Noisy waterfalls and claps of thunder can drown out even the most vocal frog. But some persistent croakers in China have a clever fix: They switch to ultrasound.
The feat, researchers say, makes the frogs the first amphibians to be placed alongside an exclusive group of mammals, such as whales and dolphins, that have ultrasonic ability.
"It shows a new example of independent evolutionary adaptation in the frogs for life in habitats filled with loud background noise," said Albert Feng, a professor of neuroscience at the University of Illinois, Urbana-Champaign.
Researchers first heard of the unusual animals in 2000 from Kraig Adler, a biologist at Cornell University.
He tipped the scientists off to Huangshan Hot Springs, a popular tourist destination near Shanghai, where he had found frogs with recessed ears.
"I noticed the frogs' sunken ears and thought they must have an odd system of communication. We had no idea they used ultrasonic sounds," Adler said.
That year a research team led by Feng reported that these frogs, called concave-eared torrent frogs (Amolops tormotus), sang like birds.
Two years later, using wide-band recording equipment, Feng and his colleagues discovered that the frogs were also croaking in ultrasound—sound vibrations beyond the limit of human hearing.
The team's findings appear in the current issue of the journal Nature.
Ultrasonic Hearing
To test the frogs' hearing prowess, Feng went to the site in China (map) last summer with devices that could play back sound in both audible and ultrasonic ranges.
The researchers played recorded sounds to eight male frogs.
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