The sight of a squirrel scampering across a snow-covered landscape often raises questions about how these small creatures survive the bitter cold. Unlike many northern mammals that disappear entirely for the winter, common tree squirrels, such as the Eastern gray squirrel and fox squirrel, remain active throughout the season. Their presence in freezing temperatures is driven by the necessity to forage for food, contrasting sharply with the deep sleep of true hibernators.
Winter Activity: Torpor vs. True Hibernation
Tree squirrels are visible in winter because they are not true hibernators. True hibernation is a state of prolonged, deep metabolic depression where an animal’s body temperature drops dramatically, and its heart rate and breathing slow significantly for weeks or months. Animals like groundhogs and some ground squirrels enter this profound state, surviving solely on large fat reserves accumulated in the fall.
Tree squirrels are homeotherms, meaning they maintain a relatively constant, high body temperature year-round, requiring a continuous energy source. They may enter a temporary state called torpor, which is a short-term reduction in metabolic rate, heart rate, and body temperature. Torpor typically lasts only a few hours or a day during periods of severe cold or food scarcity, allowing them to conserve energy.
Torpor is a shallow sleep from which the squirrel must wake frequently to raise its internal temperature and consume stored food. These frequent awakenings are energetically expensive, making it impossible for the animal to rely solely on stored body fat for the entire winter. Consequently, they must venture out of their nests at least every few days to eat, even when snow is deep. The ability to quickly warm up and become active allows the squirrel to be seen running around when a true hibernator would be dormant.
The Necessity of Foraging: Retrieving Winter Stores
A squirrel’s winter activity is driven by the immediate need for calories to fuel its high metabolism and combat heat loss. Their survival hinges on “scatter hoarding,” where they bury thousands of individual food items, like nuts and seeds, across a wide area during the autumn. This strategy ensures the squirrel has numerous backups if one cache is discovered.
When snow covers the ground, squirrels rely on a combination of spatial memory and an acute sense of smell to locate their hidden treasures. Studies suggest they create a detailed mental map of where they buried their caches, using landmarks such as trees, rocks, and fence posts as navigational cues. This memory allows them to retrieve a high percentage of their stored food.
This memory is supplemented by olfactory acuity, the ability to detect the scent of buried nuts, even through snow and frozen soil. The soil around the cache is often loosened during burial, which helps the aroma to waft upward. After locating the general area, the squirrel uses its nose to pinpoint the precise spot and retrieve the item.
The energy reward from a high-calorie nut, such as a hickory or walnut, outweighs the energy cost of the short excursion into the cold. When primary caches are inaccessible or depleted, squirrels seek alternative food sources to maintain energy balance. This includes foraging for tree buds, bark, pine cones, and fungus exposed above the snow line.
Physical Adaptations and Shelter Construction
While foraging is necessary, squirrels possess physical and behavioral adaptations that allow them to withstand the cold. Their winter coat is denser and thicker than their summer fur, providing an insulating layer of trapped air that minimizes heat loss. This thick fur acts as a natural barrier against the frigid air and snow.
The bushy tail is an important adaptation, serving as a multi-purpose tool for temperature regulation. When resting or sleeping, a squirrel wraps its tail around its body like a blanket, protecting its face and minimizing the exposed surface area from which heat can escape. Squirrels can also restrict blood flow to their extremities, such as their paws, reducing heat loss while maintaining core body temperature.
When not foraging, squirrels retreat to insulated winter nests, known as dreys, or sheltered tree cavities. A drey is a spherical nest constructed of interwoven sticks and twigs, lined with materials like dried leaves, grass, and shredded bark. These materials create a tight, insulated structure, and the interior of a well-built drey can be 20 to 30 degrees Celsius warmer than the outside air.
In the coldest conditions, several squirrels may share a single drey or tree cavity, a behavioral adaptation known as huddling, to combine body heat and conserve energy. This communal nesting increases the ambient temperature within the shelter, helping the animals endure prolonged periods of cold and minimizing the need to enter torpor.