Differential use of bank burrows and lodges by muskrats, Ondatra zibethicus, in a northern marsh environment

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Abstract
We studied the differential use of lodges and burrows by muskrats in a large (293 ha) northern marsh, during a period characterized by a 100-fold variation in apparent density. Population size (as indexed by dwelling numbers), summer heat stress, rate of collapse of lodges, and over-winter occupancy of lodges and burrows were investigated as potential determinants of dwelling selection. Muskrats selected against lodges at low population size, even when water level was experimentally controlled within a normal range. Sun-exposed lodges in summer were not subject to extreme internal temperatures capable of impairing the survival of young muskrats confined to nests. Lodges were dynamic structures that required substantial and continual upgrade during the ice-free season. Lodges also had a lower probability of remaining active through the winter compared with burrows, possibly because of the freezing of surroundings. We suggest that higher maintenance cost and greater vulnerability to predators likely constitute two critical determinants for muskrat selection against lodges at low population size.

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