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When animals are not quite what they eat: diet digestibility influences 13C-incorporation rates and apparent discrimination in a mixed-feeding herbivore

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The stable carbon isotope composition of animal tissues represents the weighted sum of the variety of food sources eaten. If sources differ in digestibility, tissues may overrepresent intake of more digestible items and faeces may overrepresent less digestible items. We tested this idea using whole blood and faeces of goats (Capra hircus L., 1758) fed different food mixtures of C3 lucerne (Medicago sativa L.) and C4 grass (Themeda triandra Forssk.). Although blood and faecal δ13C values were broadly consistent with diet, results indicate mismatch between consumer and diet isotope compositions: both materials overrepresented the C3 (lucerne) component of diets. Lucerne had lower fibre digestibility than T. triandra, which explains the results for faeces, whereas underrepresentation of dietary C4 in blood is consistent with low protein content of the grass hay. A diet switch experiment revealed an important difference in 13C-incorporation rates across diets, which were slower for grass than lucerne diets, and in fact equilibrium states were not reached for all diets. Although more research is needed to link digestive kinetics with isotope incorporation, these results provide evidence for nonlinear relationships between consumers and their diets, invoking concerns about the conceptual value of “discrimination factors” as the prime currency for contemporary isotope ecology.
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Document Type: Research Article

Affiliations: 1: Clinic for Zoo Animals, Exotic Pets and Wildlife, Vetsuisse Faculty, University of Zurich, Winterthurerstrasse 260, CH-8057 Zürich, Switzerland. 2: School of Biological and Conservation Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, 3209, Republic of South Africa. 3: Department of Anthropology, University of Colorado at Boulder, Boulder, CO 80309, USA. 4: Geological Institute, ETH-Zürich, Sonneggstrasse 5, CH-8092 Zürich, Switzerland. 5: Clinic for Zoo Animals, Exotic Pets and Wildlife, Vetsuisse Faculty, University of Zurich, Winterthurerstrasse 260, CH-8057 Zürich, Switzerland.

Publication date: 2011-05-02

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