|Scientists Discover Reason Behind Ear Canal in Chinese Frog:
A rare frog that lives in rushing streams and waterfalls of east-central China
is able to make itself heard above the roar of flowing water by communicating
ultrasonically, says new research funded in part by the National Institute on
Deafness and Other Communication Disorders (NIDCD), one of the National Institutes
of Health. According to the study, which appears in the March 16, 2006, issue
of Nature, attributes that enable the frog to hear ultrasounds are made
possible by the presence of an ear canal, which most other frogs don’t have.
The research may provide a clue into why humans and other animals also have ear
canals: to hear high-frequency sounds.
Amolops tormotus, also referred to as the concave-eared torrent frog,
is the first non-mammalian vertebrate found to be capable of producing and detecting
ultrasounds for communication, much like dolphins, bats, and some rodents. It
does so, the researchers report, to make itself heard above the din of low-frequency
sounds produced in its surroundings so that it can communicate territorial information
to other males of its species. In addition to helping researchers puzzle out
how the ear evolved, the research may one day enable scientists to develop new
strategies or technologies that help people to hear in environments in which
there is a lot of background noise.
“In the study of communication and communication disorders, researchers can
gain a great deal of insight by looking at the natural world,” says James F.
Battey, M.D., Ph.D., director of the NIDCD. “The more we can learn about the
extraordinary mechanisms that Amolops and other animals have developed
to hear and communicate with one another, the more fully we can understand the
hearing process in humans, and the more inspired we can be in developing new
treatments for hearing loss.”
Ultrasounds are high-pitched sounds more than 20 kilohertz (kHz) in frequency,
exceeding the upper limit of sounds detectable by humans, and far higher than
the 12 kHz frequencies that most amphibians, reptiles, and birds are capable
of hearing and producing. Key parts of the ear must be specially adapted to detect
ultrasounds — namely, the eardrum must be very thin to vibrate effectively
at these high frequencies, and the bones of the middle ear must be extremely
lightweight in order to transmit ultrasonic vibrations to the inner ear. The
presence of an ear canal not only protects A. tormotus’s thin and fragile
eardrum from the environment but also lessens the distance between the eardrum
and the inner ear, thus allowing the bones of the middle ear to be shorter, and
as a result, lighter in weight.
Researchers have known for several years that A. tormotus males produce
high-pitched, birdlike calls that extend into the ultrasonic range. What remained
to be tested was whether the ultrasounds were a byproduct of the frog’s sound
production system or were heard and responded to by other males of that species.
Researchers Albert S. Feng, Ph.D., an auditory neuroscientist at the University
of Illinois, Urbana-Champaign, and Peter M. Narins, Ph.D., who studies auditory
behavior, neurophysiology, and mechanics at the University of California, Los
Angeles, and collaborators conducted behavioral and physiological studies to
investigate A. tormotus’s hearing ability.
The researchers first wanted to know if A. tormotus can hear ultrasounds.
They recorded a male’s call, split it into the audible components and ultrasonic
components, and observed the responses of eight A. tormotus males to each of
the split sounds. Five of the eight frogs produced calls in response to the audible,
ultrasonic, or both components of the species call and three did not. Results
of the behavioral observations showed that males were capable of hearing and
responding to ultrasounds.
The researchers then measured the electrical activities in A. tormotus’s midbrain
that is involved in sound processing and found marked electrical responses to
sounds extending into the ultrasonic range — both in the averaged response
of a population of nerve cells in the brain and in single nerve cells — confirming
the frog’s capacity for hearing ultrasounds. (Interestingly, a different species
that lives in similar environments also demonstrated an ability to hear ultrasounds.)
The next steps for the researchers will be to study A. tormotus’s eardrum,
as well as hair cells, the sensory cells in the inner ear that are essential
for hearing, to learn how they are able to detect ultrasounds. They also are
interested in learning why only the males possess recessed eardrums while the
females do not.
Other researchers involved in the study represent the Chinese Academy of Sciences
Shanghai Institutes of Biology Sciences and Institute of Biophysics. Additional
funding sources for the study include the National Science Foundation and China’s
State Key Basic Research and Development Plan and National Natural Sciences Foundation.
| ||The concave-eared torrent frog is the first non-mammalian vertebrate found to be capable of producing and detecting ultrasounds for communication.
Credit: Used with permission of A. Feng
NIDCD supports and conducts research and research training on the normal
and disordered processes of hearing, balance, smell, taste, voice, speech and
language and provides health information, based upon scientific discovery,
to the public. For more information about NIDCD programs, see the Web site
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Agency — includes 27 Institutes and Centers and is a component of
the U.S. Department of Health and Human Services. It is the primary federal
agency for conducting and supporting basic, clinical and translational medical
research, and it investigates the causes, treatments, and cures for both common
and rare diseases. For more information about NIH and its programs, visit http://www.nih.gov.