Parts of a whole: isotopic difference between single keratin-based tissues and whole-body tissues of birds and mammals

Stable isotope analysis is a frequently used tool in ecology for unraveling dietary dynamics, physiology, and trophic relationships. In studies that use stable isotope values in mixing models to infer diet, a single tissue is commonly assumed to represent the isotopic composition of a source whole organism. In this study, we examine that assumption in keratin-based tissues for consideration in isotopic diet studies of animals that consume prey whole. By comparing carbon (δ13C) and nitrogen (δ15N) stable isotope values in single tissues of mammals and bird sources, this study aims to determine if non-invasive sampling methods in tissues such as hair and feathers accurately represent the overall isotopic composition of four mammalian species (rats, mice, rabbits, guinea pigs) and one avian species (quail). We found that keratin may be a good proxy in some cases, but keratin was not a universally accurate representation of whole-body isotopic values. Keratin-based tissue and the homogenized whole-body samples were significantly different in δ13C values in mammals (mean 𝚫13Ckeratin-whole-body = –1.93 ± 0.37 ‰), and feathers differed significantly from whole-body in δ15N values in birds (mean 𝚫15Nkeratin-whole-body = –0.32 ± 0.11 ‰). We suggest homogenizing diet sources and maintaining the lipid content especially for animals consuming whole prey. We recommend that future studies reconsider the assumption that non-invasively sampled tissues are representative of whole-body isotopic values and urge researchers to explicitly account for the true diet of their study organism.