Science + Technology

Scientists Discover Why Chinese Frog Has Ear Canal, Research That May Provide Insights Into Human Ear Canals, and Perhaps to Hearing Loss

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A rare frog thatlives in rushing streams and waterfalls in east-central China is able to make itself heardabove the roar of flowing water by communicating ultrasonically, scientistsreported March 16 in the journal Nature. Attributes that enable the frog tohear ultrasounds are made possible by the presence of an ear canal, which mostother frogs don't have.

"Our researchpoints outan elegant and novel solution to the problem of communication in high levels ofbackground noise," said Peter Narins, UCLA professorof physiological science and ecology and evolutionary biology, and co-author ofthe study. "In addition, we now add amphibians to the small group ofvertebrates (bats, whales and some rodents) that use ultrasound forcommunication. This study may provide a clue for understanding why humans haveear canals: to improve sensitivity to high-frequency sounds."

Amolops tormotus, also referred to as the concave-earedtorrent frog, is the first non‑mammalian vertebrate found to be capableof producing and detecting ultrasounds for communication, much like dolphins,bats and some rodents. It does so, the researchersreport, to make itself heard above the din of low-frequency sounds produced inits surroundings so that it can communicate territorial information to other malesof its species. In addition to helping researchers understand how the earevolved, the research may one day enable scientists to develop new strategiesor technologies that help people to hear in environments where there is substantialbackground noise.

The research wasfederally funded by the National Institute on Deafness and Other CommunicationDisorders (NIDCD), one of the National Institutes of Health, and the NationalScience Foundation.

"The more we canlearn about the extraordinary mechanisms that Amolopsand other animals have developed to hear and communicate with one another, themore fully we can understand the hearing process in humans, and the moreinspired we can be in developing new treatments for hearing loss," said JamesF. Battey, director of the NIDCD.

Ultrasounds arehigh-pitched sounds more than 20 kilohertz (kHz) in frequency, exceeding theupper limit of sounds detectable by humans, and far higher than the 12 kHz frequenciesthat 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 totransmit ultrasonic vibrations to the inner ear. The presence of an ear canalnot only protects A. tormotus' thin andfragile eardrum from the environment, but also lessens the distance between theeardrum and the inner ear, thus allowing the bones of the middle ear to beshorter, and as a result, lighter in weight.

Scientists haveknown for several years that A. tormotusmales produce high-pitched, birdlike calls that extend into the ultrasonicrange. What remained to be tested was whether the ultrasounds were a byproductof the frog's sound-production system or were heard and responded to by othermales of that species. Researchers Albert S. Feng, anauditory neuroscientist at the University of Illinois, Urbana-Champaign; Narins, who studies auditory behavior, neurophysiology andmechanics; and colleagues conducted behavioral and physiological studies toinvestigate A. tormotus' hearing ability.

The researchersfirst wanted to know whether A. tormotus canhear ultrasounds. They recorded a male's call, split it into the audiblecomponents and ultrasonic components, and observed the responses of eight A.tormotus males to each of the split sounds. Fiveof the eight frogs produced calls in response to the audible, ultrasonic orboth components of the species call, and three did not. Results of thebehavioral observations showed that males were capable of hearing andresponding to ultrasounds.

The scientists thenmeasured the electrical activities in A. tormotus'midbrain that is involved in sound processing and found marked electricalresponses to sounds extending into the ultrasonic range — both in the averagedresponse of a population of nerve cells in the brain and in single nerve cells —confirming the frog's capacity for hearing ultrasounds. (A different speciesthat lives in similar environments also demonstrated an ability to hearultrasounds.)

The next steps forthe researchers will be to study A. tormotus'eardrum, as well as hair cells, the sensory cells in the inner ear that are essentialfor hearing, to learn how the hair cells are able to detect ultrasounds. Thescientists also are interested in learning why only the males possess recessedeardrums.

Other researchersinvolved in the study represent the Chinese Academy of Sciences ShanghaiInstitutes of Biology Sciences and Institute of Biophysics. Additionalfunding sources for the study include China's State Key Basic Researchand Development Plan and National Natural Sciences Foundation.

NIDCD supports andconducts research and research training on the normal and disordered processesof hearing, balance, smell, taste, voice, speech and language and provideshealth information, based upon scientific discovery, to the public. For moreinformation about NIDCD programs, see the Web site at http://www.nidcd.nih.gov. For moreinformation about the National Science Foundation and the research it funds,see http://www.nsf.gov/.

-UCLA-

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