Freezer meals: comparative value of muskrat (Ondatra zibethicus) push-ups as late-winter forage for a northern ungulate

Abstract

One of the more unique aspects of the natural history of muskrats (Ondatra zibethicus) is that during early winter they construct push-ups—piles of frozen vegetation—on the surface of frozen lakes. Push-ups are a potentially valuable source of relatively high-quality forage for northern ungulates, which typically become increasingly nutritionally deprived during winter. Because of concerns by local people that reintroduced bison (Bison bison) were eating muskrat push-ups and causing a decline in some local populations, we assessed the comparative value of push-ups as late-winter forage for bison. We collected vegetation from 18 push-ups and 18 shoreline sites (i.e. wet sedge meadows), distributed equally from 3 lakes, and used wet chemistry techniques to assess their nutritional quality (i.e. percent crude protein, neutral detergent fibre, lignin, and total digestible nutrients, plus major and trace minerals). General linear models revealed that most variables assessed differed significantly by SITE (push-up versus shoreline; 13 of 15 variables), LAKE (3 lakes; 9 of 15), and their interaction (SITE*LAKE; 11 of 15). Our data suggest that muskrat push-ups may provide a supplement to winter forage used by bison in our study area by helping to boost protein intake as well as providing minerals that may be deficient in their late-winter diets, such as Ca, Fe, and Cu. As such, bison should seek push-ups in late winter to supplement their diet and improve their nutritional condition at a time when they may be seasonally impoverished. Our study also demonstrates that nutritional quality of push-ups varies significantly between nearby lakes—likely due to their unique biogeochemistry. As such, muskrat push-ups on some lakes may be more favoured by bison than those on other lakes. Whether our results may be extended to other species of northern ungulates or other ecological regions, however, awaits testing.