London, May 12 : Unlike other female frogs, concave-eared frogs draw their mates by producing a high-pitched chirp that to the human ear sounds like that of a bird, according to the researchers.
A female frog selects a mate from a chorus of males and then silently signals declaring her interest. However, Odorrana tormota (O. tormota) or concave-eared frogs attracts her mates through ultrasonic calls.
O. tormota frogs are found in a noisy environment on the brushy edge of streams in the Huangshan Hot Springs, in central China, where waterfalls and rushing water provide a steady din.
Lead researcher Albert Feng, a professor of molecular and integrative physiology at the University of Illinois said the frog has a recessed eardrum.
"In the world we know of only two species - the other one in southeast Asia - that have the concave ear. The others all have eardrums on the body surface," Nature quoted Feng, as saying.
Previous studies conducted by Feng have shown that O. tormota males could converse in unique chirping calls with other males.
These calls are audible, but also have energy in the ultrasonic range. The recessed ear structure protects an eardrum that is 1/30 the thickness of that of a normal frog that allows it to detect very high frequency sounds.
A single O. tormota frog transmitted its message over several frequencies at once, at harmonic intervals, like a chord strummed simultaneously on several strings.
The present study by Feng, Jun-Xian Shen at the Institute of Biophysics at the Chinese Academy of Sciences and Peter Narins at the University of California, Los Angeles discovered that female O. tormota frogs also emit a call that spans audible and ultrasonic frequencies.
The laboratory studies showed that females emitted calls only when they were carrying eggs.
And when the male O. tormota frogs were exposed to female calls recorded by the researchers, it showed that the males were quite responsive, usually chirping within a small fraction of a second and leapt directly to the source from where they could hear the female call. The accuracy was over 99 percent.
"The frog's response is instantaneous - right after the stimulus," said Feng.
"We have a lot of work to do to figure out whether she directs the signal to one male or whether she lets a bunch of males come and compete, or whether there is any kind of dueting session during which she then decides: 'OK, You're my guy. Hop on my back and I'll take you to the creek!'" he added.
The study appears this week in journal Nature.