While its been known that dolphins and bats use ultrasonic frequencies to avoid obstacles and pursue prey, up until now no animals were known to use it as a means of communication. Apparentlly, a species of squirrel does.

July 29, 2004 ó 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.