An experimental study of carbon-isotope fractionation between diet, hair, and feces of mammalian herbivores

The carbon-isotope composition of hair and feces offers a glimpse into the diets of mammalian herbivores. It is particularly useful for determining the relative consumption of browse and graze in tropical environments, as these foods have strongly divergent carbon-isotope compositions. Fecal delta(13)C values reflect the last few days consumption, whereas hair provides longer term dietary information. Previous studies have shown, however, that some fractionation occurs between dietary delta(13)C values and those of hair and feces. Accurate dietary reconstruction requires an understanding of these fractionations, but few control led-feeding studies have been undertaken to investigate these fractionations in any mammalian taxa, fewer still in large mammalian herbivores. Here, we present data from the first study of carbon-isotope fractionation between diet, hair, and feces in multiple herbivore taxa. All taxa were fed pure alfalfa (Medicago sativa) diets for a minimum period of 6 months, at which point recently grown hair was shaved and analyzed for carbon isotopes. The mean observed diet-hair fractionation was +3.2parts per thousand, with a range of +2.7 to +3.5parts per thousand. We also examined diet-feces fractionation for herbivores on alfalfa and bermudagrass (Cynodon dactylon) feeds. The mean diet-feces fractionation for both diets was -0.8parts per thousand, with less fractionation for alfalfa (-0.6parts per thousand) than bermudagrass (-1.0parts per thousand). Fecal carbon turnover also varies greatly between taxa. When diets were switched, horse (Equus caballus) feces reflected the new diet within 60 h, but alpaca (Lama pacos) feces did not equilibrate with the new diet for nearly 200 h. Thus, fecal carbon isotopes provide far greater dietary resolution for hindgut-fermenting horses than foregut-fermenting alpacas.