people
Vol.23 No.2 2009
Tracking Frogs that sing
ultrasonic duet
By XIN Ling (Staff Reporter)
In central China, unique frogs talk by emitting ultrasonic calls, male to show their virility
and female for courtship, which are received by tunable ears with amazing accuracy. Prof.
SHEN Junxian from the CAS Institute of Biophysics and his collaborators are working
diligently to explore the mysteries of unique sound communication in animal kingdom.
B
ees buzz, birds chirp, dogs bark, tigers roar.
Gentle or wild, merry or sad, the sounds of
animals are always rich and colorful. Scientists
study them not only to understand the communication
mechanisms within different species, but to gain insights
into their historical evolvement, explain laws of nature,
and find something to enlighten and serve the human
society.
As a senior researcher with the CAS Institute of
Biophysics, Prof. SHEN Junxian has been involved in this
time-honored subject for more than four decades. Working
with bat, bushcricket and mouse, he had probed into the
acoustic communication of these small creatures – from
information encoding, processing and recognition to the
organization of auditory neurons. Under an international
collaborative effort in recent years, Prof. Shen and his
US partners focus on a unique animal model called
the concave-eared torrent frogs living in the vicinity of
Huangshan Hot Springs in east China’s Anhui Province.
The frogs, brown/black-stripped and less than 35mm
across, live in a noisy environment on the bushy edges of
creeks where waterfalls and rushing water form a steady
din. They caught scientists’ attention several years ago
because they do not have external eardrums, like their
name indicates, and because the males of these torrent frog
(Amolops tormotus, later revised Odorrana tormota) can
produce bird-like melodic calls that extent to the ultrasonic
range, a feat previously found only in some mammalian
species such as the bat, dolphin, whale and some insects.
However, the detected ultrasounds will be identified
Prof. Shen works late into the night to record the sounds of
torrent frogs by Taohua Creek.
as a means of communication only when it proves to be
properly received and responded and is not an aimless
by-product of the sound-production process. Prof. Shen
and colleagues recorded a male torrent frog’s call, split it
into audible and ultrasonic components, and carried out
acoustic playback experiments along the Taohua Creek
on several summer nights, 2005. Results showed that
most male frogs as well as some sympatric species are
able to respond to the ultrasounds.
It thus proves the selective advantage, Shen explained,
of the torrent frog to use high-pitched voice to make
himself heard against the noisy springs. It also answers the
puzzle of his recessed ears, which have so evolved for the
sake of ultrasound reception. Since frogs are “a distinct
evolutionary lineage from bats and dolphins,” this research
“represents a new example of independent evolution,”
claims a Nature paper in 2006, co-authored by Shen,
Albert Feng at the University of Illinois and Peter Narins
Bulletin of the Chinese Academy of Sciences
109
BCAS
at the University of California, LA et al.
Prof. Shen devoted the following years to more indepth observation and investigation near the Taohua
Creek. Several new observations about female torrent
frogs emerged to challenge traditional notions. Of all
members of the frog world, it has always been thought
that males sing out to attract mates and therefore
dominate the communication system, while females
stay typically silent except for a few feeble reciprocal
calls during their courtship. However, when dissecting
a female O. tormota, the professor discovered to his
surprise that she has well developed vocal cords and a
large larynx which most female frogs do not have and
may enable her to sing as male croakers do. Besides, for
a female torrent frog carrying eggs, just listen carefully
between evening to daybreak and one can always hear
her short but high-pitched signals.
Many rainy nights amid grotesque dangerous stones,
Shen managed to catch gravid females and put them
in a quiet, darkened room to record their vocalizations
with the help of an ultrasonic microphone and PC-tape.
Experiments showed the females emit calls that span
audible and ultrasonic frequencies. Males exposed to
their voices would immediately chirp back and leap
toward the source with a positioning error of less than 1%.
Male O. tormota’s outstanding performance in
pinpointing sound source “rivals that of vertebrates with
the best localization ability of the barn owl, dolphin,
elephant and human.” With an interaural distance of less
than 1 cm, the frog has gained another selective advantage
in localization accuracy of high-frequency hearing to
combat the low-frequency noises in their natural habitat.
The research was reported by Nature in May, 2008.
By this July, the team of Shen, Feng and Narins
had made a novel discovery about the hearing talent of
these amazing Chinese frogs. Though previous research
proved their production of and response to ultrasonic
calls, the scientists this time were curious whether
its eardrum actually vibrates in answer to those high
frequency sounds.
According to a third paper co-authored by the three
scholars in a recent issue of Proceedings of the National
Academy of Sciences, they used a vibrometer to analyze
the vibration of the frog’s transparent eardrum, and
noticed that while the eardrum did quiver at audible or
110 Bulletin of the Chinese Academy of Sciences
Vol.23 No.2 2009
Prof. Shen shows his latest findings on ultrasonic
communication of O. tormota to Prof. T. Wiesel, a 1981
Nobel Prize laureate in Physiology or Medicine.
ultrahigh frequency sounds, its sensitivity to the latter
sometimes mysteriously disappeared at all. Further
investigation revealed the key role of the Eustachian tube,
a slender channel that connects the pharynx to the left
and right middle ears to equalize air pressure on either
side of the eardrum. To everyone’s surprise, the croaker
was able to actively open and close its Eustachian tube.
When open, the tube enables the frog to receive audible
sounds from all direction; when closed, their ability to
pick up ultrasonic sounds just comes into effect.
It means that the frog is able to shift its hearing from
one frequency to another so as to selectively choose what
it hears, just like humans tune to radio stations. By far, O.
tormota is the only known animal to achieve the feat, while
men appear to possess a slower and moderate control.
Prof. Shen and his partners have already applied
their series of research results to intelligent hearing aid
against hypacusia. Such hearing aids can spatially separate
sounds, process them the way that human brains do, and
distinguish background noises from desired sound signals.
“We aim to develop more effective and low-costing
artificial electro-cochlea so as to help more hearingdisabled people,” Shen added.
Feng has attributed much of their success by far to
the diligence and carefulness of Prof. Shen’s work. They
hope to find out more about the unique frog’s ultrasonic
communication mechanism, with strong support from the
National Natural Science Foundation of China and funding
from the US. At Huangshan Hot Springs or his Beijing
laboratory, we can still find this senior biophysicist and his
coworkers busy listening to the critter’s ultrasonic duet. n