MUSKRAT

The Muskrat is a large aquatic rodent native to North America, and introduced in parts of Europe. Adult body length is usually between 25-40 cm long, with a strong, laterally compressed tail 20-25 cm long. The body is covered in thick, brown waterproof fur; the underparts are paler. They have partially webbed hind feet and small able hand-like front feet. The weight is up to 1700 g, about four times the weight of a Brown Rat.

Identification
The muskrat (Ondatra zibethicus) is the largest microtine rodent in the United States. It spends its life in aquatic habitats and is well adapted for swimming. Its large hind feet are partially webbed, stiff hairs align the toes, and its laterally flattened tail is almost as long as its body. The muskrat has a stocky appearance, with small eyes and very short, rounded ears. Its front feet, which are much smaller than its hind feet, are adapted primarily for digging and feeding.

The overall length of adult muskrats is usually from 18 to 24 inches (46 to 61 cm). Large males, however, will sometimes be more than 30 inches (76 cm) long, 10 to 12 inches (25 to 31 cm) of which is the laterally flattened tail. The average weight of adult muskrats isfrom 1 1/2 pounds (0.7 kg) to over 4 pounds (1.8 kg), with most at about 2 1/2 pounds (1.1 kg). The color of the belly fur is generally light gray to silver to tan, and the remaining fur varies from dark tan to reddish brown, dark brown, and black.

The name muskrat, common throughout the animal’s range, derives from the paired perineal musk glands found beneath the skin at the ventral base of the tail in both sexes. These musk glands are used during the breeding season. Musk is secreted on logs or other defecation areas, around houses, bank dens, and trails on the bank to mark the area.

The muskrat has an upper and a lower pair of large, unrooted incisor teeth that are continually sharpened against each other and are well designed for gnawing and cutting vegetation. It has a valvular mouth, which allows the lips to close behind the incisors and enables the muskrat to gnaw while submerged. With its tail used as a rudder and its partially webbed hind feet propelling it in the water, the muskrat can swim up to slightly faster than 3 miles per hour (4.8 kph). When feeding, the muskrat often swims backward to move to a more choice spot and can stay underwater for as long as 20 minutes. Muskrat activity is predominantly nocturnal and crespuscular, but occasional activity may be observed during the day.

Muskrats in the wild have been known to live as long as 4 years, although most do not reach this age. In good habitat and with little competition, muskrats are very prolific. With a gestation period of between 25 and 30 days, females in the southern part of the range commonly produce 5 to 6 litters per year.

Range
The range of the muskrat extends from near the Arctic Circle in the Yukon and the Northwest Territories, down to the Gulf of Mexico, and from the Aleutians east to Labrador and down the Atlantic coast into Georgia. The muskrat has been introduced practically all over the world, and, like most exotics, has sometimes caused severe damage as well as ecological problems. Muskrats often cause problems with ponds, levees, and crop culture, whether introduced or native. Muskrats are found in most aquatic habitats throughout the United States and Canada in streams, ponds, wetlands, swamps, drainage ditches, and lakes.

Habitat
Muskrats can live almost any place where water and food are available year-round. This includes streams, ponds, lakes, marshes, canals, roadside ditches, swamps, beaver ponds, mine pits, and other wetland areas. In shallow water areas with plentiful vegetation, they use plant materials to construct houses, generally conical in shape. Elsewhere, they prefer bank dens, and in many habitats, they construct both bank dens and houses of vegetation. Both the houses of vegetation and the bank burrows or dens have several underwater entrances via “runs” or trails. Muskrats often have feeding houses, platforms, and chambers that are somewhat smaller than houses used for dens.

Burrowing activity is the source of the greatest damage caused by muskrats in much of the United States. They damage pond dams, floating styrofoam marinas, docks and boathouses, and lake shorelines. In states where rice and aquaculture operations are big business, muskrats can cause extensive economic losses. They damage rice culture by burrowing through or into levees as well as by eating substantial amounts of rice and cutting it down for building houses. In waterfowl marshes, population irruptions can cause “eat-out” where aquatic vegetation in large areas is virtually eliminated by muskrats. In some locations, such as in the rice-growing areas of Arkansas, muskrats move from overwintering habitat in canals, drainage ditches, reservoirs, and streams to make their summer homes nearby in flooded rice fields. In aquaculture reservoirs, damage is primarily to levees or pond banks, caused by burrowing.

Food Habits
Muskrats are primarily herbivores. They will eat almost any aquatic vegetation as well as some field crops grown adjacent to suitable habitat. Some of the preferred natural foods include cattail, pickerelweed, bulrush, smartweed, duck potato, horsetail, water lily, sedges, young willow regeneration, and other aquatics. Crops that are occasionally damaged include corn, soybeans, wheat, oats, grain sorghum, and sugarcane. Rice grown as a flooded crop is a common muskrat food. It is not uncommon, however, to see muskrats subsisting primarily on upland vegetation such as bermuda grass, clover, johnsongrass, and orchard grass where planted or growing on or around farm pond dams.

Although primarily herbivores, muskrats will also feed on crayfish, mussels, turtles, frogs, and fish in ponds where vegetation is scarce. In some aquaculture industry areas, this feeding habit should be studied, as it may differ significantly from normal feeding activity and can cause economic loss.

General Biology, Reproduction, and Behavior
Muskrats generally have a small home range but are rather territorial, and during breeding seasons some dispersals are common. The apparent intent of those leaving their range is to establish new breeding territories. Dispersal of males, along with young that are just reaching sexual maturity, seems to begin in the spring. Dispersal is also associated with population densities and population cycles. These population cycles vary from 5 years in some parts of North America to 10 years in others. Population levels can be impacted by food availability and accessibility.

Both male and female muskrats become more aggressive during the breeding season to defend their territories. Copulation usually takes place while submerged. The young generally are born between 25 and 30 days later in a house or bank den, where they are cared for chiefly by the female. In the southern states, some females may have as many as 6 litters per year. Litters may contain as many as 15, but generally average between 4 and 8 young. It has been reported that 2 to 3 litters per female per year is average in the Great Plains. This capability affords the potential for a prolific production of young. Young may be produced any month of the year. In Arkansas, the peak breeding periods are during November and March. Most of the young, however, are produced from October until April. Some are produced in the summer and early fall months, but not as many as in winter months. The period of highest productivity reported for the Great Plains is late April through early May. In the northern parts of its range, usually only 2 litters per year are produced between March and September.

Young muskrats are especially vulnerable to predation by owls, hawks, raccoons, mink, foxes, coyotes, and — in the southern states — even largemouth bass and snapping turtles. The young are also occasionally killed by adult muskrats. Adult muskrats may also be subject to predation, but rarely in numbers that would significantly alter populations. Predation cannot be depended upon to solve damage problems caused by muskrats.

Muskrats are hosts to large numbers of endo-and ectoparasites and serve as carriers for a number of diseases, including tularemia, hemorrhagic diseases, leptospirosis, ringworm disease, and pseudotuberculosis. Most common ectoparasites are mites and ticks. Endoparasites are predominantly trematodes, nematodes, and cestodes.

Damage and Damage Identification
Damage caused by muskrats is primarily due to their burrowing activity. Burrowing may not be readily evident until serious damage has occurred. One way to observe early burrowing in farm ponds or reservoirs is to walk along the edge of the dam or shorelines when the water is clear and look for “runs” or trails from just below the normal water surface to as deep as 3 feet (91 cm). If no burrow entrances are observed, look for droppings along the bank or on logs or structures a muskrat can easily climb upon. If the pond can be drawn down from 1 1/2 to 3 feet (46 to 91 cm) each winter, muskrat burrows will be exposed, just as they would during extended drought periods. Any burrows found in the dam should be filled, tamped in, and covered with rock to avoid possible washout or, if livestock are using less than 3' Sod cover. Where damage is occurring to a crop, plant cutting is generally evident. In aquaculture reservoirs generally maintained without lush aquatic vegetation, muskrat runs and burrows or remains of mussels, crayfish, or fish along with other muskrat signs (tracks or droppings) are generally easy to observe.

Economics of Damage and Control
Assessment of the amount of damage being caused and the cost of prevention and control measures should be made before undertaking a control program. Sometimes this can be easily done by the landowner or manager through visual inspection and knowledge of crop value or potential loss and reconstruction or replacement costs. Other situations are more difficult to assess. For example, what is the economic value of frustration and loss of a truckload of minnows and/or fish after a truck has fallen through the levee into burrowed-out muskrat dens? Or how do you evaluate the loss of a farm pond dam or levee and water behind it from an aquaculture operation where hundreds of thousands of pounds of fish are being grown? Rice farmers in the mid-South or in California must often pump extra, costly irrigation water and shovel levees every day because of muskrat damage. The expense of trapping or other control measures may prove cost-effective if damage is anticipated.

Obviously, the assessments are different in each case. The estimate of economic loss and repair costs, for example, for rebuilding levees, replacing drain pipes, and other measures, must be compared to the estimated cost of prevention and/or control efforts.

Economic loss to muskrat damage can be very high in some areas, particularly in rice and aquaculture producing areas. In some states damage may be as much as $1 million per year. Totals in four states (Arkansas, California, Louisiana, and Mississippi) exceed losses throughout the rest of the nation.

Elsewhere, economic losses because of muskrat damage may be rather limited and confined primarily to burrowing in farm pond dams. In such limited cases, the value of the muskrat population may outweigh the cost of the damage.

Muskrat meat has been commonly used for human consumption and in some areas called by names, such as “marsh rabbit.” A valuable resource, it is delicious when properly taken care of in the field and in the kitchen. Many wild game or outdoor cookbooks have one or more recipes devoted to “marsh rabbit.” Care should be taken in cleaning muskrats because of diseases mentioned earlier.

Muskrat pelts processed annually are valued in the millions of dollars, even with low prices; thus the animal is certainly worthy of management consideration. It obviously has other values just by its place in the food chain.

Legal Status
Muskrats nationwide for many years were known as the most valuable furbearing mammal — not in price per pelt, but in total numbers taken. Each state fish and wildlife agency has rules and regulations regarding the taking of muskrats. Where the animal causes significant economic losses, some states allow the landowner to trap and/or use toxic baits throughout the year. Other states prohibit taking muskrats by any means except during the trapping season. Check existing state wildlife regulations annually before attempting to remove muskrats.


Damage Prevention and Control Methods

Exclusion
Riprap the inside of a pond dam face with rock, or slightly overbuild the dam to certain specifications.

Cultural Methods and Habitat Modification
Eliminate aquatic vegetation as a food source. Draw down farm ponds during the winter months.

Frightening
Seldom effective in controlling serious damage problems.

Repellents
None are registered.

Toxicants
Zinc phosphide. Anticoagulants (state registrations only).

Trapping
Body-gripping traps (Conibear® No. 110 and others). Leghold traps, No. 1, 1 1/2, or 2. Where legal, homemade “stove pipe” traps also are effective when properly used.

Shooting
Effective in eliminating some individuals.




ACKNOWLEDGEMENTS
The above information was adapted from PREVENTION AND CONTROL OF WILDLIFE DAMAGE with permission of the editors, Scott E. Hygnstrom, Robert M. Timm, and Gary E. Larson (Cooperative Extension Division, Institute of Agriculture and Natural Resources University of Nebraska-Lincoln, United States Department of Agriculture Animal and Plant Health Inspection Service, Animal Damage Control, Great Plains Agricultural Council Wildlife Committee).