INTRODUCTION

The muskrat, Ondatra zibethicus , has been an important fur-bearing animal in many parts of North America for thousands of years. Even before settlers inhabited the United States, muskrats provided Native Americans with clothing and food. In Michigan alone, the muskrat produces millions of dollars worth of meat and fur for residents each year. The scientific name Ondatra is from a Huron Indian word and zibethicus is a Latin word which refers to the muskrat's musky odor.

DESCRIPTION

The muskrat is a semi-aquatic rodent that is slightly larger than the familiar fox squirrel. It weighs 3 pounds and is 18-25 inches long. The head is short and broad with small, dark colored eyes and short, fur covered ears. The vibrissae (whiskers) are medium-long and quite noticeable. The muskrat's body is chunky in build. Its tail is scaly and has little hair. The tail is flattened side to side. The muskrat's tail distinguishes it from the beaver or from Norway rats that frequently use urban river banks. A beaver's tail is flattened top to bottom. The Norway rat's tail is rounded. Muskrat feet are dark brown or black. The hind feet are fringed with short stiff hairs. They are large, broad, and partially or fully webbed for swimming. The front feet are small with 5 toes. The front feet are used for digging, manipulating food, and handling house-building materials.

Muskrat pelage (fur), as with all mammals, is made up of long guard hairs and short, dense, gray-colored underfur. The back and side guard hairs are usually dark brown in color. Their color may range from white or silver to tan, chestnut or black. The belly fur is a pale gray, but may be orange or cinnamon in color. Males and females have the same coloration.

Muskrats have several adaptations for aquatic life. The long glossy, dark colored guard hairs protect the underfur from matting. Its fur repels water and traps air which insulates the muskrat's body from the cold. Its mouth has valve like flaps behind the incisors (front teeth) which allow the muskrat to eat underwater without swallowing water. Heterothermia is an important adaptation for an animal that swims in icy cold water. Heterothermia is the ability to limit the flow of blood to feet and tail while in cold water. This allows the muskrat to control the amount of body heat lost. The muskrat's eyes are on the upper side of its head to allow it to see above water while swimming.

HABITAT

Muskrats use a wide variety of wetland habitats. The best habitat consists of still or slow-moving water which is at least 3 feet deep during the winter.
The most favorable habitat in my state, Michigan, is a marsh, an area of flooded herbaceous (nonwoody) plants with little or no water currents. It should have about 80% emergent vegetation (plants which grow with leaves sticking out above the water). Cattails and other vegetation serve both as food and as a source of building materials. Marshes also contain soils suitable for digging burrows and channels.

Swamps (flooded trees and shrubs) which contain pools of still water are also suitable habitat if they have a dense growth of herbaceous plants along the shoreline. Water-tolerant trees or shrubs such as buttonbush, alder, willow, and red-osier dogwood are often found in this habitat.

Arrowwood, sweet gale, cattails, rushes, grasses, sedges, and other plants provide the muskrat with food and protection. The muskrats make trails through the vegetation to reach food sources. The vegetation serves as protection from flying predators.

The least productive habitat for muskrats is a body of water with steep banks, muddy shorelines and little plant growth typically found in forested areas. Deep water (12 feet or more) is always poor habitat because few, if any, plants can grow there. The wave and ice action found in large open bodies of water destroys lodges (houses) and feeding stations.

Muskrats alter their habitat when they eat enough plants to create large areas of open water (an eat-out). Small scattered eat-outs make the marshes more suitable for waterfowl and other aquatic species. Canada geese, other waterfowl and mammals use muskrat houses or feeding structures for nesting or dens.

FOOD AND FEEDING BEHAVIOR

Cattails are a major food source for muskrats. They eat the roots, stems, shoots, leaves, and roots. In Michigan, muskrats also eat bulrushes, pondweeds, coontail, water lily roots, water milfoil, water weed, bladderwort, or burreed. Muskrats living along stream banks or lake shorelines eat willow tree bark, grasses and dandelion roots. Those muskrats living near agricultural lands often develop a taste for ear corn, clover, alfalfa, soybeans, carrots, apples and other crops. Occasionally animal matter such as mussels, crayfish, clams, snails, frogs, turtles, and fish are eaten. Rarely, poultry, birds and carrion (dead animals) are eaten. During times of food scarcity, such as winter, the muskrat may consume part of its home!

Eat-outs. High muskrat populations may create an eat-out where most of the aquatic plants (leaves, stems and roots) have been eaten. Only open water remains. Many muskrats must then leave to find another home or face starvation. However, eat-outs benefit shore birds, wading birds and other waterfowl by creating areas of open water within the marsh. The marsh eventually regrows, the muskrats return and the cycle begins anew.

Feeding platforms. Muskrats construct round feeding platforms which are hollow inside and about 1-1/2 feet tall. Feeding platforms provide both food storage and a place for muskrats to remain dry and concealed from predators while they feed. These are temporary structures that are usually abandoned after few days. During the winter, push-ups are constructed. The push-up formed by cutting a hole in ice about 12-18 inches in diameter. Then the muskrat pushes vegetation up into a pile through the hole with the edges supporting the vegetation. Push-ups provide food during the winter and then collapse when the ice thaws.

BEHAVIOR

General. Muskrats are most active at night. Darkness hides them from their predators. Much of their time during the day is spent in underground tunnels or in nests and feeding shelters. Muskrats are more active during cool weather.

Dens and lodges. Muskrats use both underground dens and lodges which are constructed in open water. Muskrats usually begin constructing new lodges and dens in late August through October. These houses must be large enough and strong enough to survive the winter. Young-of-the year muskrats usually build smaller dens or lodges than adults.

Muskrats living in flowing streams or rivers create underground dens in the banks. These dens consist of a series of tunnels, chambers and ventilation holes. A long horizontal burrow is excavated from under the water up into the bank. Then a series of chambers linked together with tunnels is created. Vertical tunnels to the surface serve as ventilation. This tunnel opening may be plugged with plant materials during bad weather. The chambers are small, 6-8 inches in diameter and are lined with shredded plant materials. The temperature in underground dens is cooler than the air temperature during the summer and warmer in the winter.

Lodges are built over open water that is approximately 2 feet deep. The base of house is made of rotted plant material and mud which is collected into a pile on the bottom of the pond or marsh. Additional layers of plants and mud are layered on top of this base until the pile extends above the surface of the water. Then the muskrat tunnels up from under the water until it reaches the water line. Just above the water line, the muskrat excavates a chamber that may range up to 1 foot in diameter. The outside dimensions of the lodge may range from 6 to 8 feet in diameter. Older lodges often contain several chambers connected by tunnels.

Breeding/courting. Muskrats deposit musk (oily secretion) from their anal glands near houses, dens, and along trails. This is the main method of communication when a muskrat is ready to breed. Little is known about actual courtship patterns. It is known that sometimes a breeding pair lives together during the nesting season. They share in house or nest building. Other muskrats meet briefly to mate and then go their separate ways.

Muskrats frequently give birth to three litters a year, mating about 10 days after the birth of each litter. The year's first litter of muskrat kits is born late in April or early in May. The next litters are born in June or July and another litter may be born in late August-September.

Gestation (pregnancy) lasts 25-30 days. The average litter size is 6 kits. They are born in a nest chamber that is lined with grass or fresh vegetation. Nests may be in the muskrat lodge or underground burrows. Males very rarely help with raising the litter.

Muskrat kits are born blind and hairless. They weigh less than 1 ounce and are less than 1 inch long. Shortly after birth the kits begin to nurse and cling to their mother's nipples constantly. After a few days, the mother carries them around by holding them with her mouth. She can even dive while holding kits in her mouth, and she holds them above the water while she swims.

The kit's eyes open at 14-16 days and they are covered with fine gray fur. They are now able to swim and dive and start eating green vegetation. Kits are weaned (no longer nursed by the mother) at 4 weeks and become independent. Muskrats are full grown at 6-7 months of age.

Territories. Muskrats may defend a territory for a short time during breeding season. The males may fight even though they are quite compatible at other times of the year. Muskrats remain close to their home sites, resting areas and feeding shelters.

Normally, young muskrats stay within 100 yards of their birth den. If a particular habitat is overcrowded with muskrats, females may violently reject their own weaned offspring. Some females even eat their offspring under these circumstances. Both male and female muskrats become quite aggressive toward other muskrats during times of overcrowding.

Locomotion. The muskrat can swim forward, backwards and underwater. It can stay underwater for up to 20 minutes at a time at a speed of 2-4 miles per hour. Muskrats typically swim with their nose out of the water. They and paddle with their hind feet. On land muskrats move by walking.

Travels, Movements and Migrations. Muskrats may travel from wetland to wetland during the spring or fall for several reasons. Young muskrats leave their birth site to seek their own homes. Water level changes may force muskrats to seek new home sites. Movements may occur when the population density in a specific location becomes too high. Muskrats roam while looking for mates.

Voice. Adult muskrats are silent most of the time. They may give low squeaks, loud squeals or chatter when cornered. Both sexes make a loud sound sounding like n-n-n-n during breeding season. Young kits are quite noisy and make a squeaky cry that quickly gets the mother's attention.

Muskrat signs. Scats (body wastes) may be seen in small piles on banks, rocks, or logs. Food remains on banks or floating on water show that muskrats are nearby. Scent (odor left on item by muskrat rubbing against that item) posts are constructed out of plant matter and mixed with mud. They frequently smell of musk.

OTHER COMMENTS

Muskrats benefit many wetland species by creating small open water areas necessary for waterfowl. Extremely abundant muskrats, however, can eat-out an active marsh, making it too open for waterfowl. Population numbers are easily managed by hunting and trapping and water level manipulation.

Muskrats are an indicator of environmental quality. They are quickly killed when exposed to crude oil in the water. Since heavy metals collect in their tissue, muskrats flesh can warn us that contamination problems exist.

Water dikes and dams are sometimes damaged by muskrat burrows. Farmers claim that muskrats damage crops grown near stream or river banks.

NOTE: All photographs are available from arttoday.com and are used in compliance with their rules.