UCLA scientists report on the only known frog species that can communicate using purely ultrasonic calls, whose frequencies are too high to be heard by humans. Known as Huia cavitympanum, the frog lives only on the Southeast Asian island of Borneo.
Ultrasounds are high-pitched sounds more than 20 kilohertz (kHz) in frequency, which exceeds the upper limit of sounds detectable by humans and is far higher than the 5 to 8 kHz frequencies most amphibians, reptiles and birds are capable of hearing or producing. Key parts of the ear must be specially adapted to detect ultrasounds.
The frogs can hear sounds up to 38 kilohertz, the highest frequency any amphibian species has been known to hear, the scientists report. Humans can hear up to about 20 kHz and typically talk at 2 or 3 kHz.
While most of the more than 5,000 frog species worldwide have eardrums that are flat on the side of the head, Huia cavitympanum has eardrums recessed in the side of the skull, similar to mammals.
Peter Narins, UCLA distinguished professor of physiological science and of ecology and evolutionary biology, and Victoria Arch, a UCLA graduate student in ecology and evolutionary biology, spent several nights in the remote area where the frogs live.
“We had very little information suggesting that they would be in this location,” said Arch, lead author of the study. “We found them our first night out.”
Huia cavitympanum produces some audible calls and others that are entirely ultrasonic.
“You look at the frog and can see it is vocalizing — you can tell because their vocal pouches pulsate — but you don’t hear any sound. It’s amazing,” Arch said. “Then you look down at the recording equipment and see the lights flashing, indicating sound. I have never seen that before. In a frog, it’s unprecedented to have purely ultrasonic vocalizations.”