| The hippopotamus (Hippopotamus amphibius), from the Greek (hippopotamos, hippos meaning 'horse' and potamos meaning 'river'), often shortened to 'hippo', is a large, mostly plant-eating African mammal, one of only two extant species in the family Hippopotamidae (the other being the Pygmy Hippopotamus). The hippopotamus is semi-aquatic, inhabiting rivers and lakes in sub-Saharan Africa in large groups of up to 40 hippos. During the day they remain cool by staying in the water or mud; reproduction and childbirth both occur in water, where territorial bulls preside over a stretch of river. They emerge at dusk to graze on grass. While hippos rest near each other in territories in the water, grazing is a solitary activity and hippos are not territorial on land. Despite their physical resemblance to pigs and other terrestrial even-toed ungulates, their closest living relatives are cetaceans—whales, porpoises and the like. The common ancestor of whales and hippos split from other even-toed ungulates around 60 million years ago. The earliest known hippopotamus fossils, belonging to the genus Kenyapotamus in Africa, date to around 16 million years ago. | 
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