Yahoo! News - Ultrasonic squirrel is a mammalian first

"Canadian zoologists noted that in addition to making audible vocalisations in the eight kilohertz (kHz) range to other members of their community, the squirrels also made 'whisper' calls to each other, which produced only faint sounds of rushing air.
Analysed closely, these calls turned out to be ultrasonic, in the 50 kHz range, placing them way beyond the hearing of humans and of predators as well.


But the signal was strong, averaging 66.8 decibels, which is the equivalent of a loud conversation among humans. "