The squirrel's secret: rodents use ultrasonic warning calls

Squirrels have been recorded using high-pitched ultrasonic "whispers" that are inaudible to the human ear but warn each other of danger.

Squirrels have been recorded using high-pitched ultrasonic "whispers" that are inaudible to the human ear but warn each other of danger.

It is the first time that any animal has been found to use ultrasound for an alarm call although high-frequency sounds well beyond the range of human hearing are widely used by bats.

The discovery that squirrels communicate using ultrasound was made by James Hare, professor of zoology at the University of Manitoba in Canada, and his student David Wilson.

They were studying the audible alarm calls of the Richardson's ground squirrel, a social animal that lives in burrows of closely related females and their offspring. This species of squirrel - sometimes called a gopher - lives on the prairies of North America and has developed a sophisticated communication system to warn of approaching predators.

Professor Hare noticed a female opening its mouth as if in alarm and emitting faint sounds of rushing air, an apparently noiseless whisper which nevertheless triggered a vigilant posture in her nearby relatives.

"I thought initially that she had lost her voice. Then I noticed other squirrels doing the same so I decided to use a bat detector which can record ultrasound," Professor Hare said.

"Sure enough, we found that the whisper call was actually full of ultrasonic frequencies and this was directed at other squirrels nearby," he said.

In their study, published today in the journal Nature, the two scientists say that ultrasonic frequencies produce highly directional sound that can help a squirrel disguise its presence from a predator yet still warn other squirrels.

"To our 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," they write.

Recordings of individual squirrels making a "whisper call" were replayed to other members of the group. The results clearly indicated that the squirrels used ultrasonic frequencies to communicate.

"It was found that the animals spent significantly more of their time on vigilant behaviour in response to the whisper calls and audible control than in response to background noise," the scientists say.

The squirrels studied by the scientists used both audible calls and whisper calls when they saw a potential threat. "Audible calls evoked a more pronounced response than whisper calls, suggesting that whisper calls either convey less urgency than audible calls or that respondents react less conspicuously," they say.

Professor Hare said that ultrasound communication may be more common among animals than previously imagined.

SOUND INSTINCTS

Bats: Echo-location is a form of sonar that bats use to move around and capture prey in the dark. They emit ultrasonic squeaks from their mouths which bounce off nearby objects

Whales: Very low frequency sound of long wavelengths - the opposite to ultrasound - carry for enormous distances underwater and are used by some whales in long-distance communication. The killer whale however uses ultrasonic clicks to communicate over short distances

Honey bees: When honey bees find a rich source of nectar they can communicate the direction and distance it is from the hive by performing a dance in the shape of a figure "8" which takes into account the height of the Sun and the angle of flight needed to be taken from the hive

Birds: Song is a ubiquitious feature of birds and the dawn chorus is believed to result from individuals needing to stake out a territorial claim each morning.