Squirrels emit 'silent scream'
By Julianna Kettlewell
BBC News Online science staff
High-pitched sounds are used by rodents in all sorts of situations
Ground squirrels make an alarm call so high pitched that we cannot even hear it,
scientists report in Nature.
While studying the little rodents, researchers noticed that some of them made
faint whispering sounds, as if they had lost their voices.
But when these "silent screams" were processed by a bat detector, an abundance
of ultrasound was detected.
The researchers believe the whispers might be "secret" alarm calls - that the
squirrels' predators cannot hear.
It is well documented that bats use ultrasonic calls to locate their prey. But,
in other animals, the use of this extremely high-pitched sound is not
particularly well understood.
Scientists know that some rodents make ultrasonic noises, but they have never
been able to work out exactly what they are for.
The problem is that the sounds are used by rodents in all sorts of situations;
and they elicit all sorts of responses. So it is hard for observers to unravel
the circumstances under which these calls are used.
Now, at last, researchers have detected an ultrasonic call with, they think, a
I came across a squirrel who had seemingly lost her voice
James Hare, University of Manitoba
James Hare, of the University of Manitoba, Canada, made the discovery while
studying Richardson's ground squirrels (Spermophilus richardsonii), which live
in the grasslands of North America.
He was looking into how the little animals - which are also called gophers -
recognise each others' audible calls, when he noticed something odd about his
subjects. They would sometimes open their mouths as if to call, but only a faint
breathy rush of air would come out.
"I was recording Richardson's ground squirrel alarm calls for research on
individual recognition of audible callers, when I came across a squirrel who had
seemingly lost her voice," Professor Hare told BBC News Online.
And she was not the only one: other squirrels were apparently having the same
"trouble". But, since their voices were coming and going, he assumed there was
more to it than a bout of sore throats.
"I began to notice more and more of this 'whisper calling' as I conducted the
work," Professor Hare said. "And I began to doubt the 'lost voice' notion when I
witnessed squirrels switching back and forth between audible and whisper calls."
He suspected the squirrels were communicating using sounds outside the human
hearing range. To test his theory he recruited some special equipment, which is
normally used to "hear" bat screams.
The instrument works by slowing the sounds down - or lowering the pitch - so
that human ears can hear them.
"Grasping at straws, I borrowed a bat detector from a student I had working with
my group that summer and lo and behold, when whisper calls were broadcast, the
detector revealed abundant ultrasound," said Professor Hare.
Detecting ultrasound in a rodent is not groundbreaking. But the exciting thing
in this case is that it seems to have a clear purpose.
James Hare and his colleague David Wilson analysed the high-pitched calls, and
found they were made in reaction to a threat; and elicited an alarm response in
It is well documented that bats use ultrasonic sound to locate their prey
Richardson's ground squirrels also make audible alarm calls, which seem to raise
a more dramatic response in the "audience".
So the researchers speculate that whisper calls might indicate a slightly lower
level of threat.
They think that the squirrels might have evolved ultrasound so they can
communicate with their neighbours without any predators knowing about it.
Also, ultrasonic screams are very targeted. So a squirrel can selectively call
to its kin without anyone else hearing.
"Such range limitations may be favoured because they reduce the detectabilty of
the caller, or because they limit the audience," said Professor Hare.
Ultrasonic communication is probably far more prevalent in the animal world than
we know because, simply, we cannot hear it.
Professor Hare said: "It may be far more common than we think, though given our
own perceptual limitations we may simply be less likely to detect such signals."