When the recordings were in the audible and ultrasonic ranges, five of the frogs gave significantly more replies than when no sound was played at all.
In response to one particular ultrasonic playback, a frog shot back with 18 rapid-fire calls.
"Males not only responded to ultrasound but also approached our loudspeaker as if to confront the frog making that sound," Feng said.
In a second test, researchers observed the brain activity of one frog during playback. The frog showed a strong response to ultrasound but no response when both its ears were blocked with modeling clay.
"This confirmed that the hearing was indeed taking place through the ear and not through sound that penetrated the skull to directly stimulate the neurons," Feng said.
Researchers theorize that there could be an evolutionary explanation for the frog's ultrasonic hearing.
Males trying to catch a female's attention need to have their calls heard over the din from other suitors, as well as over background noise, Feng noted.
Shifting the call to a much higher frequency not only ensures it is heard, he said, but it also requires less energy than generating very loud audible croaks.
The secret to the frogs' ultrasonic hearing appears to be in their unique ear structure. Their eardrums are superthin and sunken below the body surface to protect the membranes.
But the exact part of the ear that facilitates ultrasonic hearing is not known, Feng said.
"This is an interesting study for several reasons," said Michael Ryan, a zoologist at the University of Texas at Austin.
"It shows the potential for some frogs to hear in a frequency range never suspected and an unusual way to adapt to background noise."
But Feng's study begs the question as to why only males and not females have this capability, he added.
Adler, the Cornell biologist, said, "This is a fabulous paper and a carefully conducted study on the anatomy of an odd frog."
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