SEASONAL-VARIATION IN THE MICROCLIMATE AND GAS-COMPOSITION OF BEAVER LODGES IN A BOREAL ENVIRONMENT

Seasonal changes in the microenvironment of Castor canadensis in the Canadian Shield of southeastern Manitoba, Canada, are described. The construction and occupation of lodges by family groups of beavers provided these animals with year-round access to a thermoneutral microclimate. Between June 1988 and March 1989, air temperatures at sampling sites ranged from -41.4 to 32.4-degrees-C, while temperatures recorded from within the chambers of occupied lodges varied from 0 to 35.6-degrees-C. Mean monthly chamber temperature consistently exceeded mean monthly air and water temperature. Despite the large metabolic mass of resident animals, lodge occupancy resulted in only limited disturbance to the respiratory gas concentrations inside lodges. The CO2 levels of occupied lodges ranged from 0.03 to 1.8%, with no evidence of seasonal variation in either CO2 accumulation or O2 depletion in these shelters. Before freeze-up, the gaseous composition and mean internal temperature of occupied and unoccupied lodges were similar. Following freeze-up, mean CO2 accumulation, O2 depletion, and chamber temperature were all significantly higher in occupied houses. During the open-water season, water temperature accounted for 90% of the variation in chamber temperature of occupied lodges. Following freeze-up, none of the meteorological or physical variables measured were significant predictors of chamber temperature.