Do δ15N and δ13C values of feces reflect the isotopic composition of diets in small mammals?

The stable isotope composition of carbon and nitrogen in feces can be a useful tool for reconstructing diet. To examine whether the isotopic composition of feces reflect those of diet, we determined the fractionation of 15 N and 13 C along the digestive tracts of several species of small mammals. There were significant differences in the δ 15 N values of digesta in different compartments of the gastrointestinal (GI) tract, with consistent enrichment of 15 N in the stomach, intestine, and cecum, and the depletion of 15 N in the colon (i.e., feces). Although feces had lower δ 15 N values than digesta in the cecum, feces of small mammals were significantly more enriched in 15 N (by ~2.5‰) than diet. The mechanisms causing this enrichment in the GI tract may arise from the operation of different biochemical pathways within the different GI compartments. The stable carbon isotope composition of digesta in small mammals were similar along the GI tract, but the δ 13 C values of digesta and feces were significantly lower than reported previously for large ungulates (–3.5‰ vs. –0.5‰). The δ 15 N and δ 13 C values of feces did not directly reflect the isotopic compositions of these mammals’ diet. Our data offer evidence for variations in isotopic discrimination effects. This variation can probably be ascribed to metabolism of different body sizes of mammals and the complexities linked with digestive physiology of herbivores.