Diet induced differences in carbon isotope fractionation between sirenians and terrestrial ungulates

Carbon isotope differences (Delta C-13) between bioapatite and diet, collagen and diet, and bioapatite and collagen were calculated for four species of sirenians, Dugong dugon (Muller), Trichechus manatus (Linnaeus), Trichechus inunguis (Natterer), and the extinct Hydrodamalis gigas (Zimmerman). Bone and tooth samples were taken from archived materials collected from populations during the mid eighteenth century (H. gigas), between 1978 and 1984 (T. manatus, T. inunguis), and between 1997 and 1999 (D. dugon). Mean Delta C-13 values were compared with those for terrestrial ungulates, carnivores, and six species of carnivorous marine mammals (cetaceans = 1; pinnipeds = 4; mustelids = 1). Significant differences in mean delta C-13 values among species for all tissue types were detected that separated species or populations foraging on freshwater plants or attached marine macroalgae (delta C-13 values < -6%o; Delta C-13(bioapatite-diet) similar to 14%o) from those feeding on marine seagrasses (delta C-13 values > -4%o; Delta C-13(bioapatite-diet) similar to 11%o). Likewise, Delta C-13(bioapatite-collagen) values for freshwater and algal-foraging species (similar to 7%o) were greater than those for seagrass-foraging species (similar to 5%o). Variation in Delta C-13 values calculated between tissues and between tissues and diet among species may relate to the nutritional composition of a species' diet and the extent and type of microbial fermentation that occurs during digestion of different types of plants. These results highlight the complications that can arise when making dietary interpretations without having first determined species-specific Delta C-13(tissue-diet) values.