Seasonal differences in habitat use and fat reserves in an arctic muskrat population

Differences in summer and winter habitat use by muskrats (Ondatra zibethicus) were studied in the Mackenzie River Delta, Northwest Territories, Canada. Quantitative data consisting of 11 habitat variables were obtained from 73 winter and 119 summer burrow sites. The single discriminant function derived from a discriminant function analysis of summer and winter burrow sites was strongly correlated with variables interpreted as describing food, cover, and overwinter survival of muskrats. During summer, muskrat burrows were closer to shallow water, on gentler slopes with greater cover, and occurred closer to stands of horsetail (Equisetum fluviatile) than in winter. Food habit analyses revealed that high-protein E. fluviatile was the most frequently consumed species in summer. Muskrats of both sexes were found to enter the breeding season in extremely good condition when judged on the basis of body fat; the costs of breeding, lactation, and resource defense are hypothesized to account for the low-fat condition noted at the onset of winter. Prior to winter, muskrats relocated to deep-water sites, possibly to maintain access to food and (or) forage on the energy-rich roots and rhizomes of submerged macrophytes. The change in habitat use, coupled with physiological adaptations to cold, is suggested to account for the winter recovery in fat reserves. These same reserves can be mobilized in spring at the onset of the metabolically demanding breeding season.