Silent Scream

Biologists discover the first known case of an animal using an ultrasonic alarm call

WHEN A HUNGRY HAWK flies overhead, a Richardson's ground squirrel snaps to attention: To warn colony members of the imminent danger, the rodent rears up on its hind legs, opens its mouth and—silence. Or at least that's what scientists thought until University of Manitoba biologists James Hare and David Wilson switched on sensitive recording equipment and discovered that the squirrels were indeed making sounds: squeaks so high-pitched that humans, and presumably predators, cannot hear them. It's the first known case of an animal using an ultrasonic alarm call.