Effects of wetland foraging and body size on bison hair sulfur, nitrogen, and carbon isotope compositions: Implications for wildlife conservation, archaeology, and palaeoecology

Palaeoecological and archaeological reconstructions using stable isotopes rely on understanding isotopic processes in modern ecosystems. This study examines how sulfur, nitrogen, and carbon isotope values (δ³⁴S, δ¹⁵N, and δ¹³C) of modern plains and wood bison (Bison bison bison and Bison bison athabascae) hair records foraging selectivity and size-related metabolic differences in a C₃-dominated mixed woodland environment: Elk Island National Park (EINP), Alberta, Canada. Terrestrial grasses on the margins of wetlands in EINP had substantially lower δ³⁴S and higher δ¹⁵N than grasses in dry environments, signalling the potential for modern and ancient herbivore tissue δ³⁴S and δ¹⁵N values to record wetland versus dryland foraging. Bison hair δ³⁴S and δ¹⁵N differed between wood and plains bison subspecies, with lower δ³⁴S and higher δ¹⁵N values in wood bison hair suggesting they consumed more plants growing in wetland habitats. Adult females and juveniles consumed a greater proportion of wetland plants than adult males, reflecting differential group dynamics and foraging decisions. Bison body size was positively (linearly) correlated with hair δ¹³C and negatively (non-linearly) correlated with hair δ¹⁵N, but uncorrelated with hair δ³⁴S. We hypothesize that increased body size requires greater reliance on gut microbial proteins to build body tissues (increasing hair δ¹³C), and that larger-bodied individuals have greater nitrogen use efficiency and lower-quality diets (decreasing hair δ¹⁵N). Our results suggest that (palaeo)ecological and archaeological researchers should consider the extent to which habitat selectivity and metabolic effects related to sex, age and body size can influence herbivore tissue isotopic compositions.