Most female frogs can’t call, because they lack or have only primitive vocal cords. A typical female frog selects a mate from a chorus of males and then silently signals her potential mate. However, the female Concave-eared Torrent frog, Odorrana tormota, uses a direct way of showing her interest in a male.: She emits a high-pitched sound that to the human ear sounds like a bird’s chirp. This call for a mate is composed of both regular and ultrasonic frequencies.
O. tormota lives in a noisy environment on the brushy edge of streams in the Huangshan Hot Springs, in central China, where waterfalls and rushing water provide a steady loud din. So this frog has developed resessed eardrums that are allow this frog to create and hear higher pitched sound frequencies. It seems to be able to fine tune its sound much like the tuner on a radio can shift from one frequency to another. It is the only known example of an animal that can actively select what frequencies it hears. Also it can localize sound with precision.
When the male hears a female call, he leaps directly toward the sound with 99% accuracy better than any other frog tested.
The eardrum of O. tormota is transparent, so while researchers watched the eardrum move in the lab, they saw a dark shadow on the eardrum suddenly appear and disappear. They realized that they were watching the frog opening and closing its Eustachian tubes. These two narrow channels connect either side of the pharynx (part of the upper throat) to the left and right middle ear. No other animals’ hearing anatomy is arranged like this.
How do these tubes help the frog hear so acutely ?
When the Eustachian tubes were open, the laser light held by the researcher was visible through the eardrum. When the tubes closed, the circles of light glowing out through the ears disappeared. This lets the frog isolate specific sounds. So inspite of loud noises surrounding the frog, its eardrums became more sensitive to high frequency and ultrasounds when their Eustachian tubes were closed, compared with when they were open. When the Eustachian tubes were open, the eardrums responded mostly to low frequency sounds.
By unraveling the frog’s fine tuning abilities, researchers may be able to improve hear aids for humans with hearing loss.
Odorrana Tormota – Female Frog’s Ultrasonic Way Of Asking For Sex – ScientificBlogging May 11, 2008
Ultrasonic frogs can tune their ears to different frequencies ScientificBlogging July 22, 2008
Frog call courtesy of http://acp.eugraph.com/news/news06/feng.html