Ondatra ziebethicus - Muskrat

Skull Pictures:        
Anterior    Mandible
Dorsal     Posterior
Lateral

Nomenclature:

Ondatra ziebethicus has been known by many common names including the following provided by Bluett (1984):

  ∙Muskrat                            ∙Marsh Hare

 ∙Musk beaver                     ∙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 under-fur 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 scale-like 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 small-nailed 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 Reporduction:

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).  

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). 

Economy:

Muskrats have consistently played a major part in the fur-bearing 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 1970-1981 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:

McManus, J.J.  1970.  Behavior of Captive Opossums, Didelphis marsupialis virginiana.  American Midland Naturalist 84(1): 144-169.

Lyon Jr, M.W.  1936.  Mammals of Indiana.  American Midland Naturalist 17(1):  1-373.

Ladine, T.A. and Kissell Jr, R.E.  1994.  Behavior of Escape Virginia Opossums.  American Midland Naturalist 132(2):  234-238.

Allen, C.H. et al.  1985.  Movement, Habitat Use and Denning of Opossums in the Georgia Piedmont (in Notes and Discussion).  American Midland Naturalist 113(2): 408-412.

Gillette, L.N.  1980.  Movement Patterns of Radio-Tagged Opossums in Wisconsin.  American Midland Naturalist 104(1): 1-12.

Winegarner, M.S.  1982.  Seasonal Changes in the Reproductive Tract of the Male Opossum Didelphis virginiana Kerr in Florida.  American Midland Naturalist 107(2): 258-261.

 Reference written by Michael Hawley, Biol 378: Edited by Chris Yahnke and Jen Callahan.
Page last updated 4-23-04.