hippopotamus

The hippopotamus ( ; ''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" ().

After the

elephant

and

rhinoceros

, the hippopotamus is the third- largest type of land mammal and is the heaviest extant artiodactyl (in the traditional, non-cladistic sense of the term, excluding cetaceans). Despite their physical resemblance to

pig

s and other terrestrial even-toed ungulates, the closest living relatives of the Hippopotamidae are

cetacea

ns (

whale

s,

dolphin

s,

porpoise

s, 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 for males and for females. Despite its stocky shape and short legs, it is capable of running 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 birth 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.

Etymology

The

Latin

word ''hippopotamus'' is derived from the ancient Greek , ''hippopótamos'', from , ''híppos'', "horse", and , ''potamós'', "river", meaning "horse of the river". In English, the plural is "hippopotamuses", but "hippopotami" is also used.

Taxonomy and origins

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. 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.

Five subspecies of hippos have been described based on morphological differences in their skulls and geographical differences:
*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. 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.

Evolution

Until 1909,

naturalists

grouped hippos with pigs, based on molar patterns. Several lines of evidence, first from blood proteins, then from molecular systematics and

DNA

and the fossil record, show that their closest living relatives are cetaceans (

whale

s,

dolphin

s, and

porpoise

s). 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.

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 . This hypothesised ancestral group likely split into two branches around .

One branch would evolve into cetaceans, possibly beginning about , with the protowhale ''Pakicetus'' and other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic

cetacea

ns. 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