Ondatra ziebethicus
Muskrat
Nomenclature: Ondatra ziebethicus has been known by many common names including
the following provided by Bluett (1984):
∙Muskrat
∙Musk beaver
∙Marsh Hare
∙Musquash
Muskrats are called so due to their naked rat-like tail and the “musk-type” odor
they exude in their urine (Errington 1963; Von Wettberg 2001).
Geographical Range/Habitat: “The natural range of the muskrat…comprises the major
part of the North American continent” (Artimo 1960). They are found in freshwater to
slightly brackish wetlands, lakes, rivers, marshes, and bogs. Muskrats have even been
found in agricultural drainage ditches where food and water are plentiful (Errington
1963). The ideal water depth is 1.5 to 2.0 meters (Kurta 1995), and they thrive in areas
without large open expanses of water. Muskrats also need large stands of available food
and cover provided by aquatic plants (Errington 1963).
Historic Records: Early reports by settlers indicate that historically in Wisconsin
muskrats have been very plentiful. Wetland habitats were abundant before the mass
settlement of people in the state in the last 150 years. During this time period, muskrat
populations have fluctuated greatly in relation to droughts, extreme winters, but most
dramatically to the loss of habitat. To date, muskrat populations have declined from
historic numbers, but are maintaining a high viable population level (Bluett 1984).
Physical Description: Muskrats are semi aquatic organisms of the Order Rodentia, and
are considered one of the largest members of the Muridae family in North America
(Kurta 1995). Muskrats possess a waterproof brown coat comprised of dense silky underfur used for insulation overlain by “guard hairs,” which are longer, coarser, and glossy.
This creates a hydrophobic affect that water can’t penetrate (Errington 1963). Their
rudder-like tails are laterally compressed and void of all hair; possessing instead scalelike skin. Muskrats have very little eyes and ears. Their ears are indistinguishable from
its furry coat, and their eyes protrude noticeably outward from their head. Like all
rodents, muskrats have very conspicuous incisors. They can tuck their lips in back of
their teeth, in order to keep water out of their mouth while foraging underwater (Von
Wettberg 2001). The front feet of the muskrat consist of four clawed toes and a smallnailed thumb (Bluett 1984). These fore feet are small and quite suitable for the skillful
manipulation of plant material and for digging. Their hind feet contain five clawed toes
and are partially webbed (Bluett 1984; Von Wettberg 2001). They are also lined with
stiff fringes of hair increasing the surface area of the foot to help propel itself through
water (Bluett 1984).
Adult Characteristics Bluett 1984
Total Length - 470-630mm (18-25 in)
Tail length - 200-270 mm (8-10in)
Weight
- 0.8-1.5 kg (2-3 lbs)
Ontogeny and Reproduction: Muskrats are an r-selected species and produce large
quantities of young. They are monogamous breeders and come into sexual maturity the
spring after their birth (Kurta 1995). The breeding season begins in March or April
depending on climactic conditions. Females can have up to 4 litters in a breeding season
(most commonly 2) with 4-8 young per litter. The gestation time is about 30 days, and
newborns are born blind and hairless (Bluett 1984). The young weigh about 21 grams
and grow rapidly on the nourishment from their mother for 2-3 weeks. The male muskrat
contributes very little to the young. After about 15 days, the young muskrat’s eyes will
open, and they will be able to go for their first swim (Kurta 1984). After about 4 weeks,
they will have to fend for themselves, but are normally allowed to stay within the birthing
lodge for up to about 4 months (Bluett 1984). There exists an unbalanced sex ratio in
muskrat populations. In a study of 89,000 muskrats caught over an extended trapping
period in Horicon March, 55% of them were males (Mathiak 1966).
Lifespan and Mortality: The average lifespan for a muskrat is less than 2-3 years
(Kurta 1995).
Mortality
87% die 1st year
11% die 2nd year (Nearly complete population turnover)
2% die 3rd year
Food Habits: Muskrats are mid-level trophic feeders mainly consuming plant material
such as: cattails (Typha), bulrushes (Scirpus), duck potato (Sagittaria), arrowhead
(Sagittaria), pickerel weed (Pontderia) and other aquatic plants (Gashwiler 1948).
Muskrats are opportunistic feeders and will eat clams, crayfish, frogs, fish, and carrion
when in abundance. It has been estimated that 5-7% of the muskrat diet is comprised of
animal foods (Convey 1989). In the winter time, muskrats will rely on food caches, or
underwater roots and tubers for their main source of food (Bluett 1984). Muskrats prefer
to feed within 15 meters of its lodge, and rarely will venture more than 150 meters away.
(Kurta 1995).
Predation: Muskrats are an important prey species for many predators. Dogs, coyotes,
snapping turtles, eagles, hawks, owls, and other mid-large carnivores prey upon them.
(Errington 1963). Mink (Mustela vison) are one of the biggest predators of muskrats. An
early study on the relationship between these two organisms showed that of a sample size
of 297 food-containing mink carcasses, 65.92% had the remains of muskrats (Errington
1943).
Ecology and Behavior: Muskrats are very important ecologically. They help maintain
wetlands, and open up water ways with their prolific vegetation consumption. This
allows for an influx of a variety of more sensitive types of plants as well as the insect,
waterfowl, and other animal life associated with them (Bluett 1984).
Muskrats are lodge builders and tunnel diggers. They dig burrows into steep sides
of river banks or construct lodges out of mud and emergent plants. Lodges can reach 2 m
in diameter by 1.2 m in height with walls about 30 cm thick. The lodges have multiple
entrances and tunnels which lead out into the water. Lodges are insulated and may attain
an inside air temperature 20 degrees Celsius higher than the ambient temperature outside
(Kurta 1995). Muskrats also construct what is known as a “feeding hut” or “push-up.”
This is another structure located 2-8 m away from the lodge and is used for food caching
for the winter months (Bluett 1984). Muskrats dig tunnels through the mud from their
lodge to their “push-ups” for easier access (Kurta 1995).
Muskrats are amazing swimmers. On the surface, muskrats swim about 1-3 miles
per hour without the use of their tail. They use their hind limbs with vertical strokes for
propulsion while on the surface. Due to their small size, the fore-limbs are kept close
under the chin, and are not used for locomotion. Underwater, muskrats will use their tails
and resort to a horizontal locomotion. Their swimming ability allows for fast quick
movement underwater for the pursuit of prey and predator evasion (Errinton 1963).
Muskrats are also capable of lengthy dives. They have been documented going 12
minutes without air in the laboratory, and 17 minutes in the wild. Diving is a very
important predator escape behavior, because it allows for the muskrat to wait out
predators or swim to safety. Muskrats can also escape by tunneling quickly through the
mud. Muskrats can dig their way into the side of a riverbank and wait out a predator
underneath the vegetation layer, but above the water line. Muskrats also kick up mud and
other debris to confuse predators with their hind feet and claws (Errington 1963).
Huddling behavior for thermoregulatory purposes has been documented in
muskrat lodges during the winter months. Sometimes as many as six muskrats may
occupy a winter lodge. Evidence shows that huddling allows for a potentially significant
metabolic savings rate depending on how many are in the group (Bazin 1992). Muskrats
are thus more susceptible to the cold when they are by themselves. The naked tail of the
muskrat is extremely sensitive to cold, and to frostbite. In extreme cases, muskrats have
chewed off their frostbitten tail giving them a “stubby” appearance (Errington 1963).
Muskrats as Biocontrol: Muskrats have a great ecological role to play in today’s
environment. Seitman et al. (2003) noticed a relationship between muskrats and zebra
mussels (Dreissena polymorpha), and have postulated that muskrats may have success as
a biological control of the invasive species. This study was conducted in the Ohio and
Mississippi Rivers by examining muskrat “middens;” or discarded shell piles, for the
presence of zebra mussels. They also examined the number of native unionid mussels
that were encumbered by zebra mussels near high populations of muskrats. Evidence
shows that muskrats have begun to consume zebra mussels in relation to their ever
increasing population. There is strong evidence that this is a benefit to native unionids,
which seem to be less afflicted by the invasive due to muskrats exhibiting prey switching
behavior (Seitman 2003). This is important because muskrats can exhibit a strong
selective pressure on native clams and unionids (Convey 1989).
Muskrats and the Economy: Muskrats have consistently played a major part in the furbearing market of Wisconsin. In some years muskrats were the staple of what was bought
and sold in the fur industry. Some historic numbers on the harvest go all the way back to
1948. From 1948-64 muskrat harvests were recorded at 8,423,000, which averaged
approximately 500,000 muskrats being harvested annually (Mathiak 1966). Because of
this trend, and the fact that they can sustain harvests of 75-85% of the total population,
muskrats have been deemed the Wisconsin trapper’s “bread and butter.” From 19701981 there was 32.7 million dollars worth of pelts harvested from Wisconsin wetlands.
Most management practices for the state allow for a large amount of muskrat harvest, but
in most years, the animals are under harvested (Bluett 1984). Evidence suggests it is
better to over harvest muskrats rather than under harvest them so they don’t reach
harmful population levels. High population levels lead to habitat damage and the spread
of devastating disease (Mathiak 1966).
Literature Cited
Artimo, A. 1960. The dispersal and acclimatization of the muskrat, Ondatra zibethicus
(L.), in Finland. K.F. Puromiehen Kirjapaino Helsinki, Finland.
Bazin, R.C., and MacArthur. R.A., 1992. Thermal benefits of huddling in the muskrat
(Ondatra zibethicus). Journal of Mammology 73(3) 559-564
Bluett, R. 1984. The Muskrat (Ondatra zibethicus). Wisconsin Department of Natural
Resources Bureau of Wildlife Management
Convey, L.E., et al. 1989. Size-selective predation on unioind clams by muskrats.
Journal of Wildlife Management 53(3) 654-657
Errington, P. L. 1963. Muskrat Populations. Iowa State University Press, Ames, Iowa,
U.S.A.
______1943. An analysis of mink predation upon muskrats in north-central United
States. Agricultural Experiment Station Iowa State College of Agriculture and
Mechanic Arts, Ames, Iowa, U.S.A.
Gashwiler, J.S. 1948. Maine muskrat investigations. University of Maine. Onro,
Maine.
Kurta, A. 1995. Mammals of the great lakes region: revised edition. The University of
Michigan Press, U.S.A.
MacArthur, R.A., Humphries, M. M., and Jeske, D. 1997. Huddling behavior and the
foraging efficiency of muskrats. Journal of Mammology 78(3) 850-858.
Mathiak, H.A. 1966. Muskrat population studies at Horicon Marsh, Wisconsin.
Wisconsin Conservation Department 36
Sietman, B.E.; Dunn, H.L.; Tucker, J.K; and Kelner, D.E. 2003. Muskrat (Ondatra
zibethicus) predation on zebra mussels (Dreissena polymorpha) attached to
unionid bivalves. Journal of Freshwater Ecology 18(1): 25-32.
Von Wettberg, E.; 2001. Muskrat ramble. New York State Conversationalist. 56(2)
Reference written by Christopher Uitz, Biology 378 student. Edited by Christopher Yahnke.
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