SEASONAL-CHANGES IN GUT MASS, FORAGE DIGESTIBILITY, AND NUTRIENT SELECTION OF WILD MUSKRATS (ONDATRA-ZIBETHICUS)

The aim of this study was to determine whether seasonal variability in diet quality or cold stress is accompanied by compensatory changes in nutrient selection, energy intake, and digestive capacity of seasonal ly acclimatized muskrats. We hypothesized that in summer, muskrats mee t their energy and nutrient requirements by selectively consuming high -protein low-fiber aquatic plants. We also predicted that muskrats use fiber as an important energy source during those periods of the year when the nutritional value and diversity of forage species are lowest. At such rimes, muskrats should be most dependent on microbial ferment ation and should exhibit maximal gut size and digestive efficiency. As predicted, muskrats offered natural forage increased the fraction of protein while reducing the proportion of fiber in their diet during su mmer, but not during spring or winter digestibility trials. From July to December, muskrats exhibited increases in dry matter intake, gut ma ss, and forage digestibility. The increase in hindgut mass was accompa nied by an 18.5% rise in neutral detergent fiber digestibility, while the proportion of digestible energy derived from the fermentation of f iber increased from 38.4% in July to 53.2% in December. During winter, muskrats were able to reduce their dietary nitrogen requirements by 2 6.0%. Our results suggest that changes in the absorptive surface area and volume of the gut are important adaptations for promoting nutrient assimilation during periods when muskrats are challenged by both high maintenance costs and a limited choice of diets.