Nitrogen isotopes in mammalian herbivores: Hair delta N-15 values from a controlled feeding study

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 similar to3% enrichment in N-15 along each step in the food chain, resulting in carnivores having higher delta N-15 values than herbivores, which in turn have higher delta N-15 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 delta N-15 values that differ by as much as 3.6%. This is particularly striking as it suggests that interspecific physiological differences can lead to larger delta N-15 values that a shift in trophic level. We also found that diet-hair fractionation was 2.3% greater when herbivores were fed high-protein (19%) than when they were fed low-protein (9%) diets. The primary nitrogen losses in mammalian herbivores are N-15-depleted urine and N-15-enriched faeces. We reason that an increase in the ratio of urinary to faecal nitrogen efflux leads to greater diet-hair fractionation on the high-protein diet.