About
The muskrat (Ondatra zibethicus) is a member of the rodent family. It gets its common name from the musky odour produced by the two scent glands in the anal region of the male.

Distribution
The most common and widely distributed furbearer on this continent, the muskrat has contributed more than any other animal to the combined income of North America’s trappers. The muskrat’s range extends from the Arctic Ocean to the Gulf of Mexico, and from the Atlantic to the Pacific Oceans. Muskrat have also been common in Europe and Asia since they were introduced in 1905. Although distributed throughout Ontario, the greatest numbers are found in the southern part of the province.

Description
The muskrat resembles a large rat that has adapted to aquatic conditions. It has several specialized features fitting it for its life in and around the water. A vertically flattened tail works like a scull to propel the rat in the water, and valves resembling flaps of skin close off the nostrils, ears and mouth for underwater feeding and travelling. Although not webbed like those of the beaver or otter, each hind foot has a fringe of special hairs which makes swimming easier. Because it can conserve oxygen by restricting blood flow to essential organs, the muskrat can remain submerged for up to 17 minutes. The muskrat’s tail is scaled for protection and, like its feet, is comparatively hairless. This feature helps to regulate body temperature. The forefeet, which are smaller than the hind feet, have large claws used for digging and manipulating food. The four incisor teeth, two on each jaw, are about 2 cm (.75 in) long and are used for snipping off vegetation. Like those of the beaver, they grow continuously and are worn down by chewing. A rotund creature, the muskrat has a pointed head with small ears hidden beneath the fur. The average adult weighs 1 kg (2.2 lbs) and measures about 50 cm (20 in) from nose to tail. The pelt consists of long, coarse, outer guard hairs and dense, silky underfur. Pelt colour ranges from dark brown on the head and back to light grey or tan on the belly. Some muskrat may be fawn, yellow or silver, and a few albinos are found each year.

Life History
Muskrats usually reach sexual maturity by the age of one year. Under favourable conditions, they may mature by the age of six or seven months. Breeding begins as soon as the ice breaks up and, in Ontario’s climate, can continue until July or August. The gestation period is 30 days, and the female can produce as many as four litters each year. Most females produce two or three litters annually. Although a litter may contain up to 11 kits, the average number is six. One female can produce 18 to 20 kits each year. The kits are born in dwelling houses, within nest chambers lined with more finely shredded material than is found in the resting chambers. Houses containing litters can be recognized by the fresh material added to them in the spring. Trappers who spot this evidence of reproduction during the latter part of the spring season should curtail their trapping to protect the breeding stock. Kits are born blind and naked. Their eyes open after 12 to 20 days and, within two or three weeks, the young are able to leave the nest. They are weaned at one month and are completely independent by the age of six weeks. The young remain in the home area until the spring following birth, when they disperse to find their own territories.

Habitat
Good habitat conditions for muskrat consist of stands of cattail and bulrushes bordering on open water in, which there is enough submerged vegetation to supply food, and building material. Water levels should be fairly stable at minimum depths of 30 to 61 cm (1 to 2 ft) so that winter freeze-out does not occur. These habitat conditions are found in freshwater marshes and along the marshy borders of lakes. Muskrat also inhabit slow-moving streams and drainage ditches, which have good food, plants and banks soft enough to permit burrowing. Muskrat construct two types of house: the dwelling house and the feeding house. These are similar in shape to beaver houses but are not made of woody material. Instead, dead or green cattail, bulrushes and submerged vegetation are piled up on an object such as a log, stump or root mass until the desired height is reached. Chambers are then hollowed out above the water line. Dwelling houses have several chambers and are 61 to 122 cm (2 to 4 ft) high and 61 and 91 cm (2 to 3 ft) in diameter. Feeding houses are smaller and contain only one chamber. Houses are usually constructed during the fall. In winter, a small, house-like structure called a “push-up” is made along cracks or small openings in the ice. Constructed of muck and bottom vegetation, which is mounded up on the ice and hollowed out to form a chamber, push-ups are used for feeding and to supply the muskrat with air. A fourth type of muskrat structure, the burrow, is found along streams and dike banks where the soil is suitable for digging. Burrows may be as long as 46 m (150 ft), and may have several smaller tunnels branching off from the main one. Entered from below the water line, the tunnels lead to one or more chambers – located above the water line. These are lined with dry vegetation. In areas where muskrat houses exist, burrows may be used to supplement the living space and as feeding areas. They can also serve as permanent living areas, particularly along rivers where there is not sufficient material available for house construction.

Food and Feeding Behaviour
Muskrats prefer the inner parts of the stalks and roots of cattail. They will also eat bulrushes, submerged pond weeds, sedges, crayfish, frogs and even the carcasses of other muskrats. In winter, they will travel considerable distances under the ice to forage. When feeding under the ice, muskrats chew off plants and take them either to push-ups or to houses to be eaten. In open, still water, they choose a favourite spot and take their food there repeatedly. Over time, plant debris builds up in this spot and forms what is known as a “feeding platform”. These platforms make good trap sites.

Habits
Muskrats can swim at speeds of from 1.6 to 4.8 km/h (1 to 3 mph). Except during the winter when they unite to form groups, they live alone or in mated pairs. Muskrats are at their most active during the spring breeding run. At this time, they will climb onto any floating object. This should be kept in mind when setting traps. Muskrats remain active throughout the year. Although they prefer the evening and night-time hours, they can also be seen swimming, building houses and sunning themselves on logs or houses during the day. Muskrats are particularly wary and aggressive during the spring run. They will attack predators and people when cornered, and can inflict fatal wounds on other muskrats. Females with litters are especially territorial. The two musk glands beneath the skin in the anal region produce scent, which is used to mark territories and to communicate messages to other muskrat. During the breeding season, the glands enlarge and produce a yellowish substance. Home ranges in marshy areas surround the dwelling houses. On streams, home ranges extend up and down the stream from the burrow. Muskrats maintain regular travel routes, which can be observed both on land and in aquatic vegetation. These routes are good trap sites.

Population Changes
Muskrat populations appear to fluctuate in size regularly every seven to ten years. Eighty-seven per cent of muskrats under the age of one-year die of natural causes. Eleven per cent live to the age of two years, and about two per cent live to be three years old or more. Among the natural enemies of the muskrat are the mink, fox, coyote, raccoon, bobcat, owl and hawk. Muskrats also kill each other. Tularemia and infectious hemorrhagic disease frequently cause death in under-trapped and over-populated areas. White spots on the liver and spleen signal tularemia. Infectious hemorrhagic disease is indicated by the presence of blood in the intestines or lungs, or in both. Muskrat are also plagued by fleas, tapeworms, roundworms and flukes, but these seldom cause death.

Management
There are two basic methods of muskrat management. They are (1) improving the habitat, and (2) regulating muskrat numbers, principally through trapping. Habitat can be improved by stabilizing water levels and by promoting the growth of shoreline vegetation along streams and ditches. Wetlands can be created by building small dams or dikes to hold backwater over the summer. If the water conditions are suitable, the plant species muskrats require should fill in naturally. Permission from the Ministry of Natural Resources must be obtained before constructing dams or dikes. Trapping is the best means of controlling muskrat numbers. It also prevents the waste of the resource that occurs when populations become too large and epidemics of disease break out. Fall trapping removes those muskrats that would be most likely to die over the course of the winter, particularly in areas where the water is shallow and freeze-out is expected. Spring trapping must stop when evidence of shedding and cuts on the hide are observed. Trapping seasons vary across Ontario. Trappers should consult the Summary of Fur Management Regulations for the trapping dates in their areas. Trappers can assist in the management of muskrat by monitoring the status of the local population. The status is indicated by the ratio of young muskrat to adult females in the harvest. If there are many young for each female, trapping procedures are judged to be good and the population, healthy. If there are few young per female, problems involving such things as trapping pressure, disease or habitat conditions are indicated. Trappers should attempt to establish the cause of the problem and should work with the Ministry in trying to improve the situation.