July 5, 2010
Nut? What Nut? The Squirrel Outwits to SurviveBy NATALIE ANGIER
I was walking through the neighborhood one afternoon when, on turning a corner, I nearly tripped over a gray squirrel that was sitting in the middle of the sidewalk, eating a nut. Startled by my sudden appearance, the squirrel dashed out to the road — right in front of an oncoming car.
Before I had time to scream, the squirrel had gotten caught in the car’s front hubcap, had spun around once like a cartoon character in a clothes dryer, and was spat back off. When the car drove away, the squirrel picked itself up, wobbled for a moment or two, and then resolutely hopped across the street.
You don’t get to be one of the most widely disseminated mammals in the world — equally at home in the woods, a suburban backyard or any city “green space” bigger than a mousepad — if you’re crushed by every Acme anvil that happens to drop your way.
“When people call me squirrely,” said John L. Koprowski, a squirrel expert and professor of wildlife conservation and management at the University of Arizona, “I am flattered by the term.”
The Eastern gray tree squirrel, or Sciurus carolinensis, has been so spectacularly successful that it is often considered a pest. The International Union for Conservation of Nature includes the squirrel on its list of the top 100 invasive species. The British and Italians hate gray squirrels for outcompeting their beloved native red squirrels. Manhattanites hate gray squirrels for reminding them of pigeons, and that goes for the black, brown and latte squirrel morphs, too.
Yet researchers who study gray squirrels argue that their subject is far more compelling than most people realize, and that behind the squirrel’s success lies a phenomenal elasticity of body, brain and behavior. Squirrels can leap a span 10 times the length of their body, roughly double what the best human long jumper can manage. They can rotate their ankles 180 degrees, and so keep a grip while climbing no matter which way they’re facing. Squirrels can learn by watching others — cross-phyletically, if need be. In their book “Squirrels: The Animal Answer Guide,” Richard W. Thorington Jr. and Katie Ferrell of the Smithsonian Institution described the safe-pedestrian approach of a gray squirrel eager to traverse a busy avenue near the White House. The squirrel waited on the grass near a crosswalk until people began to cross the street, said the authors, “and then it crossed the street behind them.”
In the acuity of their visual system, the sensitivity and deftness with which they can manipulate objects, their sociability, chattiness and willingness to deceive, squirrels turn out to be surprisingly similar to primates. They nest communally as multigenerational, matrilineal clans, and at the end of a hard day’s forage, they greet each other with a mutual nuzzling of cheek and lip glands that looks decidedly like a kiss. Dr. Koprowski said that when he was growing up in Cleveland, squirrels were the only wild mammals to which he was exposed. “When I got to college, I thought I’d study polar bears or mountain lions,” he said. “Luckily I ended up doing my master’s and Ph.D. on squirrels instead.”
The Eastern gray is one of about 278 squirrelly species alive today, a lineage that split off from other rodents about 40 million years ago and that includes chipmunks, marmots, woodchucks — a k a groundhogs — and prairie dogs. Squirrels are found on all continents save Antarctica and Australia, and in some of the harshest settings: the Himalayan marmot, found at up to 18,000 feet above sea level, is among the highest-living mammals of the world.
A good part of a squirrel’s strength can be traced to its elaborately veined tail, which, among other things, serves as a thermoregulatory device, in winter helping to shunt warm blood toward the squirrel’s core and in summer to wick excess heat off into the air. Rodents like rats and mice are nocturnal and have poor vision, relying on whiskers to navigate their world. The gray squirrel is diurnal and has the keen eyesight to match. “Its primary visual cortex is huge,” said Jon H. Kaas, a comparative neuroscientist at Vanderbilt University, A squirrel’s peripheral vision is as sharp as its focal eyesight, which means it can see what’s above and beside it without moving its head. While its color vision may only be so-so, akin to a person with red-green colorblindness who can tell green and red from other colors but not from each other, a squirrel has the benefit of natural sunglasses, pale yellow lenses that cut down on glare.
Gray squirrels use their sharp, shaded vision to keep an eye on each other. Michael A. Steele of Wilkes University in Pennsylvania and his colleagues have studied the squirrels’ hoarding behavior, which turns out to be remarkably calculated and rococo. Squirrels may be opportunistic feeders, able to make a meal of a discarded cheeseburger, crickets or a baby sparrow if need be, but in the main they are granivores and seed hoarders. They’ll gather acorns and other nuts, assess which are in danger of germinating and using up stored nutrients, remove the offending tree embryos with a few quick slices of their incisors, and then cache the sterilized treasure for later consumption, one seed per inch-deep hole.
But the squirrels don’t just bury an acorn and come back in winter. They bury the seed, dig it up shortly afterward, rebury it elsewhere, dig it up again. “We’ve seen seeds that were recached as many as five times,” said Dr. Steele. The squirrels recache to deter theft, lest another squirrel spied the burial the first X times. Reporting in the journal Animal Behaviour, the Steele team showed that when squirrels are certain that they are being watched, they will actively seek to deceive the would-be thieves. They’ll dig a hole, pretend to push an acorn in, and then cover it over, all the while keeping the prized seed hidden in their mouth. “Deceptive caching involves some pretty serious decision making,” Dr. Steele said. “It meets the criteria of tactical deception, which previously was thought to only occur in primates.”
Squirrels are also master kvetchers, modulating their utterances to convey the nature and severity of their complaint: a moaning “kuk” for mild discomfort, a buzzing sound for more pressing distress, and a short scream for extreme dismay. During the one or two days a year that a female is fertile, she will be chased by every male in the vicinity, all of them hounding her round and round a tree with sneezelike calls, and her on top, refusing to say gesundheit. A squirrel threatened by a serious predator like a cat, dog, hawk or wayward toddler will issue a multimodal alarm, barking out a series of loud chuk-chuk-chuks with a nasally, penetrating “whaa” at the end, while simultaneously performing a tail flag — lifting its fluffy baton high over its head and flicking it back and forth rhythmically.
Sarah R. Partan of Hampshire College in Amherst, Mass., and her students have used a custom-built squirrel robot to track how real squirrels respond to the components of an alarm signal. The robot looks and sounds like a squirrel, its tail moves sort of like a squirrel’s, but because its plastic body is covered in rabbit fur it doesn’t smell like a squirrel. Yet squirrels tested in Florida and New England have responded to the knockoff appropriately, with alarm barks of their own or by running up a tree. Human passers-by have likewise been enchanted. “People are always coming over, asking what we’re doing,” said Dr. Partan. “We’ve had to abandon many trials halfway through.” An iSquirrel? Now that’s something even a New Yorker might love