Seasonal Microenvironments of the Muskrat (Ondatra zibethicus) in a Northern Marsh

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Abstract
Seasonal changes in the microenvironment of Ondatra zibethicus in Delta Marsh, Manitoba, Canada, are described. In winter, well-insulated, closely-spaced resting and feeding shelters provided an equable microclimate moderated by the presence of open water in plunge holes. Group occupation of lodges during winter resulted in chamber temperatures that averaged 20°C above external air temperature. Daily fluctuations in winter lodge temperature reflected activity patterns of resident animals. In summer, high lodge temperatures (25 to 30°C) appeared to favor the use of burrows and open nests by adults. Burrows provided the coolest, most stable microclimate in summer, with temperatures ranging from 8.5 to 20.5°C, depending on soil depth. Between November 1973 and July 1975, air temperatures in the marsh ranged from −39 to 34°C, while temperatures recorded from within shelters used by muskrats varied from −9 to 30°C. The range of mean temperatures (3 to 25°C) recorded from within occupied lodges and burrows during this period approximated the thermoneutral zone of this species. It is suggested that energy costs for thermoregulation are minimized through the construction and selective use of multiple shelters.