BEIJING, July 25 (Xinhuanet) -- A frog that lives near the noisy Yellow Mountain hot spring in Central China is the only known animal that can tune its ears to various sound frequencies.

    The species, called Odorrana tormota or concave-eared torrent frog communicates by chirping audible and ultrasonic alls.

    A new study found that the frogs have selective hearing, enabling them to listen to the high frequency range when the low frequency background noise of rushing water is too intense for them to pick out the calls of potential mates or rivals.

    The frogs do this by opening and closing canals in their eardrums called Eustachian tubes to adjust the range to which their ears are sensitive. When the researchers shined a light underneath the frog's chin, they could tell the animal's tubes were open when the light was visible through the eardrum. When the Eustachian tubes closed, circles of light glowing out through the ears disappeared.

    "We said, 'Whoa! This is bizarre!'" said University of Illinois at Urbana-Champaign researcher Albert Feng. "In all textbooks on sound communication and hearing in frogs, it is plainly stated that the Eustachian tubes are permanently open!"

    The discovery, described in this week's issue of the journal Proceedings of the National Academy of Sciences, may help researchers design better hearing aids for humans that can hone in on important frequency ranges.

    (Agencies)