O. tormota, the concave-eared torrent frog, is unusual in other ways. Most frogs have ears on the body surface, but the torrent frog's ears are recessed. Feng and his colleagues previously reported that O. tormota communicates in a noisy environment by emitting high frequency calls that include ultrasonic sounds, and can localize sound with astonishing precision. Upon hearing a female call, a male will leap directly toward the sound with an error of less than 1 percent, a feat previously unheard of in frogs.
Fortunately for the researchers, the eardrum of O. tormota is transparent, offering a view of its inner workings in a living frog.
While puzzling over the peculiar results of the eardrum vibration measurements, the researchers noticed the sudden appearance and disappearance of a dark shadow on the eardrum, Feng said.
Further investigation revealed that the frogs were actively opening and closing their Eustachian tubes, the two narrow channels that connect either side of the pharynx to the left and right middle ear. The changing state of the Eustachian tubes was more readily observed by directing a light beam at the mouth from under the frog's chin. When the Eustachian tubes were open, the light was visible through the eardrum. When they closed, the circles of light glowing out through the ears disappeared.
"We said, 'Whoa! This is bizarre!' " Feng recalled. "In all textbooks on sound communication and hearing in frogs, it is plainly stated that the Eustachian tubes are permanently open!"
Feng and his colleagues had observed that when open, the Eustachian tubes essentially couple the frog's left and right ears. This "acoustic coupling" between the ears makes them sensitive to sound direction, enabling the frog to localize sound, Feng said.
To determine the consequence of active closure of the Eustachian
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University of Illinois at Urbana-Champaign