Winnipeg (08/25/04)- Ground squirrels living in the Northern
plains of North American have been found to emit screams in the ultrasonic
range to warn others of their kind of oncoming danger. Until now, it
was not known that any mammal used ultrasound as an alarm. Researchers
have also found that while nearby squirrels can hear the high frequency
calls, predators such as hawks cannot.
The
Richardson's ground squirrel (Spermophilus richardsonii) live
in colonies and spend most of their time in underground burrows. They
are also known as prairie gophers and tawny American marmots.
"This is the first demonstration of an ultrasonic alarm signal,
period," said James Hare, PhD, a zoologist at the University of
Manitoba, in Canada. There are many examples of living things in the
animal kingdom that create and use ultrasound. For instance, bats use
it for echolocation, certain moths and butterflies use it for communication,
and even rats can create ultrasonic noises that help with navigation
in dark places. But this is the first time it has been observed to be
used for communicating danger.
The original discovery occurred in 1993 when Dr. Hare was out in the
field making recordings of the audible calls of the squirrels. He was
using a model of a predator, in the form of a beige colored hat, tossed
into the air to startle the animals and trigger them to make alarm calls.
Squirrel Whisperer
"Any item that flies quickly through the air and lands near them
will startle them. Without knowing what it is, they respond in a manner
consistent with there being a threat," he said. At one point during
his studies, he noticed a female ground squirrel going through all the
motions of making calls, but he couldn't hear anything. Thinking the
squirrel had lost its voice, he continued with his recordings -- until
he noticed other squirrels doing the same thing. At first, the researchers
called these silent calls "whisper calls".
Later, he went back to the squirrel colony with a bat-detector, a device
that detects sound in the ultrasonic frequencies. "Sure enough,
when they were doing their whisper calling, the bat detector responded
like crazy. There was ultrasound present when they were doing this calling,"
he said.
Over the ensuing years, Dr. Hare and colleagues learned more about
the squirrels and their high frequency sounds. The findings were published
in a recent edition of the journal Nature (vol. 43). The first
author is David Wilson, a student in Dr. Hare's Lab.
Another finding is that the squirrels in a colony can recognize the
calls of specific individuals. "We found they use that ability
to weigh their response according to the past reliability of those individuals,"
he said. For instance, if there is a nervous squirrel in the colony
that calls out alarms frequently for non-threatening events, the rest
of the squirrels ignore it.
"If there's Crazy Bob who calls when nothing is present, they
come to ignore Crazy Bob. It's like Chicken Little for ground squirrels.
They do assess and integrate information about individual identity and
reliability. We're not only learning about communuication, but arguably
about the cognitive ability of these animals as well," Dr. Hare
said.
The researchers also found that the ultrasonic squeals made by the
squirrels change according to how far away the danger is, suggesting
they are communicating information about the distance of danger. The
ultrasonic calls are in the range of 50 kiloHertz (KHz). The sound travels
only about 15 meters distance-wise, far enough to alert other ground
squirrels.
In addition, "avian predators are one of the more potent threats
to these animals. Red tailed hawks, owls, peregrine falcons. None of
these can detect ultrasound," Dr Hare said. Producing this high
frequency would mean the warning goes out to other squirrels, but not
doesn't alert the predator that there is a squirrel nearby.
From a biological perspective, the research highlights two things.
"One, that signals used in communication can be tailored to be
used in such a way that they suit a highly specific need," Dr.
Hare said. Two, it's a reminder that non-human animals use signals that
are well out of range of human perception.
"We have to think of it in the broader sense of what suits those
animals, not what suits ourselves," he said. It is not yet known
just how the squirrels emit the ultrasonic screams, nor whether other
varieties of squirrels can do the same.
end
* Image of Richarson Ground Squirrel. Photo by James
Hare.
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