Controlled Diet and Climate Experiments on Nitrogen Isotope Ratios of Rats

Nitrogen isotope ratios (15N/14N) increase from plants to herbivores to carnivores and can be used to estimate the degree of carnivory in human diets. Some field studies observe a greater difference in δ15N between trophic levels in dry, hot habitats than in wet, cool ones. Two hypotheses have been proposed to explain this variation in difference in δ15N between trophic levels. (1) Elevated excretion of 15N-depleted urea in heat/water-stressed animals; (2) recycling of nitrogen on protein-deficient diets. Both predict increased diet-tissue δ15N difference under stress.

Controlled diet and climate experiments with laboratory rats were performed to test these hypotheses. Litters of six or more rats were raised to maturity on diets with protein from cow milk (casein) with known δ15N values. Diets were formulated with 5%, 20% and 70% protein. Animals on 5% protein diets had slow growth rates and low adult body weights, suggesting significant protein stress. Rats were also raised to maturity on 20% and 70% protein diets, at temperatures of 36° and 20°C, with restricted water intake or water provided ad-libitum. There were no significant differences in δ15N values of bone collagen for low versus normal or high protein diets, or for water-and/or heat-stressed versus unstressed animals. The results suggest either that rats are an inappropriate animal model for such experiments, or that the protein and heat/water stress models of nitrogen isotope variation are incorrect.

Decreases in muscle and increases in hair and collagen δ15N with age were observed in litters raised on 20% protein. These results provide partial support for the hypothesis of a relationship between δ15N and age.