It will come as no surprise to squirrel lovers — and haters, for that matter — that the twitchy, resourceful rodents are exquisitely attuned to the world around them.
Researchers at Oberlin College reported Wednesday in the scientific journal PLoS One that squirrels pay attention not only to alarm calls, as many animals do, but also to the background chatter of birds, and that they relax a bit when the birds sound relaxed.
Of course they do. They’re squirrels. They pay attention to everything. I’d be willing to bet that they can tell the difference between an irate bird lover banging on a window in pajamas (no problem) and one with her snow boots on (better get ready to skitter away).
Still, that kind of unproven certainty can lead one astray. For instance, goatsuckers, a group of birds that includes the whippoorwill, do not milk goats, Aristotle’s declaration to the contrary notwithstanding. If Aristotle had done a bit more research, he might have saved goats ages of anxiety.
Keith A. Tarvin is not repeating the philosopher’s mistake. He’s a biologist at Oberlin who studies what kind of information animals derive from the many sounds around them. He and his students set out to get some hard data on what squirrels listen to.
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“I’ve been interested in alarm calls for a while,” he said. In a recent scientific paper, for instance, he reported that gray squirrels pay special attention to the alarm calls of robins. This kind of eavesdropping is widespread in nature. “Even some lizards that don’t make their own vocal sounds eavesdrop on some other animals like birds,” he said.
As he and his students discussed the soundscape squirrels live in, he said, “that led us into chatter,” the general background noise of nature — in particular the calls that birds make when nothing big is going on. Their question was not what one sparrow might be saying to another, but whether squirrels took note of the birds’ conversations when they weren’t shouting about hawks.
The researchers hypothesized that the squirrels were paying attention, and over the course of a couple of years Dr. Tarvin and two undergraduates, Mary V. Lilly and Emma C. Lucore, designed and conducted an experiment to test that idea. They played a recorded screech of a red-tailed hawk, which put the squirrels on alert and caused them to act vigilant.
Then they played recordings of desultory bird chatter, or of background noise without birds. They made all the recordings at one bird feeder at different times. Then Ms. Lilly, who did the field work (Ms. Lucore had graduated after helping to design of the experiment), had to find squirrels around Oberlin, in the winter, on her bicycle. It was cold, she said, but squirrels are easier to observe in the winter, with no leaves to obscure their actions.
She carried with her a sound system in a repurposed cat litter box. “I looked pretty ridiculous,” she said. Once she found a squirrel, she set up the sound system and played the different recordings: the hawk followed by bird chatter, and the hawk followed by birdless background noise. She kept track of what the squirrels were doing by entering data into a customized app on her phone.
She tracked six “behavioral states” and how long each squirrel was in that state: foraging, preening, resting, standing, freezing and fleeing. Standing, freezing and fleeing were the vigilance states.
The results were that when the squirrels heard the relaxed birds, they, too, relaxed. And they did so more quickly than when they heard background noise without bird chatter.
The squirrels were probably responding to the gestalt of a calm-sounding neighborhood, Dr. Tarvin said, or as Ms. Lilly put it, “the sound of no danger.” But it’s possible there are specific signals they are picking up that the researchers have not identified.
The finding is significant not just to add to the reputation of squirrels as wary and clever. Dr. Tarvin said the research also supported the idea that there may be “public information networks that exist in ecological communities.”
Human-generated sound, or sound pollution, could interrupt those networks in ways we don’t yet understand, Ms. Lilly said.
“We really can’t know the impact of the sounds that we’re creating unless we know more about the sound information that’s part of the ecosystem,” she said.
James Gorman is a science writer at large and the host and writer of the video series “ScienceTake.” He joined The Times in 1993 and is the author of several books, including “How to Build a Dinosaur,” written with the paleontologist Jack Horner.
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