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The hippopotamus (Hippopotamus amphibius), from the Greek ἱπποπόταμος (hippopotamos, ιππος hippos meaning "horse" and πόταμος potamus 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 groups of 5-30 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, porpoise, etc.). The common ancestor of whales and hippos split from other even-toed ungulates around Mya: million years ago. The earliest known hippopotamus fossils, belonging to the genus Kenyapotamus in Africa, date to around Mya: million years ago.

The hippopotamus is recognizable for its barrel-shaped torso, enormous mouth and teeth, hairless body, stubby legs and tremendous size. It is similar in size to the White Rhinoceros; only elephants are consistently heavier. Despite its stocky shape and short legs, it can easily outrun a human. Hippos have been clocked at while running short distances, faster than an Olympic sprinter. The hippopotamus is one of the most aggressive animals in the world, and are often regarded as the most dangerous animal in Africa. There are an estimated 125,000 to 150,000 hippos remaining throughout Sub-Saharan Africa, of which Zambia (40,000) and Tanzania (20,000-30,000) have the largest populations . They are still threatened by poaching for their meat and ivory canine teeth, and by habitat loss.

Taxonomy and origins

Hippopotamuses (plural hippopotami is also used ; hippos can be used as a short plural) are gregarious, living in groups of up to 40 animals; such a group is called a pod, herd, dale, or bloat. A male hippopotamus is known as a bull, a female as a cow, and a baby as a calf. They are also known as the Common Hippopotamus or the Nile Hippopotamus.

The hippopotamus is the type genus of the family Hippopotamidae. The Pygmy Hippopotamus belongs to a different genus in Hippopotamidae, either Choeropsis or Hexaprotodon. Hippopotamidae are sometimes known as Hippopotamids. Sometimes the sub-family Hippopotaminae is used. Further, some taxonomists group hippopotamuses and anthracotheres in the super-family Anthracotheroidea or Hippopotamoidea.