The Hippopotamus (Hippopotamus amphibius), or hippo, from the ancient Greek for "river horse" (ἱπποπόταμος), is a large, mostly herbivorous mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae (the other is the pygmy hippopotamus). After the elephant and rhinoceros, the 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, their closest living relatives are cetaceans (whales, 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.
Hippopotamuses are among the largest living mammals; only elephants and some rhinoceroses and whales are heavier. They are considered megafauna, but unlike all other African megafauna, they have adapted for a semiaquatic life in freshwater lakes and rivers. Hippos measure 3.3 to 5.2 m (11 to 17 ft) long, including a tail of about 56 cm (1.84 ft) in length and average about 1.5 m (4.9 ft) tall at the shoulder. Because of their enormous size, hippopotamuses are difficult to weigh in the wild. Most estimates of the weight come from culling operations that were carried out in the 1960s. The average weights for adult males ranged between 1,500 and 1,800 kg (3,300 and 4,000 lb). Females are smaller than their male counterparts, with average weights between 1,300 and 1,500 kg (2,900 and 3,300 lb). Older males can get much larger, reaching at least 3,200 kg (7,100 lb) with a few exceptional specimens exceeding 3,600 kg (7,900 lb). The heaviest known hippopotamus weighed approximately 4,500 kg (9,900 lb). Male hippos appear to continue growing throughout their lives; females reach a maximum weight at around age 25. The range of hippopotamus sizes overlaps broadly with those recorded for the white rhinoceros. While the rhinoceros reportedly is heavier on average, very large male hippopotamuses are around the same mass as the largest white rhinos. Compared to a rhinoceros, the hippopotamus has a more elongated body and shorter legs, which result in the hippopotamus having a longer overall length and shorter standing height on average. The Indian rhinoceros also attains weights that can rival those of both hippopotamuses and white rhinoceros, and the three species rank only after elephants in overall body size among extant land animals.
Hippopotamus amphibius was widespread in North Africa and Europe during the Eemian and late Pleistocene until about 30,000 years ago. Archaeological evidence exists of its presence in the Levant, dating to less than 3,000 years ago. The species was common in Egypt's Nile region during antiquity, but has since been extirpated. Pliny the Elder writes that, in his time, the best location in Egypt for capturing this animal was in the Saite nome; the animal could still be found along the Damietta branch after the Arab Conquest in 639. Hippos are still found in the rivers and lakes of the northern Democratic Republic of the Congo, Uganda, Tanzania and Kenya, north through to Ethiopia, Somalia and Sudan, west to Gambia, and south to South Africa. They inhabit both savanna and forest areas.
Hippopotamuses are by nature very aggressive animals. Hippos involved in attacks on other animals are often either mature bulls, which tend to be very territorial and indiscriminately ill-tempered, or females, which are quite protective of their young calves. Living on the African continent, hippopotamus coexist with a variety of formidable predators. Nile crocodiles, lions and spotted hyenas are known to prey on young hippos. However, due to their ill temperament and great size, adult hippopotamus are not usually subject to predation by other animals, except humans. Cases where very large lion prides or cooperating groups of Nile crocodiles have successfully preyed on adult hippopotamus have been reported, but this is typically believed to be exceptionally rare. Crocodiles are particularly frequent targets of hippo aggression, likely because they often inhabit the same riparian habitats as hippos. Crocodiles may be either aggressively displaced or killed by hippopotamuses. Hippos are also very aggressive towards humans, whom they commonly attack whether in boats or on land with no apparent provocation. They are widely considered to be one of the most dangerous large animals in Africa.