Muskrats (Ondatra zibethicus) were collected from river and creek study areas in East Tennessee from July 1972 to June 1973. An imbalance in sex ratios was found to be seasonal and an overall sex ratio approaching 1:1 is likely the actual case. Significantly heavier body weights of river versus creek animals likely reflects the more abundant and/or higher quality food supply present on the river than on the small creek. Adult male muskrats from the river tended to exhibit expected patterns of deposition and utilization of internal body fat, with increased fat levels from spring through fall and decreased amounts from winter to spring. Nonpregnant females exhibited a decrease in body fat from winter through summer while fat deposition occurred in pregnant females from spring to summer. Adults of the river contained more internal body fat than their creek counterparts over the winter, again indicating the better nutritional conditions on the river study area. Adrenals of adult male muskrats reached maximum weights following the cessation of the breeding season. Adrenal weights of nonpregnant adult females were at maximum values during the early breeding season.
