July 22, 2008 -- Just as humans tune into individual radio stations, an unusual Chinese frog can shift its hearing from one frequency to another in order to selectively choose what it hears, according to a paper published in this week's Proceedings of the National Academy of Sciences.The frog, Odorrana tormota, is the only known animal in the world that can manipulate its hearing system to select particular frequencies. Humans appear to possess a modicum of control, but our system is slow compared to that of the frog and we cannot, with precision, tune our ears to match sounds.
The rare amphibian likely evolved its hearing talent out of necessity, since its environment is so noisy.
"Their calling sites are on the steep banks of a fast-flowing body of water -- the Tao Hua Creek (at Huangshan Hot Springs in central China)," co-author Albert Feng told Discovery News.
Feng, a professor of molecular and integrative physiology at the University of Illinois at Urbana-Champaign, added that the site is especially noisy after spring showers. He pointed out that humans often cannot hear well over heavy rains either.
"We pretty much have to shout at one another," he said.
The frog doesn't shout, but it instead sings like a bird either in audible chirp-like frequencies or by emitting very high-pitched ultrasonic sounds.
Curious as to how the amphibian could hear these different vocalizations, Feng and his team analyzed the frog's hearing system, which wasn't too challenging since the frog's eardrum is completely transparent.
The scientists used a laser to measure the eardrum's vibration, and noticed that while it could respond to both audible and ultrasonic sounds, sometimes the eardrum's sensitivity to ultrasonic noise mysteriously disappeared.
Further investigation determined the frog actively opens and closes two narrow channels known as the Eustachian tubes, which connect the pharynx (part of the neck and throat) to the left and right middle ears. When open, the tubes couple the frog's left and right ears, making them sensitive to audible sounds from all directions. When closed, their ability to pick up ultrasonic frequencies kicks in.
"We said, 'Woah! 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!"
The ability to tune into specific frequencies at will isn't the frog's only claim to animal fame. It also possesses recessed ears instead of ones that, like those of most other creatures, are located on the body's surface. It additionally can localize sound with astonishing precision.
With an error of less than 1 percent, males of this species leap directly toward calling, wooing females. This level of accuracy has never before been observed in frogs.
"On the one hand, I am surprised that any frog can open and close the Eustachian tubes," Carl Gerhardt, professor of biological sciences at the University of Missouri, told Discovery News.
"But on the other hand, I am not too surprised that this frog does it because it is also the first to be shown to have ultrasonic hearing and calls with ultrasonic components."
The frog's unique hearing system is already being used as a model for "intelligent" hearing aids that can spatially separate sounds, process them the way that human brains do, and boost sound signals of interest, such as differentiating background noise from that of a desired conversation.
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