Hippopotamus (Hippopotamus amphibius). Isimangaliso Wetland Park (Greater St Lucia Wetland Park). KwaZulu Natal. South Africa. The hippo, from the ancient Greek for "river horse" ), is a large, mostly herbivorous mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae. The hippopotamus is the third largest land animal (after the elephant and the white rhinoceros) and 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.) from which they diverged about 55 million years ago.