A species of squirrel uses ultrasonic frequencies to warn fellow members of its group of impending danger, the first time an animal has been found to use high-frequency sound this way, said a study published in the weekly journal Nature. The unusual rodent is a Richardson's ground squirrel (Spermophilus richardsonii), which makes its home in the prairies of North America. Canadian zoologists noted that in addition to making audible vocalizations in the eight kilohertz (kHz) range to other members of their community, the squirrels also made "whisper" calls to each other, which produced only faint sounds of rushing air. Analyzed closely, these calls turned out to be ultrasonic, in the 50 kHz range, placing them well beyond the hearing of humans and most predators, including coyotes. In addition, it was discovered that the calls were in the range of 66.8 decibels, which is the equivalent of a loud conversation among humans. The squirrels emit these high-frequency calls when they spot low-grade threats. For urgent threats, they make audible calls.

The finding is of interest because it is the first time an animal has been found to use ultrasonic frequencies for communication. Other animals, such as bats and dolphins, use ultrasonic frequencies to locate obstacles or pursue prey.

"To our knowledge, ultrasonic alarm calls have not previously been detected in any animal group, despite their twin advantages of being highly directional and inaudible to key predators," said the authors of the study, David Wilson and James Hare of the University of Manitoba.

The limit of human hearing is about 20 kHz.


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