Whether squirrels can freeze to death in winter is a common concern. Squirrel survival depends on physiological mechanisms and learned behaviors that allow them to endure extreme cold and resource scarcity. The strategies employed vary significantly, primarily based on whether the animal is a tree squirrel, like the eastern grey squirrel, or a ground squirrel, such as the thirteen-lined ground squirrel. Understanding these different approaches provides insight into how these small rodents manage to thrive in freezing conditions.
The Primary Risks of Winter Mortality
Squirrels possess effective insulation and metabolic control, meaning they rarely die from cold temperature alone. Winter mortality is more often an indirect result of cold exposure that leads to hypothermia. This dangerous drop in internal body temperature occurs when the squirrel’s energy reserves are depleted, leaving it unable to generate sufficient body heat.
The lack of continuous access to food is a major factor, as the energy required to stay warm rapidly consumes stored fat and cached resources. Exposure to wet conditions dramatically increases the risk of hypothermia because a wet coat loses its insulating property. This forces the squirrel to expend far greater amounts of energy to remain warm. Most winter deaths are related to an inability to manage the energy balance required to counteract prolonged cold and dampness.
Behavioral Strategies for Cold Survival
To combat the cold, tree squirrels construct highly insulated homes called dreys, typically built high in tree forks or cavities. These nests are woven from twigs and leaves, then lined with soft materials like moss, shredded bark, and fur to create a thermal barrier. The structural integrity of a well-built drey defends against heat loss.
In harsh weather, tree squirrels may engage in communal nesting, sharing a drey to benefit from shared body warmth. This huddling behavior significantly reduces the surface area exposed to the cold, lowering the energy cost of thermoregulation. Maintaining a steady energy supply is also managed through diligent food caching in the autumn months.
Squirrels use different caching techniques. Grey squirrels use scatter hoarding, burying individual nuts in numerous shallow holes across a wide area, which minimizes the risk of losing a single cache to a competitor. Red squirrels are larder hoarders, accumulating large centralized stores of pine cones called middens, which they defend throughout the winter.
Physiological Adaptations and Torpor
Stored body fat is the primary fuel source used to generate heat during the winter. Squirrels build up reserves of white fat for sustained energy, and brown fat, which is specialized for non-shivering thermogenesis that rapidly produces heat. This internal heat production allows them to remain active even when external temperatures drop below freezing.
For active tree squirrels, a significant physiological adaptation is the use of torpor, a temporary state of reduced metabolic activity. Torpor involves a short-term drop in body temperature and heart rate, conserving energy during cold or stormy periods, often lasting less than 24 hours. The squirrel quickly returns to a normal body temperature once the weather stress has passed, allowing it to resume foraging.
This short-term torpor is distinct from true hibernation, which is a state of deep, prolonged metabolic suppression lasting for weeks or months. True hibernators experience a dramatic drop in body temperature, sometimes falling to just a few degrees above freezing, with their heart rate slowing to only a few beats per minute. This deep state allows them to survive long periods without eating or drinking.
How Different Squirrel Species Cope
The two main types of North American squirrels employ different strategies to survive the winter. Tree squirrels, such as the eastern grey and red squirrel, are not true hibernators and remain active throughout the winter. They rely on well-insulated dreys and extensive food caches to provide energy for daily activity. Their use of short bouts of torpor is simply an energy-saving measure used to weather brief storms.
Ground squirrels, conversely, are true hibernators and use a profound metabolic shutdown. They spend the entire winter in deep burrows, where their body temperature plummets and their metabolism nearly ceases. These animals rely almost exclusively on substantial fat reserves accumulated during preceding months, often not eating or drinking for six to eight months. This difference means tree squirrels are often visible foraging on mild winter days, while ground squirrels remain hidden deep underground until spring.