By Julianna Kettlewell
BBC News Online science staff
Ground squirrels make an alarm call so high pitched that we cannot even hear it, scientists report in Nature.
High-pitched sounds are used by rodents in all sorts of situations
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 clear meaning.
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 other squirrels.
Richardson's ground squirrels also make audible alarm calls, which seem to raise a more dramatic response in the "audience".
It is well documented that bats use ultrasonic sound to locate their prey
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."