The common hippopotamus (Hippopotamus
amphibius), or hippo, is a large, mostly herbivorous mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae, the other being the pygmy hippopotamus (Choeropsis
liberiensis or Hexaprotodon liberiensis). The name comes from
the ancient Greek for "river
horse" (ἱπποπόταμος). After the elephant and rhinoceros, the common hippopotamus is the third-largest type of land mammal and the heaviest
extant artiodactyl. Despite their physical
resemblance to pigs and other terrestrial even-toed ungulates, the closest living relatives of the Hippopotamidae are cetaceans (whales, dolphins, porpoises, etc.) from which they
diverged about 55 million years ago. 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.
Common hippos are recognisable by their barrel-shaped
torsos, wide-opening mouths revealing large canine tusks, nearly hairless
bodies, columnar legs and large size; adults average 1,500 kg
(3,310 lb) and 1,300 kg (2,870 lb) for males and females
respectively, making them the largest species of land mammal after the three
species of elephant and the white and Indian rhinoceros. Despite its stocky shape and short legs, it is capable
of running 30 km/h (19 mph) over short distances. The hippopotamus is
a highly aggressive and unpredictable animal and is ranked among the most
dangerous animals in the world. Nevertheless, they are still threatened
by habitat loss and poaching for their meat and ivory canine
teeth.
The common hippopotamus is semiaquatic, inhabiting rivers, lakes and mangrove swamps, where territorial bulls preside over a
stretch of river and groups of five to thirty 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 grasses. While
hippopotamuses rest near each other in the water, grazing is a solitary
activity and hippos are not territorial on land.