China Tree Frogs Sing Ultrasonic Duets, Study Finds

Kevin Holden Platt in Beijing, China
for National Geographic News
May 13, 2008
Tree-dwelling frogs that live near the noisy Huangshan Hot Springs of China stage intricate courtship musicals at frequencies far beyond the range of human hearing, according to a new study.

A team of Chinese and American scientists found that the frogs communicate their symphony of mating calls ultrasonically.

The researchers, led by Shen Junxian of the Chinese Academy of Science, used microphones to record the mating calls emitted by female torrent frogs on rainy nights just before ovulation.

When the ultrasonic recordings were played back to a group of male torrent frogs, they responded by singing in synchrony with the female lead and then leaping toward the source of the call with pinpoint accuracy.

The scientists say that using ultrasonic calls allows the frogs to communicate clearly despite the background of crashing mountain waterways. (Related: "Ultrasonic Frogs Discovered in China Make 'Silent' Calls [March 16, 2008].)

The frog, Odorrana tormota, joins select species of birds, along with dolphins and whales, that are known to communicate ultrasonically to circumvent background noise, said study co-author Albert Feng, a physiologist at the University of Illinois at Urbana-Champaign.

The findings appear online this week in the journal Nature.

Frog Duets

According to the authors, female frogs make the first overture with a courtship call featuring multiple harmonics that extend into the ultrasonic range—frequencies greater than 20 kilohertz, the upper limit of human hearing.

Like a soprano in a human opera, the female torrent frog's call "has a much higher fundamental frequency than her male counterpart's," said study co-author Peter Narins, a professor of physiological science at the University of California, Los Angeles.

This opening solo often triggers a quick "antiphonal response" by a nearby male, which issues a series of lower-pitched ultrasonic calls in return.

This musical dialogue resembles melodic exchanges between birds or human singers.

"A male's antiphonal response is similar to the second part of a duet," said Feng, who is also affiliated with the Beckman Institute for Advanced Science and Technology.

The mating calls of these concave-eared torrent frogs "have some musical qualities," Narins added. "Whether or not you would want to dance to this, I don't know."

Hearing Aid Aid

The frogs' calls are high enough that they could even work in sonically complex urban environments. If a male and female torrent frog were to meet in a noisy disco, for example, they could still hear each other across the dance floor.

"Disco music is limited to frequencies that humans hear, nominally from 20Hz to 20kHz. If that frequency band is saturated with disco music, then by using ultrasonic frequencies, frogs should be able to communicate despite the noise," Narins explained.

He added that the new research might one day be translated into better communications technology for humans.

Feng and colleagues at the Beckman Institute had tapped previous research on frog hearing and communication to develop an intelligent, directional hearing aid, for example.

"These directional hearing aids are great in situations like crowded parties," Narins said. "With an ordinary hearing aid, the user can only turn up the volume of all the sound around him. But with a directional device, he can focus on and amplify the voice of the person next to him."

The new study was funded in part by the National Institute on Deafness and Other Communication Disorders, one of the National Institutes of Health.

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