Nutrition and the energetic tactics of muskrats (Ondatra zibethicus): morphological and metabolic adjustments to seasonal shifts in diet quality

Abstract

Basal metabolic rate (BMR), serum thyroxine (T4) concentration, lean organ mass, and body composition were measured in 94 captive, seasonally acclimatized muskrats (Ondatra zibethicus) between May 1991 and April 1992. Seasonal measurements of oxygen consumption, body water content, and mass were obtained from an additional 124 captive or free-ranging animals in 1994-1995. Mass-independent BMRs (kJ · kg-0.67· h-1) and serum T4concentrations (nmol · L-1) varied significantly over the year (P << 0.0001), with mean values in February exceeding July values by 31.1 and 77.2%, respectively. These variables tracked seasonal changes in the neutral detergent soluble (NDS) content of broadleaf cattails (Typha latifolia), the dominant food of muskrats in the study population. From July through February, alimentary tract, liver, spleen, and heart masses increased, while kidney mass declined. Body fat stores varied significantly over both years, with peak values measured in February. However, lean body and pelt masses exhibited little seasonal variation (P >> 0.05). Stepwise multiple regression and principal component analyses suggested that variation in BMR was associated most closely with changes in heart and alimentary tract masses. Annual variations in basal energy expenditure, serum T4concentration, and organ masses of wild muskrats appear to be linked to seasonal changes in forage NDS content and energy intake, and may be important factors relating to the annual pattern of fat accretion and mobilization in this semiaquatic rodent.