Nutritional ecology and bioenergetics of muskrats, Ondatra zibethicus, in a southern Manitoba marsh

Abstract

The aim of this study was to assess the physiological mechanisms that enable muskrats (Ondatra zibethicus) to cope with seasonal changes in nutrient quality and availability. This investigation consisted of six interrelated projects. Project I entailed 30 complete digestibility-, energy-, and nitrogen-balance trials on 6 lab-acclimated muskrats fed 5 emergent plant diets. Dry matter (DM) digestibilities ranged from 61.2 to 70.6%. Microbial fermentation of fiber accounted for $>$40% of metabolizable energy intake (MEI). Project II involved testing 32 field-acclimatized muskrats maintained on mixed diets approximating those consumed in nature. From July to December, muskrats exhibited increases in DM intake and DM, energy and fiber digestibility (P $<$ 0.05). Muskrats had difficulty maintaining nitrogen (N) balance on diets composed solely of aquatic vegetation in summer, but not during winter, when their daily N requirements were 26% lower. Projects III and IV explored two potential mechanisms that could enable muskrats to meet their seasonal N requirements. The former project examined the potential nutritional benefits derived by muskrats that supplement their diet with animal tissue. Gain in body mass, intake of DM, MEI, and N digestibility all increased with rising levels of meat consumption (P $<$ 0.0001). The main focus of project IV entailed measuring the rate of $\sp{14}$C-urea hydrolysis in 32 field-acclimatized muskrats during spring, summer, fall and winter to test whether muskrats conserve body N by recycling urea. Muskrats exhibited higher rates of urea hydrolysis (ca. 400%) and a lower serum urea N/creatinine ratio in fall and winter, compared to spring and summer (P $<$ 0.0001). The final phase of my thesis entailed a two-part study of the seasonal bioenergetics of captive and free-ranging muskrats to determine to what extent wild populations are energetically and/or nutritionally stressed. The first phase examined seasonal adjustments in gut and organ morphology, MEI, basal metabolic rate (BMR), blood chemistry, and endogenous lipid and protein stores of 94 field-acclimatized muskrats. Mass-independent BMR (kJ$\cdot$kg$\sp{0.67}\cdot$hr$\sp{-1})$ varied significantly over the year (P $<$ 0.0001), with February values $>$31% higher than those collected in July. Body lipid stores were lowest from May through September ($<$2% of body mass), increased to a peak value of 9.24 $\pm$ 0.47% in February, and then were rapidly depleted in early spring (P $<$ 0.0001). The second phase involved monitoring temporal changes in the body composition (n = 129) and forage intake (n = 33) of individually marked, free-ranging muskrats using the deuterated water technique. Daily intake of DM and MEI were significantly higher in winter (75.5-76.9 g$\cdot$kg$\sp{-0.75};$ 706.9-713.1 kJ$\cdot$kg$\sp{-0.75}).$ than during mid-summer (54.9-59.7 g$\cdot$kg$\sp{-0.75};$ 406.0-438.6 kJ$\cdot$kg$\sp{-0.75}).$ Annual adjustments in basal energy expenditure, diet selection, energy intake, blood chemistry parameters, gut and organ masses, and body lipid stores appear to be closely linked to seasonal changes in forage quality and energy and nutrient availability. (Abstract shortened by UMI.)