Stable isotope ratio analysis to differentiate temporal diets of a free-ranging herbivore

Stable isotope ratio analysis (SIRA) of carbon (delta13C) and nitrogen (delta15N) in tissue samples of herbivores can identify photosynthetic pathways (C3 vs. C4) of plants consumed. We present results from free-ranging Rocky Mountain elk (Cervus elaphus) that highlight the ability to differentiate diets using tissue delta13C and delta15N. The signatures of delta13C and delta15N differed in tissues of varying metabolic activity: muscle, a short-term dietary indicator (i.e., 1-2 months) and hoof, a long-term dietary indicator (i.e., 3-12 months). We also documented that delta13C and delta15N values along elk hooves (proximal, middle, distal sections) elucidated temporal shifts in dietary selection. The carbon isotopes of the composite hoof were similar to those of the middle section, but the composite hoof differed in delta(13)C from the distal and proximal sections. The delta13C and delta15N signatures also differed among elk populations, indicating temporal dietary shifts of individuals occupying disparate native range and human-derived agricultural landscapes. Analyses of stable isotopes in various tissues highlighted carbon and nitrogen assimilation through time and differences in the foraging ecology of a rangeland herbivore.