We use the oxygen isotopic composition of tooth enamel from multiple mammalian taxa across eastern Africa to present a proxy for aridity. Here we report tooth enamel δ ¹⁸ O values of 14 species from 18 locations and classify them according to their isotopic sensitivity to environmental aridity. The species are placed into two groups, evaporation sensitive (ES) and evaporation insensitive (EI). Tooth enamel δ ¹⁸ O values of ES animals increase with aridity, whereas the tooth enamel δ ¹⁸ O values of EI animals track local meteoric water δ ¹⁸ O values, demonstrating that bioapatite δ ¹⁸ O values of animals with different behaviors and physiologies record different aspects of the same environment. The enrichment between tooth enamel δ ¹⁸ O values of ES and EI animals records the degree of ¹⁸ O enrichment between evaporated water (ingested water or body water) and source water, which increases with environmental aridity. Recognition of the ES–EI distinction creates the opportunity to use the ¹⁸ O composition of bioapatite as an index of terrestrial aridity.