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Muskrat
(Ondatra zibethicus Linnaeus)

From: Saunders, D. A. 1988. Adirondack Mammals. State University of New York, College of Environmental Science and Forestry. 216pp.

Order: Rodentia

Family: Cricetidae

Description: The muskrat in not likely to be confused with the other Adirondack mammals except the beaver, from which it differs in being much smaller and in having a laterally compressed, not flat, tail. The blackish brown, scaly tail is sparsely haired and accounts for slightly less than half the total length of 55-64 cm (22-25 in). Maximum weight is 1.8 kg (4 lb). The muskrat’s body is compact; the front feet small, the hind feet large with partially-webbed toes. Stiff hairs line each hind toe, the webs and edges of the hind feet. The eyes and ears are small, the latter barely visible. The head is both broad and relatively small. The underfur is thick and wooly; the longer guard hairs glossy. The fur is dark brown on the back, reddish to grayish brown on the sides, and a paler gray below, the throat light gray to white. Color variants (black, white, silver, tan) occur but are uncommon.

Range and Habitat: The range is most of North America except parts of the Southeast, Southwest, Texas, and nearly all of Mexico. The muskrat is an introduced and widespread species in Europe (where it has reached pest population levels) and the southern tip of South America. In the Adirondacks, this furbearer is a common resident of many marshes, beaver ponds, streams, rivers, and lakes to elevations of at least 524 m (1720 ft). Wherever they live, muskrats build conspicuous houses of mud and aquatic plants, or dig concealed burrows in the banks of ponds, lakes, and streams. Burrows are the usual homes of muskrats occupying waters where emergent vegetation is scarce or absent. A burrow is about 15 cm (6 in) in diameter with an entrance under water, and one or more living chambers or dens. The conical houses are approximately 1m (3- 4 ft) in height, twice as wide at the base, and contain one or two living chambers which also have underwater entrances. Occasionally, muskrats inhabit abandoned beaver lodges.

Food and Feeding Behavior: The roots, stems, and leaves of cattails, pickerel weed, pondweeds, arrowheads, water lilies, and other aquatic plants are the mainstay of the diet. The muskrat uses its sharp incisors and forefeet to harvest water plants, dragging or towing them to rocks, logs, or various feeding structures that it constructs (pushups, exposed on roofed feeding platforms). Muskrats may leave the water to forage on herbaceous plants, or where available, the fallen fruit of orchards, agricultural crops, and garden vegetables. The flesh of dead animals is another component of the diet, especially dead fish exposed by the spring melt. However, muskrats capture and eat live animals, for example insects, crayfish, and slow-moving fish, amphibians, and reptiles. Fresh water mussels are an important part of the winter diet, where they are available. Large piles of the shells of these bivalves on rocks and logs near the water’s edge mark feeding stations where muskrats have consumed their prey in hollows beneath shore ice.

Activity and Movement: Muskrats are primarily crepuscular or nocturnal, semi-aquatic, and active year-round. Their large hind feet propel them through the water while the tail trails behind or acts as a rudder. They usually swim on the surface, but at the slightest disturbance, or to reach the roots of water plants, dive, and can remain submerged for up to 17 minutes, traveling distances up to 46 m (150 ft). In the winter, they swim beneath the ice breathing air trapped under its surface, or use breathing holes they gnaw in the ice. Muskrats walk, hop, or run on land. Floods, drought, the lack of food, and intraspecific competition may cause some individuals to disperse over land for distances up to 34 km (21 mi). Displaced muskrats often react aggressively toward any animal approaching them.

Reproduction: The breeding season extends from March through August. Muskrats mate while partially submerged or on water-logged debris just above the surface. The gestation period is about 28 days (extremes, 22-30 days). Females bear 1-4 litters of 1-14 (average 6-7) young per litter. Newborn muskrats weigh about 22 g (0.8 oz), are pink, blind, nearly hairless, and have small round tails. They grow rapidly, their eyes opening at 14-16 days, and are weaned at 21-28 days. Most breed as yearlings. The life expectancy for an adult is 3-4 years; potential longevity is about 10 years.

Predators: Muskrats have many predators such as snapping turtles, large fish, coyotes, foxes, weasels, otters, bobcats, great horned owls, and northern harriers. Minks and raccoons, however, are the primary predators of this rodent.

Social Behavior:

• Social system - The structure of a small muskrat population changes seasonally. Population density, habitat, water levels, and food supply are important determinants of the mating system and social organization. At the onset of the breeding season, adults become aggressive, and in most cases, monogamous pairs occupy home ranges from which they exclude other adults by hostile behavior and scent marking. The home range, in effect, serves as a breeding territory. Contact between a pair may be limited to mating, and the male rarely aids in caring for the young. Occasionally, when a muskrat population reaches a high density, i.e., many adults present in a small area, dominance hierarchies form, and adults are promiscuous or polygynous during the breeding season. Outside the breeding season, muskrats are solitary except at times during the winter when several may share a den to conserve body heat. Home ranges vary in size from 30-350 m (100- 120 ft) in diameter. Individuals confine much of their activity to the area within 15 m (50 ft) of the den. Densities range from 2.5-62.5 muskrats per ha (1-25 per acre).
• Communication - Muskrats deposit on oily liquid from glands located near the anus, the “musk” of the muskrat. These chemical messages provide clues about the sexual condition and the identity of a resident. Accumulations of droppings on logs and other objects indicate scent posts. Tactile cues include “kissing”, and mutual grooming. Posturing and vocalizations such as “chirping”, and “whining” function as signals, too. Muskrats sometimes slap their tail on the water, possibly to alert neighbors of impending danger.