Fig. 119. Muskrat (Ondatra zibethicus osoyoosensis), male, Lake Washington, Seattle, October 13, 1939; weight 906 grams on November 4, 1939. (Fish and Wildlife Service photo by Victor B. [Scheffer], No. 736.)
Muskrats are found in Canada and the United States. They are lowland animals, rarely ranging into the Canadian Life-zone. Their aquatic habitat makes them relatively independent of temperature, and consequently they occur in both the Transition and Upper Sonoran life-zones.
The muskrat is well adapted to aquatic life. The toes of the large hind feet are webbed at the base, the hind feet are turned slightly outward, a fringe of stiff bristles at the rear edge of each foot furnishes additional aid in swimming, and the laterally compressed tail makes it an efficient rudder. In the water the surface of the fur flattens down to entrap air in the dense underfur, keeping the body dry.
The typical habitat of the muskrat is slow-moving water or still water, such as in lakes, ponds, marshes and sluggish rivers and streams. Muskrats occur commonly, though not in large numbers, in more swiftly moving streams. In Puget Sound and the San Juan Islands they occupy a marine habitat.
In the extensive marshes along Lake Washington, King County, muskrats are abundant. They occupy sluggish water, often water polluted by garbage and sewage. In these marshes, banks suitable for the construction of burrows are absent and houses are made of cattail stalks and leaves. The food of these marsh-living muskrats consists principally of cattail and other marsh vegetation.
In contrast to the marsh-occupying muskrats, muskrats along the open, marsh-free shores of the lake live in deep clear water where waves lap the shore. These muskrats live exclusively in burrows dug in the banks and feed upon fresh-water mussels.
In still greater contrast were muskrats living 20 miles away, near Cottage Lake, King County. Here we found them in small clear streams, 4 to 10 feet wide. Depth of the water varied from a few inches to three feet. The streams flowed through meadows, pastures and junglelike, deciduous woods. Muskrats were slightly more common along wooded stretches than in open areas. Some were trapped where the streams flowed at high velocity over shallow, gravel bottoms. The animals lived in burrows and fed upon fresh-water mussels and a variety of plants.
Near Richmond Beach, Snohomish County, muskrats took up residence in a small tidal pool along Puget Sound. The nearest fresh water stream large enough to support a muskrat was two miles away. Two muskrats were trapped here. Investigation of a tidal pool a mile to the north disclosed unmistakable muskrat signs. Traps set in the culvert connecting the pool with Puget Sound at high tide took several specimens. Study showed that the muskrats were not living in the pool but among the large boulders forming the breakwater for the Great Northern Railroad, along the sound itself. They were feeding on marine mussels (Mytilus). These mussels lived in the salt water of the sound, not in the tidal pool.
At Peavine Pass, Blakely Island, in the San Juan Islands, muskrats were living in the swift tidal current and deep, marine waters. Several were seen in late afternoon. All were swimming parallel to the shore about 50 feet out. Here also they fed on Mytilus, but their homes were not discovered. Certainly they were not living in the tidal pool at Flat Point, a half-mile away.