RATIONALE: The ratios of stable nitrogen isotopes (δ¹⁵N values) in excreta have been used to examine aspects of trophic and nutritional ecology across taxa. Nitrogen fractions in feces of herbivores include endogenous (e.g., sloughed intestinal cells, unresorbed digestive secretions, and microbial debris) and dietary sources. For animals such as large herbivores, that have diets and feces with high concentrations of indigestible fiber, endogenous ¹⁵N may constrain the use of fecal δ¹⁵N values to estimate dietary δ¹⁵N values and reconstruct diets.
METHODS: We compared two techniques (detergent and detergent-free) to isolate fractions of plant fibers in the forages of caribou (Rangifer tarandus) and muskoxen (Ovibos moschatus); estimated the discrimination factors between the δ¹⁵N values of fecal fiber residues and of the diets of captive animals; and used the more effective isotopic tracer of dietary δ¹⁵N values to examine the relationships between the δ¹⁵N values of fecal residues and diet composition in several populations of wild caribou and muskoxen throughout North America in winter.
RESULTS: The detergent-based approach contaminated the fractions of plant fibers in forages and feces with ¹⁴N, whereas the detergent-free method was a good proxy to estimate δ¹⁵N values of plant fibers (r²= 0.92) and provided a better estimate of the fecal-fiber to diet discrimination factor for both species (caribou = 3.6‰; muskoxen = 2.8‰). In wild populations, the δ¹⁵N values of fecal fibers reflected diet composition in muskoxen (adjusted R²= 0.43) but not caribou (adjusted R²= 0.06).
CONCLUSIONS: Contamination from detergent residues prohibited the use of detergent extraction in isolating forage ¹⁵N from endogenous ¹⁵N in the feces of herbivores. Although δ¹⁵N values in fecal fibers can be used to track dietary δ¹⁵N values in wild herbivores, discrimination between fecal extracts and diet may vary with the contribution of endogenous nitrogen (N), and, therefore, residual endogenous ¹⁵N in feces may limit dietary reconstructions from fecal δ¹⁵N values for some large herbivores.