The muskrat is about the same size as a cottontail rabbit. Adults vary from 16 to 25 inches in total length and weigh from 1 1/2 to 4 pounds. The average weight is about 2 1/2 pounds. Muskrats have small eyes and ears. The front legs are short while the hind legs are longer, stronger and have partially webbed feet. The black, scaly tail is flattened vertically (like the rudder of a ship) and is almost as long as the body. The back is dark or chocolate brown and fades gradually to a lighter brown with a reddish tinge on the sides. The underparts are still lighter, shading to almost white on the throat.
Distribution & Abundance
Muskrats are common and live in every county in Illinois. Some of the highest numbers are found in the northeastern and northwestern regions. Statewide, numbers are lower than they were 50 to 100 years ago because of habitat loss caused by channelization (straightening) of streams, drainage of wetlands and changes in stream flow caused by man-made drainage systems under agricultural fields.
Muskrat populations can change greatly from year to year depending on weather conditions and other factors. A bad winter, drought, flood or outbreak of hemorrhagic plague can cause their numbers to plummet. However, they bounce back quickly when conditions improve because they produce many young. Fall populations can exceed eight muskrats per acre in good marsh habitats.
Aquatic habitats like rivers, streams, drainage ditches, marshes, lakes and ponds are home to muskrats. Those that live in areas with shallow, stable water (like marshes) often build dome-shaped houses by cutting and piling up cattails, bulrushes or other aquatic vegetation. Some are 8 feet or more in diameter at the base and have walls 1 to 2 feet thick. Tunnels angle upward from the underwater entrances (usually two) to an inside chamber that's hollowed out above the water level. This allows the muskrats to stay warm and dry in their nest chamber while resting and raising their young. Similar but smaller houses with thinner walls are built for resting and protection from predators while muskrats venture away from their dwelling to look for food.
Muskrats that live in rivers, ditches and ponds usually don't build houses. Instead, they burrow into the bank, beginning underwater and angling upward until the tunnel clears the water level. Here they hollow out a living chamber. Trails hollowed out by muskrats swimming in and out of their den entrances are sometimes visible from above, especially along well-used travel routes. Burrows dug into the dams of man-made ponds can wash out during high water, causing the ponds to drain.
Muskrats are most active at night, but it's not uncommon to see one feeding or building a house in the early morning or late afternoon. They spend most of their time within a few hundred feet of their dens or houses. However, some move away from their homes in late summer or early fall and can be seen traveling through areas far from water. Similar overland movements can occur in the spring during breeding season.
When populations get so high that food is in short supply, muskrats become aggressive, often fight with one another, and sometimes eat their young. This is one natural control that helps to balance muskrat numbers with their food supply.
In marshes, muskrats eat the roots and stems of cattail, bulrush, arrowhead, duckweed and water-lily. Clover, corn and grasses are common foods in agricultural areas. Although they are considered to be herbivores (plant-eaters), muskrats sometimes eat freshwater clams, snails, crayfish, fish and frogs. Feeding piles or platforms of shredded vegetation often accumulate where muskrats make a habit of stopping to eat their meals.
The breeding season starts in late winter and ends in September. Pregnancy averages 28 days. Females usually have two litters, sometimes three, per year. A typical litter has four to seven young.
The young are blind, nearly helpless and practically naked at birth. By one week of age they're covered with coarse gray-brown fur. Their eyes open between 14 and 16 days of age. About this time the young can swim, dive, and climb on low, floating objects. They're weaned at 3 to 4 weeks, and can take care of themselves at about one month of age. It's possible for young born in early spring to breed in late summer, but most breed for the first time the spring following their birth.
Trapping is the best way to solve specific problems caused by muskrats (for example, when they burrow in a pond dam) and manage their numbers for ecological benefits. Trapping is allowed for only two to three months during the fall and winter so that no babies or mothers with dependent young are taken. Wetland protection, restoration and enhancement are important practices, especially in fast-developing areas of northeastern Illinois where marshes were once abundant. Filter strips (typically 30- to 60-foot-wide strips of grass planted along streams and rivers) improve water quality, reduce erosion, stabilize banks and provide food for muskrats.
Breaches in pond dams can be avoided by building the dam with a wide base and a crest at least three feet above the water line. Dams that slope gently toward the water are less prone to damage than those with a steep face. Fencing should be used to keep livestock off the sides of the dam so that they don't cave-in muskrat burrows that could weaken the dam. A thick layer of hand-sized rocks dumped on the entire face of the dam can help to prevent erosion and discourage burrowing.