The hippopotamus (Hippopotamus amphibius), from the Greek ‘ιπποπόταμος (hippopotamos, hippos meaning "horse" and potamos meaning "river"), 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 a semi-aquatic mammal, 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 mya. The earliest known hippopotamus fossils, belonging to the Kenyapotamus in Africa, date to around 16 mya.
The hippopotamus is recognizable for its barrel-shaped torso, hairless body, stubby legs and tremendous size; similar in size to the White Rhinoceros, only elephants are consistently larger. Despite its popularity in zoos and cuddly portrayal in fiction, the hippopotamus is among the most dangerous and aggressive of all mammals. Although there are an estimated 125,000 to 150,000 hippos throughout Sub-Saharan Africa, they are still threatened by poaching and habitat loss.