Muskrats don’t hibernate. They stay awake and busy through the coldest, iciest months, just out of sight beneath the ice.1Duluth News Tribune. Klemek Column: Wildlife’s winter survival strategies are fascinating, perplexing Staying active in freezing water is costly, though, so a muskrat’s winter is a daily balancing act between eating enough and not losing too much heat.
What Muskrats Do All Winter Instead of Sleeping
Muskrats are active in bouts spread through the day and night, with a new bout roughly every 6 hours, in winter as in summer.2Ecography. Daily and seasonal activity patterns of the muskrat Ondatra zibethicus as revealed by radiotelemetry They don’t keep a strict day or night schedule, which is one reason you rarely catch them. In winter, a typical bout means leaving a lodge or bank burrow, swimming under the ice to feed, then returning to rest. A lodge is a dome of mud and plant stems built in shallow water, with a dry chamber above the waterline.
A bout starts at an underwater entrance, so the muskrat never has to cross open ice. It swims along the pond bottom to a feeding spot, such as a patch of roots or a push-up, and eats there rather than carrying food home each time. Afterward it returns to the dry chamber to rest and dry off until the next bout. Once ice forms, it hides all this swimming and feeding, and because muskrats come and go through underwater entrances, they rarely appear on top.
Muskrats do prepare for winter in some of the same ways hibernators do. Before winter, they build up fat and grow thick fur.3Duluth News Tribune. Klemek Column: Wildlife’s winter survival strategies are fascinating, perplexing They also store food to eat later.4Duluth News Tribune. Klemek Column: Wildlife’s winter survival strategies are fascinating, perplexing But hibernators drop their body temperature and heart rate for weeks, live off fat, and barely move. Muskrats keep a normal body temperature and wake every few hours, so they have to keep eating all winter.
How Muskrats Stay Warm in Icy Water
A muskrat’s fur doesn’t soak through, so it holds a thin layer of air against the skin underwater.5Acta Theriologica. Thermoregulation of a Semi-aquatic Mammal, the Muskrat, in Air and Water That air layer insulates the body and helps keep it warm in near-freezing water.6Acta Theriologica. Thermoregulation of a Semi-aquatic Mammal, the Muskrat, in Air and Water The fur only works while it stays unsoaked, which is why muskrats spend time grooming and oiling it.
Heat also leaks from the feet and tail, and muskrats control that. They can narrow blood vessels in their feet and tail so those parts cool close to the water’s temperature.7Comparative Biochemistry and Physiology Part A: Physiology. Thermoregulation in the muskrat (Ondatra zibethicus): The use of regional heterothermia Less warm blood reaching the surface means less body heat lost.8Comparative Biochemistry and Physiology Part A: Physiology. Thermoregulation in the muskrat (Ondatra zibethicus): The use of regional heterothermia When they need to shed heat, such as in summer, they widen those vessels and let the extremities warm up.9Comparative Biochemistry and Physiology Part A: Physiology. Thermoregulation in the muskrat (Ondatra zibethicus): The use of regional heterothermia
The scaly tail and webbed hind feet have little fur, so they would lose heat fast if kept warm. Heat flows out faster the bigger the temperature gap between a body part and the water around it, so when the feet and tail cool to near water temperature, very little escapes.
Even so, the body cools a little on each trip. During winter foraging trips under the ice, a muskrat’s body temperature usually drops by no more than about 2 °C.10Canadian Journal of Zoology. Seasonal patterns of body temperature and activity in free-ranging muskrats (Ondatra zibethicus) That drop stays about the same even on trips longer than 40 minutes.11Canadian Journal of Zoology. Seasonal patterns of body temperature and activity in free-ranging muskrats (Ondatra zibethicus) The reason is that muskrats raise their body temperature before entering the water, and they warm back up in feeding shelters partway through their outings.12Canadian Journal of Zoology. Seasonal patterns of body temperature and activity in free-ranging muskrats (Ondatra zibethicus)
How Muskrats Breathe Under the Ice
Muskrat heart and leg muscles hold far more of an oxygen-storing protein than a lab rat’s do.13Canadian Journal of Zoology. Oxygen transport, tissue glycogen stores, and tissue pyruvate kinase activity in muskrats Their muscles therefore carry an oxygen reserve that lets them keep working on a single breath.14Canadian Journal of Zoology. Oxygen transport, tissue glycogen stores, and tissue pyruvate kinase activity in muskrats This extends a dive but doesn’t make it unlimited; the muskrat still has to surface or reach air within minutes. That limit is why breathing spots matter so much to where a muskrat can feed.
Those spots are called push-ups. A push-up is a small mound of plants and mud that muskrats build over a hole in the ice. It serves as a place to breathe, eat, and rest away from the home lodge.15Alaska Department of Fish and Game. Muskrat Species Profile From shore, it looks like a small dark heap of vegetation sitting on the ice, usually a foot or two across, smaller than a lodge.
Building one starts with gnawing or pushing up through thin new ice, then piling cut plants over the hole so it is covered and insulated. The muskrats keep adding material and clearing the hole from below as the ice thickens. The covering keeps cold air off the opening, which slows the hole from freezing shut.
Muskrats have also been seen putting their noses into air bubbles under the ice and apparently breathing from them.16Prairie Public. Muskrats in Winter This bubble-breathing is estimated to roughly double how long they can stay in the water.17Prairie Public. Muskrats in Winter The two tools work together: push-ups act as rest stops along a feeding route, and bubbles give a quick breath between stops.
What Muskrats Eat in Winter
Before winter, muskrats often move to deeper water.18Canadian Journal of Zoology. Seasonal differences in habitat use and fat reserves in an arctic muskrat population Deeper water stays unfrozen and holds energy-rich roots and underground stems of submerged plants.19Canadian Journal of Zoology. Seasonal differences in habitat use and fat reserves in an arctic muskrat population Shallow marsh edges can freeze to the bottom, shutting off the plants growing there. They also eat parts of their own lodges and push-ups, which are made of plants.
Muskrats aren’t strict vegetarians, though they mostly eat plants year-round. They will eat freshwater mussels, leaving shell piles at feeding spots.20The American Midland Naturalist. Consumption of Freshwater Bivalves by Muskrats in the Green River, Kentucky They leave fewer shells when fresh plants are plentiful, so eating mussels is mainly a fallback when plant food runs low.21The American Midland Naturalist. Consumption of Freshwater Bivalves by Muskrats in the Green River, Kentucky Crayfish and snails may also be taken.
Why Muskrats Huddle Together in Winter
Winter shelter air can sit between about 0 °C and −10 °C.22Journal of Mammalogy. Thermal Benefits of Huddling in the Muskrat (Ondatra zibethicus) At those temperatures, four muskrats resting together use about 11 to 14% less energy than one alone.23Journal of Mammalogy. Thermal Benefits of Huddling in the Muskrat (Ondatra zibethicus) The reason is that as a group they lose body heat about 8 to 10% more slowly.24Journal of Mammalogy. Thermal Benefits of Huddling in the Muskrat (Ondatra zibethicus) A shared lodge therefore stretches food supplies a little further over a long winter.
The animals sharing a lodge are often a family group from the past season. Several curl together in the dry chamber, and their body heat warms the chamber above the outside air. Each animal in a cluster exposes less of its body surface to the cold, so less heat is lost per animal.
Grouping may not change how well they forage. Grouped muskrats tended to spend more time in the water and use less energy than solitary ones, but the difference was too small to be sure of.25Journal of Mammalogy. Huddling Behavior and the Foraging Efficiency of Muskrats So the clearest benefit of grouping is while resting, not while feeding.
What Kills Muskrats in Winter
Very cold winters with little snow are the worst combination. Push-ups often freeze solid,26Alaska Department of Fish and Game. Muskrat Species Profile and muskrats lose their breathing holes and their places to eat and rest.27Alaska Department of Fish and Game. Muskrat Species Profile Such winters can cause large die-offs.28Alaska Department of Fish and Game. Muskrat Species Profile
Thick ice shrinks the food supply too. As ice thickens, less water is left open for feeding.29Alaska Department of Fish and Game. Muskrat Species Profile Muskrats leave shallow ponds for deeper ones, which often have fewer plants.30Alaska Department of Fish and Game. Muskrat Species Profile Food then runs out faster, leading to fighting, starvation, or animals leaving to search elsewhere.31Alaska Department of Fish and Game. Muskrat Species Profile Open homes are rarely available, so many wanderers freeze, starve, or are caught.32Alaska Department of Fish and Game. Muskrat Species Profile
Predators strike hardest late in winter. Mink are the muskrat’s main predator, and foxes, coyotes, raccoons, and birds of prey also take them.33Ecological Engineering. Muskrats (Ondatra zibethicus) in treatment wetlands The heaviest killing happens in late winter and early spring, when muskrats come out on the ice.34Ecological Engineering. Muskrats (Ondatra zibethicus) in treatment wetlands Most of those killed are sick or young animals.35Ecological Engineering. Muskrats (Ondatra zibethicus) in treatment wetlands
How to Spot Muskrats in Winter
The animals themselves are hard to see, so look for what they leave behind. A used lodge usually has fresh, green or recently cut plant material on top, while an abandoned one looks weathered and sagging. A clear or open spot in the ice around the base suggests animals are moving in and out. One clue alone isn’t conclusive, so a few signs together give a better answer.
Tail-drag tracks in snow near holes show recent outings. If you watch, try dawn and dusk near open water, though activity can happen any time.36Ecography. Daily and seasonal activity patterns of the muskrat Ondatra zibethicus as revealed by radiotelemetry
Muskrat and beaver signs are easy to mix up. Beaver lodges are much larger and built with sticks, while muskrat lodges are smaller and made of soft plants and mud. Beavers also leave chewed trees; muskrats don’t.
Watching from shore with binoculars works best. Keep off thin ice near lodges, and don’t break open push-ups, which can freeze the hole. Spring thaw is when you’ll see the muskrats again openly.