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Rare Chinese frogs communicate
by means of ultrasonic sound
Life Sciences Editor
photo to enlarge
by L. Brian Stauffer
S. Feng, a professor of molecular and integrative
physiology, is the corresponding author of a paper
to be published in Nature on March 16 that presents
the first documented case of an amphibian being able
to communicate like bats, whales and dolphins.
— First came word that a rare frog (Amolops tormotus) in China
sings like a bird, then that the species produces very high-pitched
ultrasonic sounds. Now scientists say that these concave-eared torrent
frogs also hear and respond to the sounds.
The findings, to appear in the March 16 issue of Nature, represent the
first documented case of an amphibian being able to communicate like
bats, whales and dolphins, said corresponding author Albert S. Feng,
a professor of molecular
and integrative physiology at the University of Illinois at Urbana-Champaign.
Feng, a researcher at the Beckman
Institute for Advanced Science and Technology, was introduced to
the frog species by Kraig Adler, a Cornell University biologist who
had learned about it while conducting a survey of amphibians in China.
Feng continues to study frogs and bats to understand how the brain processes
sound patterns, especially in sound-cluttered environments in which
filtering is required to allow for communication.
photo to enlarge
rendering of Amolops tormotus.
Feng and colleagues
previously reported that males of the species make these high-pitched
bird-like calls, with numerous variants in terms of harmonics and frequency
sweeps. Some sounds exceeded their recording device’s maximum
capability of 128 kilohertz. Human ears hear sound waves generally no
higher than 20 kilohertz. The frogs studied inhabit Huangshan Hot Springs,
a popular scenic mountainous area, alive with noisy waterfalls and wildlife
west of Shanghai.
“Nature has a way of evolving mechanisms to facilitate communication
in very adverse situations,” Feng said. “One of the ways
is to shift the frequencies beyond the spectrum of the 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.”
Adler had drawn attention to the species because the frogs do not have
external eardrums, raising the possibility of unusual hearing abilities.
“Now we are getting a better understanding of why their ear drums
are recessed,” Feng said. “Thin eardrums are needed for
detection of ultrasound. Recessed ears shorten the path between eardrums
and the ear, enabling the transmission of ultrasound to the ears.”
To test if the frogs actually communicated with their ultrasonic sounds,
Feng and colleagues returned to China with their recording equipment
and a special device that allowed playback of recorded frog calls in
the audible or ultrasonic ranges. They observed eight male frogs under
three experimental conditions (no sounds, playback of calls containing
only audible parts and playback of just ultrasonic frog calls).
During playback, the researchers watched for evoked calling activity
in which a male frog begins calling upon hearing calls from other frogs
in the area. Five frogs responded to ultrasonic and audible sound ranges,
with four responding with calls in both ranges. One frog called 18 times
to ultrasonic calls, including four very telling rapid responses, Feng
said. Another frog did not respond to ultrasonic stimulation but produced
calls 18 times to an audible prompt.
Clearly, Feng said, some of the frogs indeed communicated ultrasonically.
They have the ability to do so, but for some reason some frogs do and
some don’t, he said. “We believe that all of them have the
capacity to respond to the ultrasound.”
Ultrasonic communication likely will be found in other amphibians and
birds, Feng said, but, until now, no one has bothered to look into it.
“Humans have always been fascinated by how some animals can discern
their world through a sensing system vastly different from our own,”
Feng said. “The electromagnetic sense in fishes and homing pigeons,
polarized light vision in ants, chemical sensing of pheromones in insects
and rodents, echolocation by ultrasound in bats and dolphins, are just
a few examples.
“That frogs 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,”
he said. “This suggests that there are likely many other examples
of unexpected forms of communication out there.”
The eight authors were Feng; Wen-Yu Lin, a senior research scientist
in Feng’s lab; Peter M. Narins of the University of California
at Los Angeles; Chun-He Xu of the Shanghai Institutes of Biological
Sciences, Chinese Academy of Sciences, in Shanghai; and Zu-Lin Yu, Qiang
Qiu, Zhi-Min Xu and Jun-Xian Shen of the State Key Laboratory of Brain
and Cognitive Science, Institute of Biophysics, Chinese Academy of Sciences,
Feng and Narins received funding from the National Institute on Deafness
and Other Communication Disorders, one of the National Institutes of
Health. Feng also was funded by the National Science Foundation. Additional
Chinese grants from the State Key Basic Research and Development Plan
and the National Natural Sciences Foundation to Chun-He Xu and Shen,
respectively, supported the work.