Hippos are born with sterile intestines, and require bacteria obtained from their mothers' feces to digest vegetation. Hippos are also very aggressive towards humans, whom they sometimes attack whether in boats or on land, commonly with no apparent provocation, and are widely considered to be one of the most dangerous large animals in Africa. Hippos mark their territory by defecation.
Until 1909, naturalists grouped hippos with pigs, based on molar patterns. Several lines of evidence, first from blood proteins, then from molecular systematics and DNA and the fossil record, show that their closest living relatives are cetaceans – whales, dolphins and porpoises.