Ondatra zibethicus
(Muskrat)

Description:
The muskrat is well-named; it has musk glands near the base of the tail which give the species a musky odor and its name. It is well adapted to its aquatic existence. Even though it is much larger, it resembles its relatives, the voles, in that it has a large head with small ears and eyes. It has a very dense underfur which actually traps air and forms a waterproof barrier. Its lips seal closed behind its incisors, which allows underwater foraging without gulping in the pond or stream. Its pelage is dark brown to reddish-brown on top and silvery-gray underneath. Its tail is laterally compressed to form a rudder for swimming, and it has stiff hairs forming a fringe around its feet, and partial webbing, which help for swimming. They are large compared to other members of this family; total length is 18 to 22 inches (460-550 mm), tail length is 8 to 10.2 inches (200-254 mm), and they weigh 1.5 to 4 pounds (0.7-1.8 kg).

Range:
Throughout North America north of Mexico, except portions of southwestern U.S. and Florida.

Habitat:
Prefers fresh or brackish marshes, lakes, ponds, swamps, and other bodies of slow-moving water. Most abundant in areas with cattails. Rare or absent from large, artificial impoundments where fluctuating water levels eliminate littoral zone plants (food supply). In Idaho, occurs primarily in lowland ponds, lakes, marshes and streams.

Diet:
Diet consists primarily of aquatic plants, particularly cattails, cordgrass, and bulrushes. Also eats crustaceans and mollusks; in some areas may eat large numbers of mussels.

Ecology:
They are active year-round and during the winter are able to swim out of their den under the ice to acquire food. They can swim up to 180 feet (55 m) under water and can stay submerged for up to 20 minutes. They are mainly nocturnal, but frequently seen in daylight. Activity seems to peak twice daily: between 1600 and 1700 hours, and between 2200 and 2300 hours. They construct dens in bank burrows or conical houses of vegetation in shallow, heavily vegetated water. They build rooted feeding platforms in their houses. Their home range is relatively small as they usually do not forage more than about 35 feet (11 m) from their home site (in marginal areas, foraging excursion areas are greater). Populations fluctuate as density may reach up to 90 per 2.5 acres (1 ha), but it is usually much less. Individuals are generally solitary, but several may use same general area, and in winter several may congregate in a single den. However, territoriality is common. The species can cause damage to river banks and the banks of irrigation canals. Young often build small nest near their parents house shortly after becoming independent. But, in the fall, they are often driven from the “home” pond by the parents and are forced to disperse. They are vulnerable to predators during this time. Predators include mink, which are known to heavily utilize muskrats, other large carnivores such as coyotes, most likely, large raptors and humans. Their fur is very fine and valuable and the muskrat is one of the most heavily exploited furbearers in North America.

Reproduction:
Breeding occurs in the spring and early summer. Gestation lasts 28 to 30 days and females produce an average of 2 to 3 litters per year. litter size varies from 1 to 12 young but in Idaho the average is about 7. Young are weaned and fairly independent after about 1 month, and reach sexual maturity in 4 to 6 months. A high rate of mortality exists in young.

Conservation:

Status:	Game species
Global Rank:	G5
State Rank:	S5

Important State References:
Reeves, H.M. and R.M. Williams. 1956. Reproduction, size, and mortality in the Rocky Mountain muskrat. J. Mammal. 37:494-500.