Female Concave-eared Frogs Draw Mates With Ultrasonic Calls

Excerpt from ScienceDaily.com

Most female frogs don't call; most lack or have only rudimentary vocal cords. A typical female selects a mate from a chorus of males and then --silently -- signals her beau. But the female concave-eared torrent frog, Odorrana tormota, has a more direct method of declaring her interest: She emits a high-pitched chirp that to the human ear sounds like that of a bird.

This is one of several unusual frog-related findings reported recently in the journal Nature. O. tormota lives 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. The frog has a recessed eardrum, said Albert Feng, a professor of molecular and integrative physiology at the University of Illinois and team leader on the new study. "In the world we know of only two species -- the other one in southeast Asia -- that have the concave ear," Feng said. "The others all have eardrums on the body surface."

Earlier studies, conducted 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, found that O. tormota males emit -- and respond to -- unusual chirping calls from 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, allowing it to detect very high frequency sounds. The unusual ear structure and the high-pitched calls are likely an adaptation to the noisy environment, Feng said.

The waterfalls and streams produce a steady racket predominantly in a lower frequency range than that used by the frogs. Laboratory experiments showed that the frogs could hear most of the audible and ultrasonic frequencies emitted by other O. tormota frogs. The only other animals known to use ultrasonic communication are bats, dolphins, whales and some insects. The calls are quite complex. A single O. tormota frog broadcasts its message over several frequencies at once, at harmonic intervals, like a chord strummed simultaneously on several strings.

The new analysis, conducted by Shen, Feng and Narins, found that female O. tormota frogs also emit a call that spans audible and ultrasonic frequencies. The team has not observed females vocalizing in the wild (these frogs are nocturnal and can leap up to 30 times their body length), but in laboratory settings the females emitted calls only when they were carrying eggs.