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Isotopic nitrogen in fecal fiber as an indicator of winter diet in caribou and muskoxen

February 18, 2014

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

Publication Year 2014
Title Isotopic nitrogen in fecal fiber as an indicator of winter diet in caribou and muskoxen
DOI 10.1002/rcm.6825
Authors David D. Gustine, Perry S. Barboza, Jennifer Addison, Rachel Shively, Lola Oliver
Publication Type Article
Publication Subtype Journal Article
Series Title Rapid Communications in Mass Spectrometry
Index ID 70094149
Record Source USGS Publications Warehouse
USGS Organization Alaska Science Center