The diet of African hippopotamids can be documented through stable carbon isotope ratios (13C/12C) analyses of enamel and other tissues. The common hippopotamus Hippopotamus amphibius is widely assumed to be a pure grazer; however, the 13C/12C ratios of modern H. amphibius show a higher fraction of dietary C3 biomass than estimated from traditional observations. Isotope profiles of modern hair and modern tooth enamel confirm that H. amphibius has a variable diet in both the short- (seasonal) and long- (sub-decadal) time scales. Isotopic analyses of extant mammals from the same parks as the analyzed hippos provide comparative examples for diets of C3-browsers and C4-grazers. Oxygen isotope ratios (18O/16O) show that the hippo is consistently the most 18O-depleted mammal in any one ecosystem; this directly reflects its semi-aquatic habitat.