ABSTRACT
The muskrat, Ondatra zibethica (L.), occurs over almost the whole of North America and has been introduced into, or has thereafter spread into, many countries in the northern parts of Eurasia. The species is active the year-round and occupies a wide variety of aquatic habitats. It is most abundant in localities with a stable water level and a rich aquatic vegetation, e.g., ponds, marshes, and the slow-flowing reaches of watercourses. Where the muskrat population lives solely in houses and where these are destroyed annually, e.g., due to high water levels during the spring floods following the snow-melt, house counts represent the most appropriate method of making population censuses. The muskrat builds houses throughout the ice-free season of the year, but such house-building activity becomes intensified as winter approaches.
