily cover the croak of a frog and this is why, in most cases, many frogs living near flowing water have quiet croaks and lack vocal sacks. However, a Chinese frog came up with the perfect solution for this tiny predicament: ultrasound croaks! We're talking about the small concave-eared torrent frog (Amolops tormotus = Odorrana tormota) from eastern China, which thus manages to overcome the noise of the waterfalls it lives in. The only other vertebrates known to use ultrasounds are all mammals: bats, dolphins, whales and a few rodents. A new research published in the Nature journal details how these frogs use ultrasounds in their mating system.

Ultrasounds have frequencies of over 20 kHz and cannot be heard by the human ear. These frogs emit ultrasounds of over 128 kHz! As far as we know, they do not have visible external eardrums. Recessed ears shorten the length between the eardrums and the ear, favoring the transmission of ultrasound to the ear. Some frogs from tropical America, that are usually preyed on by bats, can detect ultrasounds emitted by bats and immediately become quiet when they perceive them; however, it has not yet been proven if they can also emit ultrasounds. Their croaks are clearly audible by humans.

The ultrasonic frogs live near Yellow Mountain in eastern China. "The background noise covers the entire human hearing range. Just talking to my colleagues there, you really have to raise your voice to be heard. We think these animals are under evolutionary pressure to avoid that channel and go above it," said Peter Narins, a biologist at the University of California, Los Angeles.

The ultrasounds are mainly courtship calls: males show off their quality, attracting females, while the latter emit calls just before they ovulate to announce their partners about it. The American-Chinese team recorded the squeaks of female Odorrana frogs with an ultrasonic microphone; when these ultrasounds were played back to male frogs, they responded with ultrasounds and jumped toward the speakers. The team was surprised by the precision of the males in determining the exact location from where the female calls were coming.

"From a meter away you can play the sound of a female, and in some cases, in one leap the males jump right to the center of the sound. Their precision was remarkable - it rivals that of the best vertebrate performers, such as barn owls, dolphins and humans. Male Odorrana frogs can locate the source of a female's mating call to within about one degree on average. In contrast, most other amphibians can only pinpoint sound to within 16 to 23 degrees," Narins told LiveScience.

Animals have to compare the intensity of the sounds captured by their two ears to determine where the source of a sound is located. Because the signal detected by one ear is stronger than to the other (a phenomenon called "sound shadow"), the animal can determine the source of the noise.

The size ratio of the animal's head connected to the wavelength of the sound does count: when sound waves are much smaller than the head, many sounds are reflected and one ear receives a much stronger signal than the other. When the sound waves are larger than the head, the two ears get comparable signals, and locating the source of the noise becomes difficult.