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Nitrogen isotope analysis is a common technique for investigating dietary behaviour in modern and archaeological populations. One of its primary uses is to provide trophic level information. This application is possible because of a ∼3‰ enrichment in ¹⁵N along each step in the food chain, resulting in carnivores having higher δ¹⁵N values than herbivores, which in turn have higher δ¹⁵N values than plants. Much variation has also been observed within a trophic level, although the reasons for this are poorly understood. Here we present the results of a controlled feeding study designed to test the effects of gut anatomy and dietary protein levels on hair δ¹⁵N values within a trophic level. The data reveal that mammalian herbivores eating identical diets can have hair δ¹⁵N values that differ by as much as 3.6‰. This is particularly striking as it suggests that interspecific physiological differences can lead to larger …