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Squirrels Emit Ultrasonic Noise, Warn Others of Danger

By Pippa Wysong, Access Excellence


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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