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

Abstract

Knowledge of the seasonal energy and forage requirements of free-ranging muskrats (Ondatra zibethicus) is essential for evaluating the habitat requirements and potential effect of this species on aquatic vegetation. We measured rates of water influx with the deuterated water (D 2 O) technique and assessed the accuracy of this method for estimating dry-matter intake (DMI) of captive muskrats fed a natural diet of cattail (Typha latifolia) rhizomes. Water influx in laboratory feeding trials was highly variable, ranging from 97 to 430 mL.kg -1 .day -1 (x = 243.5 ± 24.0). Over this range, DMI estimated from water influx exceeded measured DMI by an average of 52.2%. This error was reduced substantially as water influx increased, with a mean error of only 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 conditions (423-915 mL.kg -1 .day -1 ), the error in estimating DMI is predicted 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 vegetation were highest from spring through fall. However, because of the lower water content and higher digestibility of the winter diet, daily intake of DM and AE were higher (P < 0.05) in winter (76.9 g/kg 0.75 , 713.1 kJ/kg 0.75 ) than midsummer (54.9 g/kg 0.75 , 438.6 kJ/kg 0.75 ). If corrections are made for wastage and use of vegetation for house construction, these food consumption estimates can be used to assess the potential effect of muskrats on the primary productivity of prairie marsh ecosystems.