The muskrat Ondatra zibethicus is
a fairly large rodent commonly found in the wetlands and waterways of North
America. It has a rotund, paunchy appearance. The entire body, with the
exception of the tail and feet, is covered with a rich, waterproof layer
of fur. The short underfur is dense and silky, while the longer guard hairs
are coarser and glossy. The colour ranges from dark brown on the head and
back to a light greyish-brown on the belly. A full-grown animal weighs
on the average about 1 kg but this varies considerably in various parts
of North America. The length of the body from the tip of the nose to the
end of the tail is usually about 50 cm. The tail is slender, flattened
vertically and up to about 25 cm long. It is covered with a scaly skin
that protects it from physical damage.
Only a minimal amount of hair grows
on the feet. The hand-like front feet are used in building lodges, holding
food, and digging burrows and channels. Although the larger hind feet are
used in swimming, they are not webbed like those of the beaver and otter.
Instead, the four long toes of each foot have a fringe of specialized hairs
along each side, giving the foot a paddle-like effect. The rather small
ears are usually completely hidden by the long fur. The four chisel-like
front teeth (two upper and two lower incisors), each up to 2 cm long, are
used in cutting stems and roots of plants.
The muskrat’s name is derived from
the fact that the animal has two special musk glands—also called anal glands—situated
beneath the skin in the region of the anus. These glands enlarge during
the breeding season and produce a yellowish, musky-smelling substance that
is deposited at stations along travel routes used by muskrats. Common sites
of deposition are "toilets," bases of lodges, and conspicuous points of
land. The biology of musk glands has not been studied extensively, but
the odour produced is believed to be a means of communication among muskrats,
particularly during the breeding season.
Signs and sounds
In the spring, during the mating season,
sharp whining noises and occasional sounds of fighting may be heard.
Habitat and habits
Muskrats typically live in freshwater
marshes, marshy areas of lakes, and slow-moving streams. The water must
be deep enough so that it will not freeze to the bottom during the winter,
but shallow enough to permit growth of aquatic vegetation—ideally between
1 and 2 m. Areas with good growths of bulrushes, cattails, pondweeds, or
sedges are preferred.
Compact mounds of partially dried
and decayed plant material can frequently be seen scattered among the cattails
and bulrushes. These dead-looking heaps are homes of the muskrat. Bulrushes
and cattails are most important, particularly in lakes. As well as being
eaten, they are used as building material in the construction of lodges
and feeding stations, and as shelter from winds and wave action. In northern
regions, horsetails can be important in muskrat habitat.
If bulrushes or cattails are not available,
muskrats dig burrows in firm banks of mossy soil or clay. Because muskrats
require easy access to deep water, water depths must increase fairly rapidly
from the shore where burrows are situated. This provides muskrats with
an opportunity to escape from predators, and with a food supply under the
ice during the winter.
Some people refer to muskrats as "house
rats" and "bank rats" because the animals build lodges in certain areas
and bank burrows in others. Often, these names are used in a way that suggests
that these two "types" of muskrats possess inherited biological differences.
This is not the case. The type of habitation used is simply a response
to local conditions.
With the shortening of days and the
coming of colder weather in September, preparations for winter begin. The
fall is spent building and reinforcing lodges for winter occupancy, and,
in some regions, storing food for winter use. Lodge building behaviour
is an extremely important aspect of the ecology of muskrats. The lodge
permits them to live in areas surrounded by water, far away from dry land.
It protects them from enemies and gives them shelter from the weather.
A muskrat builds a lodge by first
heaping plant material and mud to form a mound. A burrow is then dug into
the mound from below the water level, and a chamber is fashioned at the
core of the mound. Later, the walls of the lodge are reinforced from the
outside with more plants and mud. A simple lodge of this type is about
0.5 to 1 m high and 0.5 to 1 m in diameter. It contains only one chamber
and has one or two plunge holes, or exit burrows. More complex lodges,
containing several separate chambers and plunge holes, may be up to 1.5
m high and 1.8 m in diameter.
Shortly after freeze-up, muskrats
chew holes through the ice in bays and channels up to 90 m away from the
lodge to create "push-ups." After an opening has been created, plant material
and mud are used to make a roof over it, resulting in a miniature lodge.
Typically there is just enough room for one muskrat in the push-up. It
is used as a resting place during underwater forays, and as a feeding station.
The winter is a period of relative
inactivity. The muskrat is safe from the cold and from most predators.
It spends most of its time sleeping and feeding until breeding activities
begin after spring break-up.
The muskrat is well adapted to a semi-aquatic
life style. Although fully functional on land, it has evolved characteristics
that make it at home in the water. At three weeks of age it is a capable
swimmer and diver. As an adult, it swims effortlessly and can do so for
long periods of time. This ability is greatly facilitated by the buoyant
qualities of the thick waterproof fur. When swimming on the surface, the
muskrat tucks its front feet slightly forward against the upper chest while
using the back feet in alternate strokes to propel the body. The tail is
used at most as a rudder. When the muskrat is swimming under water, however,
the sculling action of the tail probably provides as much propulsive force
as do the hind feet.
In the late evening during ice-free
periods of the year, muskrats can be seen swimming, sitting at feeding
stations such as logs or points of land, and busily improving lodges.
Although the muskrat builds lodges
near the water and is an accomplished swimmer, it is not a close relative
of the beaver, as is sometimes thought. Nor is it a true rat. Instead,
it is basically a large field mouse that has adapted to life in and around
The muskrat, together with the beaver
and several other mammals, is capable of remaining submerged up to 15 minutes
if in a relaxed state. Non-aquatic mammals cannot do this because they
need a constant supply of oxygen and must continually expel carbon dioxide.
The muskrat is able to partially overcome this problem by reducing its
heart rate and relaxing its muscles when submerged; this reduces the rate
at which oxygen is used. Also, it stores a supply of oxygen in its muscles
for use during a dive and is less sensitive to high carbon dioxide levels
in the blood than are non-diving mammals. This ability for extended dives
is important in escaping enemies, digging channels and burrows, cutting
submerged stems and roots, and travelling long distances under the ice.
The muskrat’s front teeth are especially
modified for underwater chewing. Non-aquatic mammals such as dogs or humans
would have great difficulty in trying to chew on a large object under water,
because water would enter the mouth, throat, and nasal passages. This problem
has been overcome in the muskrat through the evolution of incisors, or
cutting teeth, that protrude ahead of the cheeks and of lips that can close
behind the teeth. This adaptation permits the muskrat (and the beaver)
to chew on stems and roots under water "with its mouth closed."
The muskrat is more widely distributed
in North America than almost any other mammal and in this respect is a
very successful species. It is found from the Arctic Ocean in the north
to the Gulf of Mexico in the south and from the Pacific Ocean in the west
to the Atlantic Ocean in the east. This broad distribution is closely related
to the muskrat’s use of aquatic environments, which are common in North
America. Human activities in North America during the last two centuries
have not significantly affected the distribution of muskrats. In some cases,
however, the draining of marshes or swamps for agricultural or other purposes
has completely exterminated local populations. In others, the building
of irrigation ditches and canals has increased populations.
Until the early part of this century,
muskrats occurred only in North America. In about 1905, they were introduced
to Europe, where they quickly established themselves as permanent residents.
They spread northward and eastward, and today are common in Europe and
Of all plants available in marshes,
cattails are most preferred as a food item. However, muskrats appear to
thrive equally well on a diet of bulrushes, horsetails, or pondweeds, the
last two constituting the basis of the diet in northern latitudes. They
also eat a variety of other plants, including sedges, wild rice, and willows.
During the winter a thick layer of
ice restricts the muskrat to the interior of the lodge or burrow and the
watery environment beneath the ice. The animal’s highly developed diving
abilities and its use of push-ups become critical in procuring food under
those conditions. It covers considerable distances under the ice searching
for food. When the muskrat reaches a feeding area it chews off portions
of plants and carries them to the nearest push-up, where it eats. This
foraging activity under perhaps a metre of ice and snow, in ice-cold water
and almost total darkness, is truly a remarkable feat.
When their normal food items are scarce
or unavailable, and food of animal origin is abundant, muskrats are known
to be highly carnivorous, or meat-eating. Under these circumstances muskrats
most commonly consume animals such as fish, frogs, and clams. However,
muskrats rarely do well on this type of diet and consuming such foods is
generally taken to be evidence of hard times.
Mating activity occurs immediately
following spring break-up in March, April, or May. Mating pairs do not
form lasting family ties; instead, the muskrat appears to be promiscuous,
or have many mates. Males compete fiercely for females. The birth of the
litter, containing five to 10 young, occurs less than a month after the
female has been mated. The same female normally has another litter a month
after the first, and sometimes yet another a month after the second.
The young at birth are blind, hairless,
and almost completely helpless, but they develop rapidly. They are covered
with thin fur at the end of the first week, their eyes open at the end
of the second week, and they normally begin leaving the lodge on short
trips at about two to three weeks of age. Weaning, or making the transition
from the mother’s milk to other foods, occurs at about three weeks, and
juveniles are essentially independent of their parents at six weeks.
Breeding continues throughout the
summer, with the last litters born about August. Food is plentiful during
the summer and the young grow rapidly.
Few rodents live to old age; they
are usually killed by other animals while still quite young, or they die
accidentally. The limited information available suggests that muskrats
become old at three or four years of age. When they reach this age, they
lose much of their natural alertness and fall easy prey to mink, foxes,
and other predators.
The muskrat is a vicious fighter when
provoked. It stands its ground courageously if an escape route to deep
water is not available and can inflict considerable damage on an attacker
with its long incisors, or cutting teeth. In spite of this, it is often
preyed upon by other species. The mink occupies much of the same habitat
as muskrats and can be the cause of heavy mortality among juveniles under
certain conditions. Mink use the same burrow systems, dig into muskrat
lodges, and may enter lodges through plunge holes. The snapping turtle
and the northern pike also inhabit marshes and prey on the muskrat. When
muskrats wander on dry land in search of new habitat, they are subject
to predation by members of the dog family—wolves, coyotes, foxes, and domestic
dogs—as well as by typical predators such as badgers, wolverines, fishers,
racoons, and lynx.
The muskrat has long been hunted by
humans, probably the major enemy or predator of this species. Prior to
the colonization of North America by Europeans, it was hunted occasionally
for food. With the coming of the early settlers and the introduction of
guns and traps, the muskrat was hunted intensively for its fur. This activity
has persisted to the present day—muskrat fur is still in demand. Also,
the muskrat is still used as food by people in some parts of North America.
Muskrats, like many other wildlife
species, show large fluctuations in numbers that follow what appears to
be a regular pattern. In the case of the muskrat, numbers decrease drastically
about every seven to 10 years. At such times, few or no muskrats can be
found where two or three years earlier there had been thousands. These
catastrophes are often blamed on predators or on over-trapping. However,
scientists do not believe that these are the real causes. Instead, for
some as-yet-unknown reason, the health of individuals deteriorates, causing
widespread death and reproductive failure. Reproductive and death rates
return to normal one or two years following such a population decline,
leading to an increase in muskrat numbers once more.
The muskrat contributes more to the
total combined income of North American trappers than any other mammal.
Because of its important role in the trapping industry, it has been studied
extensively. The first major studies were conducted by the Canadian Wildlife
Service on the Mackenzie River Delta in the far north and the Athabasca–Peace
Delta in northern Alberta during the late 1940s. A thorough understanding
of habitat requirements, food habits, reproduction, longevity, causes of
mortality, long-term changes in numbers, and the effect of weather on all
these factors is essential to put management procedures on a sound scientific
basis. The single most important contribution to our understanding of the
biology and ecology of the muskrat was Paul Errington’s Muskrat Populations
(1963), which combined the results of years of study by the author with
information on the muskrat throughout North America. More recent studies
in eastern Canada and central and eastern United States have augmented
what is now a comprehensive body of knowledge on the dynamics and management
of muskrat populations.
There are two major methods of managing
muskrat populations: the first is to improve habitat, and the second is
to regulate the commercial harvest by trappers. The most common method
of improving habitat is to regulate water levels between about 1 and 2
m of depth over large areas by building dams at strategic points in lake
outlets and streams. Sometimes this occurs as a natural side effect of
Regulation of commercial harvest is
based on current population sizes and future population trends. Usually
the harvest is maintained at the highest possible level that will not adversely
affect population sizes and harvests in future years.
The future of the muskrat in Canada
is bright. In spite of heavy trapping pressure, the draining of marshes
for agricultural purposes, and unprecedented industrial activity, the species
has never been endangered in Canada. Indeed, population numbers today are
probably almost as high as they were a thousand years ago.