Ground squirrels scream ultrasonic warning

Zoologists in Winnipeg find ground squirrels emit shriek to warn fellow squirrels to predators. Non-squirrels can't hear alarm.

Ground squirrels let out a "silent", high-frequency shriek to warn each other about predators, Canadian researchers have found.

Other than echolocation in bats and dolphins, scientists know little about the use of ultrasound in the animal world.

Since ultrasonic alarm calls are highly directional yet inaudible to key predators, zoologists David Wilson and James Hare of the University of Manitoba speculated animals may use the system to signal danger.

"This is the first actual demonstration of animals using an ultrasonic alarm signal," said Hare.

The researchers noticed some ground squirrels moved as if they were making alarm calls, although they produced only faint sounds of rushing air.

The pair designed an experiment to test the idea among rodents called Richardson's ground squirrels (Spermophilus richardsonii). The ground squirrels are common on the Prairies.

Wilson and Hare recorded the animals using video cameras and a special ultrasonic device. In the field, they played back the chirps that humans can't hear, and kept track of how the squirrels responded.

"I think they do things that most people find shocking," said Wilson. "They're able to discriminate among callers based on their calls, and they can communicate fairly specific information."

The limit of human hearing is about 20 kilohertz. The squirrels' alarm calls are in the 50 kilohertz range, well beyond the hearing of predators, the researchers said in Thursday's issue of the journal Nature.

"The fact that these animals that were right in our backyard are communicating for I guess some purpose we're just finding out about is really quite exciting," said Bob Wrigley, curator of Winnipeg's Assiniboine Park Zoo.

The researchers plan to learn more about what the rodents are saying in their ultrasonic language. They suspect it may include detailed information about the squirrels' predators.