The hippopotamus (/ˌhɪpəˈpɒtəməs/ HIP-ə-POT-ə-məs;[3] Hippopotamus amphibius), also called the hippo, common hippopotamus or river hippopotamus, is a large, mostly herbivorous, semiaquatic mammal and ungulate native to sub-Saharan Africa. It is 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, both of which are found in Africa, the hippopotamus is the third-largest type of land mammal and the heaviest extant artiodactyl (in the traditional, non-cladistic sense of the term, not including cetaceans). 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.

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) for males and 1,300 kg (2,870 lb) for females. Despite its stocky shape and short legs, it is capable of running 30 km/h (19 mph) over short distances.

Hippos inhabit rivers, lakes, and mangrove swamps, where territorial males preside over a stretch of river and groups of five to thirty females and young hippos. 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 hippos rest near each other in the water, grazing is a solitary activity and hippos are not territorial on land. The hippo is among the most dangerous animals in the world due to its highly aggressive and unpredictable nature. They are threatened by habitat loss and poaching for their meat and ivory canine teeth.


Classification
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 subfamily Hippopotaminae is used. Further, some taxonomists group hippos and anthracotheres in the superfamily Anthracotheroidea.[8]:39 Hippopotamidae are classified along with other even-toed ungulates in the order Artiodactyla. Other artiodactyls include camels, cattle, deer and pigs, although hippos are not closely related to these groups.


Detail of the head
Five subspecies of hippos have been described based on morphological differences in their skulls and geographical differences:[8]:3

Great northern hippopotamus or Nile hippopotamus H. a. amphibius – (the nominate subspecies) which stretched from Egypt, where they are now extinct, south up the Nile River to Tanzania and Mozambique
East African hippopotamus H. a. kiboko – in Kenya in the African Great Lakes region, and in Somalia in the Horn of Africa. Broader nasals and more hollowed interorbital region
Cape hippopotamus or South African hippopotamus H. a. capensis – from Zambia to South Africa, most flattened skull of the subspecies
West African hippopotamus or Tchad hippopotamus H. a. tschadensis – throughout Western Africa to, as the name suggests, Chad, slightly shorter and wider face, with prominent orbits
Angola hippopotamus H. a. constrictus – in Angola, the southern Democratic Republic of Congo and Namibia, named for its deeper preorbital constriction
The suggested subspecies were never widely used or validated by field biologists; the described morphological differences were small enough that they could have resulted from simple variation in nonrepresentative samples.[8]:2 Genetic analyses have tested the existence of three of these putative subspecies. A study examining mitochondrial DNA from skin biopsies taken from 13 sampling locations, considered genetic diversity and structure among hippo populations across the continent. The authors found low, but significant, genetic differentiation among H. a. amphibius, H. a. capensis, and H. a. kiboko. Neither H. a. tschadensis nor H. a. constrictus has been tested.[9][10]

Evolution

Evolutionary relationships among hippo and Cetacea (whales, dolphins).[11]
Until 1909, naturalists grouped hippos with pigs, based on molar patterns. Several lines of evidence, first from blood proteins, then from molecular systematics[12] and DNA [13][14] and the fossil record, show that their closest living relatives are cetaceans – whales, dolphins and porpoises.[15] The common ancestor of hippos and whales branched off from Ruminantia and the rest of the even-toed ungulates; the cetacean and hippo lineages split soon afterwards.[13][16]

 

Anthracotherium magnum from the Oligocene of Europe
The most recent theory of the origins of Hippopotamidae suggests that hippos and whales shared a common semiaquatic ancestor that branched off from other artiodactyls around 60 million years ago.[13][15] This hypothesised ancestral group likely split into two branches around 54 million years ago.[12]

One branch would evolve into cetaceans, possibly beginning about 52 million years ago, with the protowhale Pakicetus and other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans.[16] The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of which in the late Eocene would have resembled skinny hippos with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved into Hippopotamidae, became extinct during the Pliocene without leaving any descendants.[15]

A rough evolutionary lineage can be traced from Eocene and Oligocene species: Anthracotherium and Elomeryx to the Miocene species Merycopotamus and Libycosaurus and the very latest anthracotheres in the Pliocene.[17] Merycopotamus, Libycosaurus and Hippopotamidae can be considered to form a clade, with Libycosaurus being more closely related to hippos. Their common ancestor would have lived in the Miocene, about 20 million years ago. Hippopotamidae are therefore deeply nested within the family Anthracotheriidae.

Hippopotamidae are believed to have evolved in Africa; the oldest known hippopotamid is the genus Kenyapotamus, which lived in Africa from 16 to 8 million years ago. While hippopotamid species spread across Asia and Europe, no hippos have ever been discovered in the Americas, although various anthracothere genera emigrated into North America during the early Oligocene. From 7.5 to 1.8 million years ago, an ancestor to the modern hippo, Archaeopotamus, lived in Africa and the Middle East.[18]

While the fossil record of hippos is still poorly understood, the two modern genera, Hippopotamus and Choeropsis (sometimes Hexaprotodon), may have diverged as far back as 8 million years ago. Taxonomists disagree whether or not the modern pygmy hippopotamus is a member of Hexaprotodon – an apparently paraphyletic genus, also embracing many extinct Asian hippopotamuses, that is more closely related to Hippopotamus – or of Choeropsis, an older and basal genus.[17][18]


Choeropsis madagascariensis skeleton with a modern hippopotamus skull.
Extinct species
Three species of Malagasy hippopotamus became extinct during the Holocene on Madagascar, one of them within the past 1,000 years. The Malagasy hippos were smaller than the modern hippo, likely through the process of insular dwarfism.[19] Fossil evidence indicates many Malagasy hippos were hunted by humans, a likely factor in their eventual extinction.[19] Isolated members of Malagasy hippos may have survived in remote pockets; in 1976, villagers described a living animal called the kilopilopitsofy, which may have been a Malagasy hippo.[20]