The muskrat is the largest species in the arvicoline subfamily; which includes 142 other species of rodents, mostly voles and lemmings. Muskrats are called "rats" in a general sense because they are medium-sized rodents with an adaptable lifestyle and an omnivorous diet. They are not, however, so-called "true rats", that is members of the genus Rattus.
An archaic name in English for the animal is musquash, derived from the Abenaki native word mòskwas (New Oxford American Dictionary).
An adult muskrat is about 40 to 60 cm (16 to 24 inches) long, almost half of that tail, and weighs from 0.7 to 1.8 kg (1.5 to 4 lb). That is about four times the weight of the brown rat (Rattus norvegicus), though an adult muskrat is only slightly longer. Muskrats are much smaller than beavers (Castor canadensis), with whom they often share their habitat. Adult beavers weigh from 14 to 40 kg (30 to 88 lb). The nutria (Myocastor coypus) was introduced to North America from South America in the early twentieth century. It shares the muskrat's habitat but is larger, 5 to 10 kg (11 to 22 lb) and its tail is round, not flattened. It cannot endure as cold a climate as can the muskrat and beaver, and so has spread only in the southern part of their ranges in North America (Caras 1967, Nowak 1983).
Muskrats are covered with short, thick fur which is medium to dark brown in color with the belly a bit lighter. The fur has two layers, which helps protect them from the cold water. They have long tails which are covered with scales rather than hair and are flattened vertically to aid them in swimming. When they walk on land the tail drags on the ground, which makes their tracks easy to recognize (Caras 1967, Nowak 1983).
Muskrats spend much of their time in the water and are well suited for their semi-aquatic life, both in and out of water. Muskrats can swim under water for up to 15 minutes. Their bodies, like those of seals and whales, are less sensitive to the build up of carbon dioxide than those of most other mammals. They can close off their ears to keep the water out. Their hind feet are semi-webbed, although in swimming the tail is their main means of propulsion (Voelker 1986).
Muskrats continue to thrive in most of their native habitat and in areas where they have been introduced. While much wetland habitat has been eliminated due to human activity, new muskrat habitat has been created by the construction of canals or irrigation channels and the muskrat remains common and wide-spread. They are able to live alongside streams which contain the sulfurous water that drains away from coal mines. Fish and frogs perish in such streams, yet muskrats may thrive and occupy the wetlands. Muskrats also benefit from human persecution of some of their predators (Nowak 1983).
Muskrats are most active at night or near dawn and dusk. They feed on cattails and other aquatic vegetation. They do not store food for the winter, but sometimes eat the insides of their lodges or steal food that beavers have stored. Plant materials make up about 95 percent of their diets, but they also eat small animals such as freshwater mussels, frogs, crayfish, fish, and small turtles (Caras 1967, Nowak 1983).
Muskrats provide an important food resource for many other animals including mink, foxes, coyotes, wolves, lynx, bears, eagles, snakes, alligators, and large owls and hawks. Otters, snapping turtles, and large fish such as pike prey on baby muskrats. Caribou and elk sometimes feed on the vegetation which makes up muskrat lodges during the winter when other food is scarce for them (MU 2007).
Muskrats, like most rodents, are prolific breeders. Females can have 2 to 3 litters a year of 6 to 8 young each. The babies are born small and hairless and weigh only about 22 grams (0.8 oz). In southern environments young muskrats mature in 6 months, while in colder northern environments it takes about a year. Muskrat populations appear to go through a regular pattern of rise and dramatic decline spread over a 6 to 10 year period. Some other rodents, including famously the muskrat's close relatives the lemmings, go through the same type of population changes (MU 2007).
Muskrats have sometimes been a food resource for humans. Muskrat meat is said to taste like rabbit or duck. In the Roman Catholic Archdiocese of Detroit, there is a longstanding dispensation allowing Catholics to consume muskrat on Ash Wednesday and the Fridays of Lent (when the eating of meat, except for fish, is prohibited): because the muskrat lives in water, it is considered equivalent to fish (Lukowski 2007).
Lenten dinners serving muskrat are traditional in parts of Michigan. The meat is also commonly consumed in Belgium and The Netherlands, and is traditional dish on the Delmarva Peninsula and in certain other areas and population segments of the United States.
Muskrat fur is very warm and of good quality, and the trapping of muskrats for their fur became an important industry in the early Twentieth Century, especially in the state of Louisiana. At that time muskrats were introduced to Europe as a fur resource. Muskrat fur was specially trimmed and dyed and called "hudson seal" fur, and sold widely in the United States in the early twentieth century (Ciardi 1983). They spread throughout northern Europe and Asia. Some European countries such as Belgium and the Netherlands consider the muskrat to be a pest that must be exterminated. Therefore the animal is trapped and hunted to keep the population down. The muskrat is considered a pest because its burrowing causes damage to the dikes and levees that these low-lying countries depend on for protection from flooding. Muskrats also sometimes eat corn and other farm and garden crops (Nowak 1983).