Muskrats

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MUSKRATS

James E. Miller

Program Leader, Fish and Wildlife

USDA Extension Service

Natural Resources and Rural

Development Unit

Washington, DC 20250

Fig. 1. Muskrat, Ondatra zibethicus

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.

Other Methods

Integrated pest management.

Identification

The muskrat (Ondatra zibethicus, Fig. 1)

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 (Fig. 2), 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 some-

times 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 is

PREVENTION AND CONTROL OF WILDLIFE DAMAGE — 1994

Coop erat ive 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

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from 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 through-

out 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 rud-

der 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 feed-

ing, the muskrat often swims back-

ward to move to a more choice spot

and can stay underwater for as long as

20 minutes. Muskrat activity is pre-

dominantly nocturnal and crespuscu-

lar, 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

habitats throughout the United States

and Canada in streams, ponds, wet-

lands, 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 shal-

low water areas with p lentiful vegeta-

tion, they use plant materials to

construct hou ses, generally conical in

shape (Fig. 4). Elsewhere, they prefer

bank dens, and in many habitats, they

construct both bank dens and houses

of vegetation. Both the houses of veg-

etation and the bank burrows or dens

have several underwater entrances via

“runs” or trails. Muskrats often have

feeding houses, platforms, and cham-

bers 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 styro-

foam 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 c an

cause “eat-out” where aquatic

Tail mark sometimes shows

Walking

Front foot

Hind foo t

habitat and with little competition,

muskrats are very prolific. With a ges-

tation period of between 25 and 30

days, females in the southern part of

the range commonly produce 5 to 6

litters p er 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 M exico, and from the

Aleutians east to Labrador and down

the Atlantic coast into Georgia (Fig. 3).

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

problem s with ponds, levees, and crop

culture, whether introduced or native.

Muskrats are found in most aquatic

Fig. 2. Muskrat tracks

Fig. 3. Range of the muskrat in North Americ a.

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vegetation in large areas is virtually

eliminated by muskrats. In some loca-

tions, such as in the rice-growing areas

of Arkansas, muskrats move from

overwintering habitat in canals, drain-

age 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 bur-

rowing.

Food Habits

Muskrats are primarily herbivores.

They will eat almost any aquatic vege-

tation 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 aqu atics.

Crop s that are occasionally dam aged

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, johnson-

grass, and orchard grass where

planted or growing on or around farm

pond dams.

Although primarily herbivores, musk-

rats will also feed on crayfish, mussels,

turtles, frogs, and fish in ponds where

vegetation is scarce. In some aquacul-

ture industry areas, this feeding habit

should be studied, as it may differ sig-

nificantly from normal feeding activity

and can cause economic loss.

duced from October until April. Some

are produced in the summer and early

fall months, but not as many as in win-

ter months. The period of highest pro-

ductivity 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 be-

tween March and September.

Young muskrats are especially vulner-

able to predation by owls, hawks, rac-

coons, 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 de-

pended upon to solve damage p rob-

lems 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 dis-

eases, leptospirosis, ringworm disease,

and pseudotuberculosis. Most com-

mon ectoparasites are mites and ticks.

Endoparasites are predominantly

trematodes, nematodes, and cestodes.

Damage and Damage

Identification

Damage caused by muskrats is prima-

rily 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 shore-

lines 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 pos-

sible washout or, if livestock are using

Cross section of

muskrat house

showing nest

cavity and tunnel

leading to water.

General Biology,

Reproduction, and

Behavior

Muskrats generally have a small home

range but are rather territorial, and

during breeding seasons some dispers-

als are common. The apparent intent

of those leaving their range is to estab-

lish 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 popula-

tion 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 territo-

ries. 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. Lit-

ters 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 pro-

duction of young. Young may be pro-

duced any month of the year. In

Arkansas, the peak breeding periods

are during November and March.

Most of the young, however, are pro-

Fig. 4. Muskrat house

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rip-rapping of the dam. Serious dam-

age often can be prevented, if antici-

pated, by constructing dams to the

following specifications: the inside face

of the dam should be built at a 3 to 1

slope; the outer face of the dam at a 2

to 1 slope with a top width of not less

than 8 feet (2.4 m), preferably 10 to 12

feet (3 to 3.6 m). The normal water

level in the pond should be at least 3

feet (91 cm) below the top of the dam

and the spillway should be wide

enough that heavy rainfalls will not

increase the level of the water for any

length of time (Fig. 5). These specifica-

tions are often referred to as over-

building, but they will generally

prevent serious damage from burrow-

ing muskrats. Other methods of exclu-

sion can include the use of fencing in

certain situations where m uskrats may

be leaving a pond or lake to cut valu-

able garden plants or crops.

Cultural Methods and Habitat

Modification

The best ways to modify habitat are to

eliminate aquatic or other suitable

foods eaten by muskrats, and where

possible, to construct farm pond dams

to previously suggested specifications.

If farm pond dams or levees are being

damaged, one of the ways that dam-

age can be reduced is to draw the

pond down at least 2 feet (61 cm) be-

low normal levels during the winter.

Then fill dens, burrows, and runs and

rip-rap the dam with stone. Once the

water is drawn down, trap or other-

wise remove all muskrats.

Frightening Devices

Gunfire will frighten muskrats, espe-

cially those that get hit, but it is not

effective in scaring the animals away

from occupied habitat. No conven-

tional frightening devices are effective.

Repellents

No repellents currently are registered

for muskrats, and none are known to

be effective, practical, and environ-

mentally safe.

Toxicants

The only toxicant federally registered

for muskrat control is zinc phosphide

at 63% concentrate. It is a Restricted

Use Pesticide for making baits. Zinc

phosphide baits for muskrats generally

are made by applying a vegetable oil

sticker to cubes of apples, sweet pota-

toes, or carrots; sprinkling on the toxi-

cant; and mixing thoroughly. The bait

is then placed on floating platforms

(Fig. 6), in burrow entrances, or on

feeding houses. Use caution when

mixing and applying baits treated with

zinc p hosphide. Carefully follow

instructions on the zinc phosphide

container before using.

Some states have obtained state regis-

trations for use of anticoagulant baits

Sod cover

Normal water level

(does not rise more than 6")

20' minimum

23' preferred

Flooded den

Less than 3'

Less than

20'

High water level

Normal water level

More than 6"

Steep bank encourages burrowing

Proper Construction of Embankments

Improper Construction of Embankments

New den

the pond, to prevent injury to a foot or

leg.

Where damage is occurring to a c rop,

plant cutting is generally evident. In

aquaculture reservoirs generally main-

tained without lush aquatic vegetation,

muskrat runs and burrows or remains

of mussels, crayfish, or fish along with

other muskrat signs (tracks or drop-

pings) are generally easy to observe.

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 be-

fore attempting to remove mu skrats.

Damage Prevention and

Control Methods

Exclusion

Muskrats in some situations can be

excluded or prevented from digging

into farm pond dams through stone

Fig. 5. Proper dam construction can reduce muskrat damage to the structure.

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1/4" holes for bolts

Top

Fir edge

Drain

Concrete block

Side

3" thick

Pulley system

Co ncrete block

Rafts can be anchored in three ways.

Tie-down

1" thick board

1" hole

Top

1/2 x 1 1/2"

styrofoam

roofing nails

Bottom

Bamboo pole for anchor

Finishing nails

Side

Fig. 6. A bait platform for controlling muskrats.