Silent scream. Ground squirrels use high-frequency shrieks to warn others of impending danger, according to a report in the 29 July Nature. The discovery marks the first known use of ultrasound as an alarm call. The researchers say the calls are likely above the hearing range of predators, making them a safe way to sound a warning without attracting unwelcome attention.
If I'm not mistaken, elephants communicate at subsonic frequencies.