The hippopotamus (Hippopotamus amphibius) or hippo (Greek: ἱπποπόταμος, hippopotamos, from ἵππος, hippos, "horse", and ποταμός, potamos, "river") is a large, mostly plant-eating mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae (the other is the Pygmy Hippopotamus). The hippopotamus is the heaviest extant artiodactyl, despite being considerably shorter than the giraffe.
The hippopotamus is semi-aquatic, inhabiting rivers and lakes where territorial bulls preside over a stretch of river and groups of 5 to 30 females and young. During the day they remain cool by staying in the water or mud; reproduction and childbirth both occur in water. They emerge at dusk to graze on grass. While hippopotamuses rest near each other 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, etc.). The common ancestor of whales and hippos split from other even-toed ungulates around 60 . The earliest known hippopotamus fossils, belonging to the genus Kenyapotamus in Africa, date to around .
The hippopotamus is recognizable for its barrel-shaped torso, enormous mouth and teeth, nearly-hairless body, stubby legs and tremendous size. It is the third-largest land mammal by weight (between 1½ and 3½ tons), behind the white rhinoceros (1½ to 4 tons) and elephants (3 to 7 tons). Despite its stocky shape and short legs, it can easily outrun a human. Hippos have been clocked at 18 mph (29 km/h) over short distances. The hippopotamus is one of the most aggressive creatures in the world and is often regarded as the most ferocious animal in Africa. There are an estimated 125,000 to 150,000 hippos remaining throughout Sub-Saharan Africa; Zambia (40,000) and Tanzania (20,000–30,000) possess the largest populations.They are still threatened by habitat loss and poaching for their meat and ivory canine teeth.