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Digesta Retention and Digestive Performance in Sciurid and Microtine Rodents: Effects of Hindgut Morphology and Body Size

To examine the effects of hindgut morphology and body size on the digestive performance of rodents, we fed diets of three different fiber contents (7%, 22%, and 28%) to three species of sciurids-yellow-pine chipmunks (Eutamias amoenus) (55-60 g body mass), Columbian ground squirrels (Spermophilus columbianus) (600-700 g), and hoary marmots (Marmota caligata) (2,000-3,000 g)-and to Townsend voles (Microtus townsendii) (55-60 g), a microtine rodent. All species generally maintained body mass on all three diets, except that chipmunks lost mass on the high-fiber diet. We measured intake and digestibility of the three diets and compared rates of passage of fluid digesta (marked with Co-EDTA) and large particles (marked with Cr-mordanted plant cell walls) on the 7% fiber diet. For voles, the mean retention time (MRT ± SE) of fluid (14.8 ± 1.9 h) was greater (P = 0.02, paired t-test) than that of large particles (13.1 ± 2.4 h). This indicates separation of digesta phases in the proximal colon and selective retention of fluid digesta, probably in the cecum, which means that small, highly digestible food particles as well as bacteria are preferentially held in the cecum, which results in a more efective digestion by this system. In contrast, in the three sciurids, MRTs of fluid were the same or less than those of large particles, indicating no selective fluid retention. Among all species, MRTs increased with increasing body size. Dry matter digestibility was greater in the voles than in the chipmunks for all three diets and was greater in marmots than in voles for the low-fiber diet. Our results confirm the general prediction that voles can utilize more fibrous foods than chipmunks because of separation of digesta in a more complex proximal colon and selective retention of fluid in a larger cecum. Within the Sciuridae, increasingly larger body size (and hence a larger absolute gut capacity relative to metabolic rate) offsets the disadvantage of the relatively simple sciurid hindgut.