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 dige stive performance of rodents, we fed diets of three different fiber co ntents (7%, 22%, and 28%) to three species of sciurids-yellow-pine chi pmunks (Eutamias amoenus) (55-60 g body mass), Columbian ground squirr els (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-f iber 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 lar ge particles (13.1 +/- 2.4 h). This indicates separation of digesta ph ases in the proximal colon and selective retention of fluid digesta, p robably in the cecum, which means that small, highly digestible food p articles as well as bacteria are preferentially held in the cecum, whi ch results in a more effective 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 digesti bility was greater in the voles than in the chipmunks for all three di ets and was greater in marmots than in voles for the low-fiber diet. O ur results confirm the general prediction that voles can utilize more fibrous foods than chipmunks because of separation of digesta in a mor e complex proximal colon and selective retention of fluid in a larger cecum. Within the Sciuridae, increasingly larger body size (and bence a larger absolute gut capacity relative to metabolic rate) offsets the disadvantage of the relatively simple sciurid hindgut.