Ground squirrels emit an ultrasonic shriek to warn others that a predator is nearby. In the current issue of the journal Nature, University of Manitoba researchers report that while bats and whales use ultrasound for echolocation and to track prey, to their 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." From a New Scientist article about the study:
"Ultrasonic alarm calls might be beneficial because many of the birds-of-prey that catch and eat squirrels cannot hear them. Conveniently, ultrasound also has a shorter range than audible sound.Link (Thanks, Gabe!)
'It may be used to secretly warn others without alerting a more distant predator,' says (researcher David) Wilson.