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

Carbon isotope diVerences ( 13C) between bioapatite and diet, collagen and diet, and bioapatite and collagen were calculated for four species of sirenians, Dugong dugon (Müller), 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 13C values were compared with those for terrestrial ungulates, carnivores, and six species of carnivorous marine mammals (cetaceans = 1; pinnipeds = 4; mustelids = 1). SigniWcant diVerences in mean 13C values among species for all tissue types were detected that separated species or populations foraging on freshwater plants or attached marine macroalgae ( 13C values < ¡6‰; Cbioapatite–diet »14‰) from those feeding on marine seagrasses ( 13C values > ¡4‰; Cbioapatite–diet »11‰). Likewise, Cbioapatite–collagen values for freshwater and algal-foraging species (»7‰) were greater than those for seagrass-foraging species (»5‰). Variation in 13C 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 diVerent types of plants. These results highlight the complications that can arise when making dietary interpretations without having Wrst determined species-speciWc Ctissue–diet values.