The question of whether squirrels hibernate is a common source of confusion, largely because the term “squirrel” refers to a diverse family of rodents with vastly different winter behaviors. Some squirrels are true hibernators, while many of the most commonly observed species are not. The survival strategy an individual squirrel employs depends entirely on its specific species and whether it lives in a tree or underground. Understanding the fundamental difference between true hibernation and other states of reduced activity is necessary to appreciate the varied ways these animals survive the cold months.
Defining True Hibernation
True hibernation is a profound physiological state of metabolic depression that allows an animal to conserve energy over long periods, typically weeks or months, when food is scarce and temperatures are low. This state is characterized by a significant drop in body temperature, often approaching the ambient temperature of the burrow. The animal’s metabolic rate can decrease to as little as one to two percent of its active rate, while the heart rate may slow dramatically, sometimes to just a few beats per minute. Hibernators sustain themselves solely on the substantial fat reserves they accumulate in the fall, minimizing the need to wake up for foraging.
The Squirrels That Truly Hibernate
The squirrels that engage in true hibernation are overwhelmingly members of the ground squirrel group, such as the Arctic Ground Squirrel and the Thirteen-lined Ground Squirrel. The Arctic Ground Squirrel is an obligate hibernator, meaning it must hibernate to survive the harsh winter conditions, often spending seven to nine months underground. This species exhibits the lowest body temperature ever recorded in a mammal, sometimes reaching -2.9 degrees Celsius, a phenomenon known as supercooling, where the blood remains liquid despite the sub-zero temperature.
Physiological changes in the Thirteen-lined Ground Squirrel during hibernation are also extreme, with heart rates dropping to a mere three to five beats per minute. These bouts of deep torpor, which can last for days or weeks, are periodically interrupted by short, spontaneous arousals, lasting about 12 to 24 hours, where the body temperature returns to a normal state. The energy spent on these periodic warm-ups accounts for the majority of the fat reserves consumed during the entire hibernation cycle. Ground squirrels prepare for this prolonged state by rapidly increasing their body weight to build the necessary fat stores.
Winter Survival Strategies of Tree Squirrels
Tree squirrels, such as the Gray Squirrel and Fox Squirrel, do not engage in true hibernation. These species remain active throughout the winter, relying on different methods to survive the cold and scarcity of food. They significantly reduce their activity levels, staying sheltered in their nests for extended periods, but they are not in a sustained dormant state.
A key survival tactic is the use of torpor, a short, temporary reduction in body temperature and metabolic rate that lasts only a few hours. This shallow state of inactivity is typically entered during a cold night or a harsh, short cold spell, and it allows them to save energy without requiring a massive physiological overhaul. These squirrels often venture out during the warmest part of the day to forage because they do not rely on a single, massive fat reserve.
Their primary long-term strategy involves extensive food caching, which they begin in the fall. Gray and Fox Squirrels practice scatter hoarding, burying individual nuts and seeds in numerous locations across their territory. They depend on their excellent sense of smell and spatial memory to retrieve these hidden caches, even when they are buried under a layer of snow.
For shelter, tree squirrels construct sturdy nests called dreys, often high in tree branches, which are insulated with materials like leaves, moss, and grass to retain warmth. They may also use tree cavities or share a communal den with other squirrels to pool body heat and conserve energy.