Are elevated δ15N values in herbivores in hot and arid environments caused by diet or animal physiology?

P>1. Mammalian species from hot and arid environments often have elevated nitrogen isotope values compared to animals from similar trophic levels in more temperate climates. This pattern has most often been explained as the result of a physiological response by animals to heat and water stress. However, a positive correlation between rainfall and the delta 15N values of plants and herbivorous mammalian species suggests that diet may be responsible. 2. This study uses the horn keratin of desert-adapted bovids (Dorcas gazelle and Nubian ibex) to test whether the delta 15N values of herbivore body tissues are determined by heat and water stress or by the isotopic composition of their diet. The delta 15N values of horn keratin are compared against several climatic factors that affect heat and water stress to determine if a relationship exists. In addition, the range of delta 15N values measured in desert vegetation is used to evaluate the contribution of the diet to the isotopic values of bovid body tissue. 3. The delta 15N values of desert bovid horn keratin were correlated against individual climatic factors that induce water stress and were not found to be significant. When climatic factors were combined, a significant positive correlation was found between the delta 15N values of Dorcas gazelles and temperature, humidity, and rainfall. This observation contradicts the physiological stress hypothesis that predicts a negative correlation between rainfall and humidity and delta 15N values. Instead, this correlation is likely attributable to denitrification processes in the soil that directly affect the isotopic values of the plants. Values for delta 15N of horn keratin fall within the range predicted by discrimination between diet and consumer (Delta = 4 center dot 1 parts per thousand) which supports the diet hypothesis. 4. The results suggest that the isotopic composition of the diet is the dominant factor determining the delta 15N values of herbivore body tissue.