NUTRITION AND THE ENERGETIC TACTICS OF MUSKRATS (ONDATRA-ZIBETHICUS) - MORPHOLOGICAL AND METABOLIC ADJUSTMENTS TO SEASONAL SHIFTS IN DIET QUALITY

Basal metabolic rate (BMR), serum thyroxine (T-4) concentration, lean organ mass, and body composition were measured in 94 captive, seasonal ly acclimatized muskrats (Ondatra zibethicus) between May 1991 and Apr il 1992. Seasonal measurements of oxygen consumption, body water conte nt, and mass were obtained from an additional 124 captive or free-rang ing animals in 1994-1995. Mass-independent BMRs (kJ . kg(-0.67) . h(-1 )) and serum T-4 concentrations (nmol . L-1) varied significantly over the year (P < 0.0001), with mean values in February exceeding July va lues by 31.1 and 77.2%, respectively. These variables tracked seasonal changes in the neutral detergent soluble (NDS) content of broadleaf c attails (Typha latifolia), the dominant food of muskrats in the study population. From July through February, alimentary tract, liver, splee n, and heart masses increased, while kidney mass declined. Body fat st ores varied significantly over both years, with peak values measured i n February. However, lean body and pelt masses exhibited little season al variation (P > 0.05). Stepwise multiple regression and principal co mponent analyses suggested that variation in BMR was associated most c losely with changes in heart and alimentary tract masses. Annual varia tions in basal energy expenditure, serum T-4 concentration, and organ masses of wild muskrats appear to be linked to seasonal changes in for age NDS content and energy intake, and may be important factors relati ng to the annual pattern of fat accretion and mobilization in this sem iaquatic rodent.