Can You Hear Me Now? Frogs in roaring streams use ultrasonic calls

A small frog species from China is the first amphibian shown to use ultrasonic calls, says an international research team.

Equipment designed for studying bats picked up high-pitched chirrups in the calls of the concave-eared torrent frog Amolops tormotus, says Albert S. Feng of the University of Illinois at Urbana-Champaign. He and his colleagues also measured nerve activity in the hearing centers of the frog brains in response to ultrasonic tones.

The small brown frogs live beside water splashing over rocks on a mountainside. Ultrasonic perception may have developed as the frogs struggled to hear each other over the din, the researchers suggest in the March 16 Nature.

High-pitched signaling is “absolutely unexpected,” says Michael Ryan of the University of Texas at Austin, who studies frogs and frog-eating bats.

The Chinese frog calls in a wide range of frequencies, including those that people hear. The frog sounds like a bird, says Feng. The species made headlines in 2002 for its diverse whistles and chirps (SN: 9/14/02, p. 173: Available to subscribers at Singing frog in China evokes whales, primates).

While studying sonograms of the calls, Feng and his colleagues noticed strong bursts above 20 kilohertz, which people can’t hear. Tests showed that these bursts were in a range higher than the water’s roar.

Such high-pitched sounds don’t mean much by themselves. “If you scratch a table, there are lots of ultrasonic elements,” says Ryan.

So, to see whether the frogs could actually hear the sounds, Feng and his colleagues visited the frogs’ home at China’s Huangshan Hot Springs in 2005. The researchers played various segments of a recorded frog call. When they broadcast just the ultrasonic section, a male frog called immediately after the burst. Several other males seemed to respond, but with less-precise timing.

Such responses were “suggestive” that frogs use ultrasound, says Ryan. What he found more convincing, he says, were measurements of nerve activity.

The researchers inserted electrodes into the part of the frog brain involved in hearing and found that the frogs are sensitive to sounds up to 34 kHz, says Feng.

“I think that the results are both surprising … and robust,” says Carl Gerhardt of the University of Missouri–Columbia. Frog hearing is usually confined to much lower frequencies than other vertebrates’, he says.

The high-pitched calls may solve a long-standing question that Feng has had: Why did males evolve unusual, recessed ears? Now, Feng speculates that the structures improve sensitivity to high pitches.

---