Frog's Ear Canal May Provide Insights for Understanding Human Hearing Loss
A rare frog that lives in rushing streams and waterfalls in east-central China is able to make itself heard above the roar of flowing water by communicating ultrasonically, scientists reported March 16 in the journal 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.
"Our research points out an elegant and novel solution to the problem of communication in high levels of background noise," said Peter Narins, UCLA professor of physiological science and ecology and evolutionary biology, and co-author of the study. "In addition, we now add amphibians to the small group of vertebrates (bats, whales and some rodents) that use ultrasound for communication. This study may provide a clue for understanding why humans have ear canals: to improve sensitivity to 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 understand how the ear evolved, the research may one day enable scientists to develop new strategies or technologies that help people to hear in environments where there is substantial background noise.
The research was federally funded by the National Institute on Deafness and Other Communication Disorders (NIDCD), one of the National Institutes of Health, and the National Science Foundation.
"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," said James F. Battey, director of the NIDCD.
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' 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.
Scientists 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, an auditory neuroscientist at the University of Illinois, Urbana-Champaign; Narins, who studies auditory behavior, neurophysiology and mechanics; and colleagues conducted behavioral and physiological studies to investigate A. tormotus' hearing ability.
The researchers first wanted to know whether 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 scientists then measured the electrical activities in A. tormotus' 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. (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' eardrum, as well as hair cells, the sensory cells in the inner ear that are essential for hearing, to learn how the hair cells are able to detect ultrasounds. The scientists also are interested in learning why only the males possess recessed eardrums.