Muskrats are semi-aquatic rodents highly adapted to life in wetlands, marshes, and along the banks of rivers and lakes. Muskrats do not hibernate; instead of entering a deep, prolonged sleep with drastically lowered body temperature, they remain active throughout the winter. Their survival relies on physiological adaptations and engineering to find food and stay warm beneath the ice.
The Winter Survival State
Muskrats maintain a nearly normal body temperature throughout winter, unlike true hibernators whose body temperature plummets close to freezing. Their survival strategy centers on reducing activity and conserving body heat, but they must still forage daily. The muskrat’s dense, two-layered coat is an adaptation, with guard hairs repelling water and a thick underfur trapping air for insulation, even in near-freezing water.
To further manage heat loss, muskrats employ a mechanism called regional heterothermia, which restricts blood flow to less-insulated extremities like their tail and feet. They also exhibit a physiological response just before diving into cold water, where they may slightly elevate their core body temperature by about 1°C. When submerged, their heart rate decreases, and oxygen is drawn from stores in their muscle tissue, allowing them to remain underwater for up to 15 to 20 minutes.
Constructing Winter Lodging
Muskrats build two primary types of shelters to endure the winter: bank burrows and dome-shaped lodges. In areas with stable banks, they excavate dens, always ensuring the entrance is below the waterline to prevent it from freezing shut. In marshes and open water, they construct lodges, which are mounds of mud and aquatic vegetation like cattails and reeds, rising above the water surface.
These lodges can be up to three feet high and contain a dry living chamber situated above the water level, which remains warm due to the surrounding insulation. Multiple muskrats, often a family group, may occupy a single lodge during the winter, huddling together to share body heat and reduce energy expenditure. The thick walls of vegetation and mud protect the occupants from the cold and predators like mink and coyotes.
Accessing Food Sources Under Ice
Muskrats must actively forage for aquatic plants and roots throughout the winter. They utilize a system of trails and tunnels created just beneath the ice surface to travel safely between their main lodge and feeding areas. Their primary diet consists of the starchy roots and tubers of cattails, bulrushes, and other marsh plants, which they dig up from the pond or marsh bottom.
To aid in their foraging, muskrats construct smaller, temporary shelters called “push-ups” by chewing a hole in the thin ice and pushing up a mound of vegetation and mud to cover it. These push-ups, which can be up to 300 feet away from the main lodge, serve as resting spots, feeding stations, and breathing holes during long underwater excursions. Using these structures, muskrats can safely surface for air and consume gathered food without being exposed to predators or the harsh winter air.