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Digestion and assimilation strategies of herbivorous mammals are diverse but fall into two major categories (Moir, 1968; Gartner and Pfaff, 1979; Hume and Warner, 1980). The most complex of which occurs in the lagomorphs and has been well studied. In contrast rodents exhibit great variability in diet and nutritional biology (Landry, 1970; Baker, 1971; Kenagy and Hoyt, 1980). Early reports of coprophagy by rodents were incidental or descriptive, and indicated it was infrequent (Howell and Gersh, 1935; Ingles, 1961; Wilkes, 1962; Hoover et al., 1969; Jarvis, 1981). Rats mechanically prevented from reingesting feces showed reduced growth rates (Barnes et al., 1963). Presumably, some nutritive elements unavailable in the diet (such as B-complex vitamins and amino acids) are supplied through ingestion of products synthesized by endoflora harbored in the cecum of the lower digestive tract (Fridericia et al., 1927; Daft et al., 1963; Fitzgerald et al., 1964). Unfortunately, these micronutrients have not been determined with precision (Barnes, 1962; Barnes et al., 1963), but the nutritional benefits of corophagy are probably quite similar for both rodents and lagomorphs (Kenagy and Hoyt, 1980). Coprophagy in rodents is considered to be nutritionally beneficial, yet only a small amount of work has attempted to indicate the extent to which it occurs or the accompanying digestive mechanism in a rodent species (Kenagy and Hoyt, 1980). A recent investigation does indicate that reingestion behavior appears to be most frequent in herbivorous species, such as the microtines (Kenagy and Hoyt, 1980). Microtine rodents subsist primarily on forbs and grasses (Batzli and Cole, 1979) but do eat a wide variety of forage types (Zimmerman, 1965; Fleharty and Olson, 1969; Gill, 1977). Although few studies have closely examined the nutritional value of microtine forages, some dietary components appear to fluctuate with growing season (Cole and Batzli, 1979; Servello, 1981). Additionally, the nutritional quality of available forages may be affected by habitat manipulation (Cengel et al., 1978). Few researchers have considered a relationship between changes in diet quality and the use of the cecum-coprophagy system found by rodents. Since free ranging animals may have a highly unpredictable diet which varies in quality over time, a mechanism compensating for dietary changes may involve coprophagy and postgastsric fermentation processes. This study examined coprophagic behavior in response to diet quality for the meadow vole (Microtus pennsylvanicus) and the pine vole (Microtus pinetorm). The nutritional response of these animals to high and low quality diets was assessed after the coprophagic component was eliminated from the digestive process. The nature of the nutritional response was determined by measuring food consumption, fecal production, diet digestibility, energy intake, and body weight dynamics.