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

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 C-3 lucerne (Medicago sativa L.) and C-4 grass (Themeda triandra Forssk.). Although blood and faecal delta C-13 values were broadly consistent with diet, results indicate mismatch between consumer and diet isotope compositions: both materials overrepresented the C-3 (lucerne) component of diets. Lucerne had lower fibre digestibility than T. triandra, which explains the results for faeces, whereas underrepresentation of dietary C-4 in blood is consistent with low protein content of the grass hay. A diet switch experiment revealed an important difference in C-13-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.