Influence of diet on the distribution of nitrogen isotopes in animals

The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition.The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ₁₅N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ¹⁵N values. The variability of the relationship between the δ¹⁵N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ¹⁵N relative to the diet, with the difference between the δ¹⁵N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ¹⁵N values ofcollagen and chitin. biochemical components that are often preserved in fossil animal remains, are also related to the δ¹⁵N value of the diet.The dependence of the δ¹⁵N values of whole animals and their tissues and biochemical components on the δ¹⁵N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ¹⁵N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources.The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ¹⁵C and δ¹⁵N values of bone collagen suggest that C₄ and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.