Diets of modern and late Miocene hippopotamids: Evidence from carbon isotope composition and micro-wear of tooth enamel

Carbon isotope composition and micro-wear analyses of tooth enamel were used to reconstruct the diet of late Miocene hippopotamids unearthed in the Toros-Ménalla area, Chad, contemporary to the oldest known hominids. A large sample of wild modern Hippopotamus amphibius from various locations in Africa was also analysed for comparison. Isotopic analyses showed that the modern hippo, reputedly a strict grazer, has a more varied diet than usually thought, including a significant amount of C3 plants in closed to moderately open environments. Enamel formed before weaning was on average 3‰ depleted in 13C compared to post-weaning enamel, a pattern that could be partially explained by milk consumption. The observed micro-wear pattern of the modern hippo differs from that of other modern ungulates. We explain the very abundant fine scratches and small pits found on these hippo teeth by the preference for fresh short grasses with low silicon content. The diet of the late Miocene hippopotamid was probably close to that of the modern Hip. amphibius, but included a larger amount of C3 plants. This contradicts previous palaeoecological findings based on relative hypsodonty degree and indicates that the modern feeding behaviour of large hippos was already developed at the end of the Miocene, when C4 grass exploitation by large ungulates became much more frequent in Africa. Finally, it also indicates that C4 grasses were a significant component in late Miocene environments of Central Africa.