This aquatic rodent is found across Canada and throughout the United States but not in areas of Arctic tundra. The muskrat is also found in much of Europe and Asia since being introduced by humans.
Fully grown, muskrats measure about 60 cm (2 ft.) in overall length and weigh about 1.5 kg (3 lb.). They are excellent swimmers, capable of travelling 90 m (295 ft.) under water and staying submerged, when necessary, for more than 15 minutes at a time.
Their houses are constructed of cattails or bulrushes, weeds and packed mud, with a dry sleeping platform inside. Usually, only one rat lives in each house, although several may live together harmoniously (usually the female with her most recent litter) over the winter. They also build dens in stream banks with the entrance underwater. The rats become more aggressive during the breeding season, which is from late winter to early September. Very quarrelsome, even among themselves, they are vicious fighters and have been known to attack humans without obvious provocation.
As well as a wide variety of vegetation, they feed on freshwater mussels and are reported to eat frogs and small turtles. Their chief predator is the mink, but while on land they also fall prey to foxes, coyotes and lynxes as well as some of the larger avian predators.