Muskrats

Figure 1: Photo Credit - John White

Overview

Muskrats (Ondatra zibethicus, Fig. 1) get their common name from their resemblance to stocky rats and from the musky odor that they produce with their scent glands.

Muskrats weigh two to 4 four and reach lengths of 18 to 25 inches, including the eight- to 11-inch, sparsely haired tails. The coat color is generally dark brown, but individuals can range from black to almost white. Muskrats have partially webbed hind feet that function as paddles and much smaller front feet used primarily for digging.

Muskrats are found throughout still or slow-moving waterways, including marshes, beaver ponds, reservoirs, and marshy borders of lakes and rivers.

Muskrats make a valuable contribution to aquatic communities. By harvesting plants for food and den sites, they create open water for ducks, geese, shorebirds and other wildlife. In addition, a variety of animals – including snakes, turtles, frogs, ducks and geese – use muskrat lodges and platforms to rest and nest. Muskrats are, however, considered pests when their burrowing activity damages dams and dikes, and when their feeding activity damages new plantings and crops.

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. (Fig. 1)

Facts About Maine's Muskrats

Food and Feeding Behavior

  • Muskrats eat a wide variety of plants, including cattails, sedges, bulrush, arrowhead, water lilies, pondweed and ferns. They also eat alfalfa, clover, corn and other crops that happen to be in their territory.
  • Although muskrats will eat shellfish, snails, fish, frogs and salamanders, these animals are a small part of the diet and are generally consumed when plant foods are scarce.
  • Muskrats normally feed within 150 feet of their main dwelling; however, they will travel much farther in search of food.
  • Muskrat habitat in Maine is likely limited by the social structure of muskrats.

Figure 2

Figure 3

Den Sites

  • Depending on site conditions, muskrat dens are located in banks or lodges.
  • In dams, dikes and banks, muskrats tunnel upward from below the water surface into the soil to make dens that remain dry. (Fig. 2)
  • Bank dens range from a short tunnel leading to an enlarged nest chamber, to a long and complex system of chambers, air ducts and entrances.
  • In marshes and other areas lacking steep banks, muskrats build dome-shaped lodges from leaves, stems, roots and mud.
  • Lodges are constructed in open water that is two to four feet deep, and are built high enough to keep the den above high-water levels.

In marshes and other areas lacking steep banks, muskrats build dome-shaped lodges from leaves, stems, roots and mud. (Fig. 2)

Reproduction and Family Structure

  • Muskrats are prolific breeders and under favorable conditions may raise 15 young per season. (Fig. 3)
  • The first litter is born in April/May; a second litter may follow in June/July.
  • An average of five to eight kits are born after a 30-day gestation period. Kits are dependent on their mother for approximately 30 days, after which time they can swim, dive and eat green vegetation.
  • At about six weeks of age, kits leave the den or live in a separate chamber.
  • Adult females are thought to overwinter with surviving offspring from the last litter and one or more adult males. In spring, the young seek out their own territories, generally within 300 feet of the maternal female's home range.

Mortality and Longevity

  • Muskrats have many predators, including mink (a major predator), otters, bobcats, house cats, domestic dogs, coyotes, foxes, bald eagles, large hawks and owls, and even largemouth bass.
  • Muskrats are fierce fighters and combat between males is common when densities are high and food supplies are low.
  • Spring flooding can drown early litters and inundate burrows and lodges, exposing muskrats to predators.
  • Historically, muskrats have been one of the most commonly trapped animals in Maine.
  • Muskrats have been reported to live more than four years in the wild; most muskrats, however, do not live more than one year.

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Viewing Muskrats

Muskrats are active throughout the year. They are most active at twilight and throughout the night, although they will feed during the day when food is scarce or bask in the sun when temperatures are low.

Muskrats are usually seen swimming and they are rarely found very far from water. They usually swim with their narrow, pointed tails snaking behind them or arched out of the water.

When startled, muskrats enter the water with a loud splash, and, being strong swimmers, they may swim long distances underwater before surfacing. They can remain motionless under sparse vegetation, with only their noses and eyes above water, for 20 minutes.

When cornered or captured, muskrats are aggressive biters and scratchers and can seriously injure pets and humans.

Living Areas

In marshes, ponds and swamps, prominent muskrat lodges are sure indicators of a present muskrat population.

Look for entrances into their bank dens along dams, dikes, and stream banks. Entry holes are particularly evident where muskrats are living in tidewater areas near the mouths of rivers. When the tide recedes, the entrances are exposed until the tide comes back in.

Similarly, in dry years the water in ponds and reservoirs can drop and expose den entrances. Muskrats will then usually dig new dens farther out in the pond.

Entry holes are five to eight inches in diameter and are located three to 36 inches below the surface of the water.

Feeding Areas

Evidence of muskrat feeding includes plants gnawed to a stubble, floating cattail roots or other vegetation that has been clipped, and piles of clipped vegetation under overhanging vegetation or in a well-concealed spot at the water's edge.

Muskrats sometimes use feeding huts or eating platforms that they create from mud and compacted vegetation. Feeding huts, which are hollow inside, look like small lodges about one foot above the water level. Feeding platforms look like small piles of cut vegetation.

Figure 4: Drawing Credit - WDFW

Tracks

Muskrat tracks can be found in mud or sand along shorelines (Fig. 4). Sometimes a dragged tail mark is visible.

Muskrat prints are small and hand-like. The animals have long toes. The rear print is two to three inches long and may look like a smaller version of a raccoon track. The front print may appear four-toed, as the inner toe is extremely small and barely shows in the track. (Fig. 4)

Droppings

Muskrat droppings can be found floating in the water, along shorelines, on objects protruding from the water, and at feeding sites. The animals may repeatedly use these spots, and more than one muskrat may use the same spot. Droppings are dark green, brown, or almost black. They are slightly curved, cylindrical, and about half inch long and three eighths of an inch in diameter.

Slides

Muskrats, like other water animals, make slides where they enter and leave the water. These narrow muddy trails are about the width of a hand. Sometimes the animals slide down them on their bellies, giving them a slicked down appearance.

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Preventing Conflicts

Although muskrats are important contributors to natural aquatic systems, their burrowing may threaten the safety of dams, dikes, and other human-created embankments. Muskrats may also undermine retaining walls that shore up homes, bridges and other structures. Muskrats occasionally eat new wetland plantings and agricultural crops growing in their territories.

Their numbers may increase to the point where an area is denuded of aquatic plants. After foraging on entire plants, including the roots, they leave the area pitted with digging sites and deep swimming canals. This feeding behavior can destroy existing root mats that bind and secure a wetland, and the area can be quickly eroded by wind and wave action.

The following suggestions will help to reduce conflicts. You can do the work yourself or hire a company to do all or part of the work yourself.

Figure 5: Drawing Credit - Jenifer Rees

Water-level management:

Muskrats (and occasionally voles and Old World rats) dig into dams, dikes and other embankments to make dens. (Fig. 5) Typically these dens have two feet or more of earth above them. When fluctuating water levels flood their initial den, however, muskrats burrow farther into the bank or dig new den chambers closer to the surface, an action that can weaken the bank. Or, livestock and other large animals can make holes in the bank, starting the erosion process.

In dams, dikes and banks, muskrats tunnel upward from below the water surface into the soil to make dens that remain dry. When fluctuating water levels flood their initial den, they burrow farther into the bank or dig new, higher den chambers. (Fig. 5)

To prevent muskrats from tunneling higher in an embankment or dam, minimize changes in the water level. You may need to monitor the spillway frequently to ensure an unobstructed flow and/or widen the spillway to carry off surplus water so that it never rises more than six inches.

You can also manipulate the water level to make muskrats move to other suitable habitat. Raising the water level in the winter so that it is close to flood level – and keeping it there – will force the animals out of their dens. Similarly, dropping water levels during the summer will expose muskrat dens to predators, forcing them to seek a more secure area.

Slope management:

Muskrats prefer to burrow on steep slopes covered with vegetation. Hence, they can be discouraged by keeping side slopes at or below a 3:1 ratio, and by controlling vegetation growth. Managing vegetation by hand can be difficult in large areas, but routine mowing or cutting with a weed whacker can be effective. Only herbicides registered for use next to water should be used, and then only per the manufacture's recommendations. If possible, keep livestock off embankments so that they will not put a hoof through a den chamber. If a roof is pierced, immediately fill in the cavity with soil, rocks or a mudpack (see below).

Embankment barriers:

You can prevent muskrats from burrowing into an earth embankment by installing a barrier one foot above to three feet below normal water level. Use one-inch mesh hardware cloth (aluminum and stainless steel are also available), or heavy-duty plastic or fiberglass netting. Place the barrier flat against the bank and anchor it every few feet along all edges. To extend the life of galvanized hardware cloth, spray it with automobile undercoat paint or other rustproof paint before installing it. As the wire will eventually corrode, do not use this material where people are likely to swim.

Riprapping areas with stone creates an effective barrier and protects slopes from wave action. Stone should be at least six inches thick. Do not use rocks larger than six inches in diameter because when piled they tend to form cavities, providing hiding places for muskrats and Old World rats.

In situations where muskrats are burrowing into existing rock walls, place gravel or concrete between the rocks to block up the holes. A permit from DEP may be needed when working the shoreland zone.

Where a burrowing problem is extreme, use a gas-powered trenching machine (available at rental stores) to dig a narrow trench along the length of the embankment. Hand digging will be required to dig to the recommended depth – three feet below the high-water level. Next, fill the trench with a mudpack. A mudpack is made by adding water to a 90 percent earth and 10 percent cement mixture until it becomes a thick slurry. The resulting solid core will prevent muskrats from digging through the embankment. Work of this type will definitely require a permit from DEP.

Floating dock barriers:

Muskrats will burrow into floating docks (generally those floating on Styrofoam) and scattering the broken pieces along the shoreline. These pieces are dangerous to birds and small mammals that eat them. To remedy the situation, pull the dock on shore and cover the Styrofoam with one-inch mesh hardware cloth (aluminum and stainless steel versions are available).

Figure 6: Drawing Credit - Jenifer Rees

Figure 7: Drawing Credit - Jenifer Rees

Fences and other barriers:

Muskrats are not climbers. A properly designed and maintained two-foot tall wire fence will prevent muskrats from entering an area. The fence must be taller if snow or other materials are likely to build up near it.

Because muskrats are diggers, the fence must extend at least a foot below ground. Alternately, a tight fit to the ground and an L extension that runs two feet out on the soil surface toward the muskrat will also prevent entering the animal from digging underneath. (Fig. 6)

Welded-wire cylinders around individual plants are often used where only a few plants need to be protected.

Note: Lightweight plastic seedling protectors do not work because muskrats can chew through them.

A floppy fence can be constructed as a barrier between an active muskrat colony and a large area needing protection. (Fig. 7) To prevent muskrats from walking around the fence, connect each end to an existing, impenetrable solid fence or structure.

Muskrats are diggers and a fence will need to extend at least 12 inches below ground. Alternately, a tight fit to the ground and an L chicken wire or hardware cloth extension that runs 24 inches out on the soil surface toward the animal will also prove effective. (Fig. 6)

A mini floppy fence constructed of one-inch mesh wire or heavy plastic needs to be at least two feet high and staked so that it's wobbly to prevent an athletic muskrat from making the climb. Constructing the fence so that it leans slightly toward the muskrat will increase its effectiveness. (Fig. 7)

Harassment and repellents:

Muskrats are wary animals and will try to escape when threatened. When new burrows are discovered early on, the entry holes can be stuffed with rocks, balled-up window screen, and/or rags sprinkled with predator urine (mink, coyote, or bobcat – available from trapper supply outlets and over the Internet) or ammonia. Some people have had success applying used cat litter in this way. Exposing their tunnels from above may also work. The success of this type of control depends on continued effort and thus is often short-lived.

Large dogs that are awake during the night can be effective at keeping muskrats out of areas.

Commercially available taste repellents may be effective at preventing damage to crops and other plants.

Crop location:

Unfenced crops and gardens located close to water will be more attractive to muskrats than those further from water. If you have a choice regarding the location of your garden, consider the potential for muskrat damage. Natural vegetation buffers next to water bodies can provide feeding areas and reduce the attractiveness of vegetation farther from the water.

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Lethal Control

The exclusion methods described above are often the best long-term solution. Lethal control can be effective in areas where the local population of muskrats is still small and there are no replacement animals from upstream or downstream. If there are many muskrats, then lethal control becomes an ongoing program rather than a one-time action. The moment surveillance stops, the effort fails.

Lethal trapping has traditionally been the primary form of control. See "Legal Status" for information on trapping muskrats.

Note: State wildlife offices do not provide trapping services, but they can provide names of individuals or companies that do.

No fumigants are currently registered for muskrat control.

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Public Health Concerns

Tularaemia

Muskrats can be infected with the bacterial disease tularemia, which is transmitted by ticks and biting flies as well as contaminated water. Animals with this disease may be sluggish, appear tame, or be unable to move quickly when disturbed. Tularemia is fatal to animals.

Tularemia may be transmitted to humans who drink contaminated water; eat undercooked, infected meat; are bitten by an infected tick or biting fly; inhale dust from contaminated soil; or allow an open cut to contact an infected animal. The most common route of infection occurs when someone who is skinning or gutting an infected animal is cut or nicked by a knife.

A human who contracts tularemia commonly has a high temperature, headache, body ache, nausea, and sweats. A mild case may be confused with the flu and ignored. Humans can be easily treated with antibiotics.

Giardia

Because muskrats defecate in water, they can be one of several vectors (transmitters) of a flu-like infection called giardiasis, more commonly referred to as giardia, derived from giardia, the single-cell protozoan that causes the disease. Giardia has been found in many animal species, including pets, other wildlife, and livestock. It is one of the most common water-born pathogens in fresh water. It's main source is fecal material from birds and animals, as well as humans. Never drink untreated water from any source in the wild.

Individuals handling a dead or live muskrat should wear rubber gloves and wash their hands well when finished.

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Legal Status

Muskrats are classified as furbearers. A trapping license is required; they may only be trapped during seasons set by the state. Because legal status, trapping restrictions and other information about muskrats can change, contact your Inland Fisheries and Wildlife Regional Office for updates.

If a muskrat is causing damage or is a nuisance, consult Maine's laws on this subject: http://www.mainelegislature.org/legis/statutes/12/title12ch921sec0.html

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Additional Information

Books

New England Wildlife, Habitat, Natural History, and Distribution
Written by: Richard DeGraff, and Mariko Yamasaki
University Press of New England, 2001.
(Available from: www.upne.com)

Prevention and Control of Wildlife Damage
Written by: Scott E. Hygnstrom, et al.
University of Nebraska-Lincoln, Institute of Agriculture and Natural Resources, 1994.
(Available from: University of Nebraska Cooperative Extension, 202 Natural Resources Hall, Lincoln, NE 68583-0819; phone: 402-472-2188; also see Internet Sites below.)

Landscaping for Wildlife in the Pacific Northwest
Written by: Russell Link
University of Washington Press and the Washington Department of Fish and Wildlife, 1999.

Internet Resources

U.S. Forest Service Wildlife Species Life Form Information

Adapted from: "Living with Wildlife in the Pacific Northwest"
(see Washington Department of Fish and Wildlife)

Written by: Russell Link, Wildlife Biologist, Email Russell Link, with assistance from WDFW Biologists Rich Beausoleil and Rocky Spencer

Design and layout: Peggy Ushakoff, ITT2

Illustrations: As credited

Copyright 2005 by the Washington Department of Fish and Wildlife


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