SEASONAL-CHANGES IN WATER FLUX, FORAGE INTAKE, AND ASSIMILATED ENERGYOF FREE-RANGING MUSKRATS

Knowledge of the seasonal energy and forage requirements of free-rangi ng muskrats (Ondatra zibethicus) is essential for evaluating the habit at requirements and potential effect of this species on aquatic vegeta tion. We measured rates of water influx with the deuterated water (D2O ) technique and assessed the accuracy of this method for estimating dr y-matter intake (DMI) of captive muskrats fed a natural diet of cattai l (Typha latifolia) rhizomes. Water influx in laboratory feeding trial s was highly variable, ranging from 97 to 430 mL.kg(-1).day(-1) ((x) o ver bar = 243.5 +/- 24.0). Over this range, DMI estimated from water i nflux exceeded measured DMI by an average of 52.2%. This error was red uced substantially as water influx increased, with a mean error of onl y 9.2% when water influx averaged 349 mL.kg(-1).day(-1) (DMI > 40 g.kg (-1).day(-1)). At rates of water influx obtained under field condition s (423-915 mL.kg(-1).day(-1)). the error in estimating DMI is predicte d to be <10%. We obtained 33 seasonal estimates of the daily intake of water, fresh vegetation, dry matter (DM), and assimilated energy (AE) of 27 free-ranging muskrats. Water influx and consumption of fresh ve getation were highest from spring through fall. However, because of th e lower water content and higher digestibility of the winter diet, dai ly intake of DM and AE were higher (P < 0.05) in winter (76.9 g/kg(0.7 5), 713.1 kJ/kg(0.75)) than midsummer (54.9 g/kg(0.75), 438.6 kJ/kg(0. 75)). If corrections are made far wastage and use of vegetation for ho use construction, these food consumption estimates can be used to asse ss the potential effect of muskrats on the primary productivity of pra irie marsh ecosystems.