Ground squirrels emit an ultrasound squeal to warn other animals that a predator is approaching, researchers have discovered.
Bats and toothed whales are already known to use ultrasound for echolocation, but little is known about ultrasound communication in the rest of the animal kingdom. Some rodents, including rats and hamsters, emit ultrasound calls, but no one is sure what they mean.
To investigate, David Wilson and James Hare of the University of Manitoba in Winnipeg, Canada, studied how Richardson鈥檚 ground squirrels (Spermophilus richardsonii), a burrowing rodent, responded to the approach of humans.
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Usually, the animals made audible alarm calls, opening their mouths wide and thrusting their bodies forward. But occasionally, a squirrel would do this but make no sound, its open mouth producing only a faint whisper of rushing air.
Pure tone
When Wilson and Hare recorded these whispering calls using a bat detector and analysed the frequencies, they found that the calls contained pure ultrasound tones with frequencies of about 48 kilohertz, far too high for people to hear.
And when they broadcast pure tones of this frequency to other ground squirrels, the animals recognised them as a warning and became more vigilant than usual, tilting their heads to listen and look for predators.
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. 鈥淚t may be used to secretly warn others without alerting a more distant predator,鈥� says Wilson.
Journal reference: Nature (vol 430, p 523)