Seasonal Changes in Water Flux, Forage Intake, and Assimilated Energy of Free-Ranging Muskrats

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

“…However, our findings suggest that energy intake may not be the primary factor regulating kidney mass in wild muskrats. Rather, the seasonal trend in kidney mass is consistent with the observation that muskrats exhibit substantially higher rates of urine output and water flux in summer than in winter (Campbell and MacArthur 1997;Campbell et al 1998).…”

“…It has been suggested that fat storage in this rodent may be facilitated by a winter reduction in thyroid activity and lean body mass, and hence a reduction in basal metabolic costs (Aleksiuk and Frohlinger 1971;Messier 1992a, 1995). In a recent study of free-ranging muskrats (Campbell et al 1998), however, we found that the intake of assimilated energy was substantially higher (>60%) in winter than in summer. Consequently, it is still not clear whether lipid deposition in muskrats during winter results from an increase in energy intake, a reduction in metabolic activity, or perhaps a combination of these factors (Salsbury and Armitage 1994).…”

“…However, few attempts have been made to collect both BMR and daily energy intake data from the same individuals, or to test for this relationship on a seasonal basis in either captive or free-ranging animals (Weiner 1992;Salsbury and Armitage 1994). Our results clearly indicate that seasonal changes in metabolizable energy intake (MEI) of both captive and freeliving muskrats (Campbell and MacArthur 1996;Campbell et al 1998) closely follow seasonal adjustments in BMR (Fig. 5).…”

“…Not surprisingly, we found that annual changes in forage quality (Fig. 1) and energy intake (Campbell and MacArthur 1996;Campbell et al 1998) were accompanied by changes in basal energy expenditure, serum T 4 concentration, body lipid stores, and organ sizes of seasonally acclimatized muskrats Fig. 3.…”

“…Serum samples were distilled by vacuum sublimation (Stansell and Mojica 1968) and the deuterium concentration was measured at 2510.0 cm -1 (wavelength 3.8 µm) using a Model 881 Perkin-Elmer dual-beam infrared spectrophotometer and standard calcium fluoride cells. As the D 2 O technique can overestimate the TBW content of mammals by up to 12.0% (Nagy and Costa 1980), a correction factor of 0.929 obtained from 15 muskrats with known water contents (Campbell et al 1998) was applied to all TBW content estimates. To further correct for preformed water in the gut, and thus calculate the ingesta-free TBW content, we then multiplied the adjusted estimate of TBW content by a second correction factor (0.978) obtained by subtracting the average water content of the ingesta from the known TBW content of the above 15 animals.…”