In bone, the spacing between δ13C in collagen and bioapatite carbonate is greater in herbivores than carnivores, with implications for understanding animal dietary ecology from surviving hard tissues. Two explanations have been proposed: varying diet composition or differences in physiology between herbivores and carnivores.

We measured the isotopic effects of carnivorous and herbivorous diets on a single species, to test the effect of diet composition alone. Protein δ13C and δ15N and carbonate δ13C were measured on egg and bone from hens on different diets. Herbivorous hens had a +14.3‰ spacing between egg albumen and shell δ13C, compared to +12.4‰ for omnivorous hens, and +11.5‰ for carnivorous hens. Bioapatite-collagen Δ13C spacing was measured as +6.2‰ for herbivorous hens, and calculated as +4.3‰ for omnivorous hens, and +3.4‰ for carnivorous hens – similar to observed mammalian herbivore and carnivore bioapatite-collagen Δ13C differences.

We conclude that a shift in diet composition from herbivory to carnivory in a single species does alter the bioapatite-collagen carbon isotopic spacing. Our data strongly suggest that this results from differences in the Δ13Cbioapatite-diet spacing, and not that of Δ13Ccollagen-diet.