Common Muskrat, Ondatra zibethicus
Names: musquash, muskrat, house rat,
bank rat or musk beaver
Species: Ondatra zibethicus
General Description: The common muskrat is a fairly large and mainly aquatic North American rodent. It is the largest member of the rat and mouse family (Cricetidae). Female muskrats are the same size or slightly smaller than male muskrats. The typical adult muskrat has a head-body length of 25-36 cm, tail length of 20-28 cm, and shoulder height of approximately 13 cm. It has a weight ranging from 908-1,816 g. Common muskrats have a plump, rat shaped body. Their entire body, with the exception of the tail and feet, is covered with a waterproof fur. The color of the fur ranges from dark brown on the head and back to a light greyish-brown on the belly. The coat consists of a short, dense, and silky underfur as well as longer, coarser, and glossy guard hairs. The muskrat has small ears and eyes that are hidden within the fur. The muskrat has small fore feet on short legs which are used for grasping food, burrowing, and lodge building. The hind feet are partially webbed and aid in swimming. The tail is scaly, black, and flattened vertically so that it can be used as a rudder while swimming. The skull contains sixteen teeth of which four are large incisors (2 cm). Two incisors are located on the upper anterior jaw and two on the lower anterior jaw. They are used for cutting plant material. The presence of muskrats can be associated with the presence of conical houses made of partially dried and decaying plant vegetation. These houses are scattered among the cattails of wetlands.
Geographical Distribution and Habitat Requirements: The common muskrat first appeared in North America approximately 1.8 million years ago. Within this time period, it has become distributed throughout suitable habitats in most of North America and all of New Brunswick. It is one of the most widely distributed mammals and in Canada there are 7 subspecies known to exist. Within North America the muskrat is only uncommon in the extreme north, Florida, central Texas, Mexico, and parts of California. The subspecies common to New Brunswick is Ondatra zibethicus zibethicus. The large range that muskrats inhabit is related to the use of aquatic environments, which are common in North America. Locally, within this vast distribution, muskrats require a marshy area in which to live. This is, typically, a fresh or salt water marsh, marshy area of a lake, or a slow-moving body of water. The water depth should be approximately 1-2 m. This depth of water will not fully freeze in winter and is shallow enough that vegetation successfully grows. The common muskrat prefers areas with cattails, pondweeds, and bulrushes. These types of vegetation provide food and building materials for the muskrat. If these types of vegetation are not present than the muskrat requires steep mud and clay banks for burrowing underground homes. Fast moving water or water bodies that change in depth too quickly are unsuitable year-round habitats, but muskrats may occupy them for shorter periods of time. Muskrats also require travel corridors of open water which makes dense marshes unsuitable for a habitat. In prime habitats, muskrats attain a density of 25/acre. In less suitable habitats (such as shallower marshes), muskrats attain a density of 17.5/acre.
General Biology: The muskrat derives it’s name because of the two special musk glands situated under the skin in the region of the anus. These glands enlarge during the breeding season and produce a yellowish, musky-smelling substance that is thought to be a means of communication during the breeding season. The common muskrat, being a generalist, feeds mainly on aquatic vegetation, but will occasionally feed on small animals. Cattails, duckweed, pickerel weed, bulrushes, and horsetails comprise the majority of the vegetarian diet. Small fish, clams, frogs, crayfish and snails comprise the carnivorous portion of the diet. Muskrats prefer cattails and are usually only carnivorous when vegetation is scarce or unavailable. The muskrat is a crepuscular organism and therefore is most active at dawn, dusk, and night. In winter muskrats remain active and feed outside of their dens in nearby push-ups (weed and stick houses above holes in ice). They usually emerge from their bank burrows, into the water under the ice, and go on long foraging trips (up to 90 m with an average of 7-30 m) in search of food. When food is located it is brought back to the push-up, which is closer than the den, and consumed. The push-up is also used as a resting place. The push-up resembles the true den of the muskrat. Habitat dictates the type of home that is built. When marshes and marsh vegetation is present the den is built in a conical form in marshy shallows. It is usually built of cattails, reeds, and mud. The den closely resembles the push-up. The muskrat builds the lodge by heaping plant material and mud together to form a mound in the shallow water. A burrow is then dug up into the mound from below the water level and the mound is hollowed out, forming a chamber on the water. The lodge is usually around one meter high and one meter wide. The chamber also contains exit holes into the water. When aquatic vegetation is not present, muskrats build burrows, with underwater entrances, into watershed banks.
The muskrat is normally a solitary dweller, but it may form pairs during the breeding season. Mating usually occurs between April and August in the north and year-round in the south. Males compete fiercely for females. Mating usually occurs two or three times a season. The gestation period is from 22 to 30 days and the litter size is between 1 and 11 (larger litter sizes being in the south). The young are weaned for approximately a month and reach sexual maturity within a year. After weaning the young move overland to locate new territory. The average life span of the muskrat is about four years. Predators of muskrat include mink (Mustella vison), foxes (Vulpes vulpes), coyotes (C. latrans), raccoons (Procyon lotor), snapping turtles (Chelydra serpentina), great horned owls (Bubo virginianus), and humans. The muskrat possesses special features suited to its mainly aquatic environment. These characteristics include the ability to stay underwater for up to fifteen minutes (through specially adapted eyes, nose, and respiratory system), waterproof fur, rudder-like tail, and front teeth that are capable of allowing the muskrat to chew underwater without drowning. This ability is achieved through the placement of the fleshy and furry lips behind the incisors so that the mouth opening is actually closed while chewing.
Conservation: The common muskrat, as it’s name suggests, is quite common. In Canada the species has never been endangered and is presently considered stable and abundant. The muskrat population shows a large fluctuation in numbers every 7-10 years, but this appears to be a natural process that is probably related to the abundance of food resources. Human activities have not affected the population of muskrats. In some cases, the draining of land for agricultural or other purposes has exterminated local populations. The increased amount of irrigation ditches along waterways has counteracted this affect by producing new habitats. The muskrat is not listed as threatened by the IUCN. However, muskrat is trapped for food, fur and musk. The muskrat is one of North America’s most valuable fur animals. The fur is predominately used in women’s coats and the musk in perfume. From 1982-1983 approximately seven million muskrats were trapped and their fur yielded a profit of 28 million dollars. Surprisingly, the muskrat is very successful and therefore heavy trapping pressure, drainage of wetlands, and unprecedented industrial activity have not affected the population. Actually, muskrat population numbers today are almost as high as they would have been a thousand years ago.