Hippopotamidae Temporal range: Late Miocene-Recent
Common hippopotamus
Pygmy hippopotamus
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Artiodactyla
Suborder:	Whippomorpha
Superfamily:	Hippopotamoidea
Family:	Hippopotamidae Gray, 1821
Subtaxa
†Trilobophorus Geze, 1985 Hippopotaminae Gray, 1821 †Archaeopotamus Choeropsis †Hexaprotodon Hippopotamus †Saotherium †Kenyapotaminae Pickford, 1983 Kenyapotamus Palaeopotamus

Hippopotamidae is a family of stout, naked-skinned, and semiaquatic artiodactyl mammals, possessing three-chambered stomachs and walking on four toes on each foot. While they resemble pigs physiologically, their closest living relatives are the cetaceans. They are sometimes referred to as hippopotamids.

There are two living species of hippopotamid in two genera; the pygmy hippo, Choeropsis liberiensis of the forests of west Africa, and the common hippo, Hippopotamus amphibius. The term hippopotamus can also be applied to hippopotamids in general, although it is most frequently used for the common hippo and its respective genus.

Characteristics

Hippopotamids are large mammals, with short, stumpy legs, and barrel-shaped bodies. They have large heads, with broad mouths, and nostrils placed at the top of their snouts. Like pigs, they have four toes, but unlike pigs, all of the toes are used in walking. Hippopotamids are unguligrade, although, unlike most other such animals, they have no hooves, instead using a pad of tough connective tissue on each foot. Their stomachs have three chambers, but they are not true ruminants.

The living species are smooth-skinned and lack both sebaceous glands and sweat glands. The outer epidermis is relatively thin, so hippos dehydrate rapidly in dry environments.^[1]

Both the incisors and canines are large and tusk-like, although the canine tusks are by far the larger. The tusks grow throughout life. The postcanine teeth are large and complex, suited for chewing the plant matter that comprises their diets. The number of incisors varies even within the same species, but the general dental formula is given in the table below:

Evolution

Hippopotamus skeleton at Għar Dalam

The hippopotamids are descended from the anthracotheres, a family of semiaquatic and terrestrial artiodactyls that appeared in the late Eocene, and are thought to have resembled small- or narrow-headed hippos. The hippos split off from the anthracotheres some time during the Miocene. The oldest records of Hippopotamidae are from Afro-Arabia and date to the late Miocene, approximately 7.4 million years ago, expanding into Eurasia around 6 million years ago.^[2] After the appearance of the hippopotamids, the remaining anthracotheres went into a decline brought about by a combination of climatic change and competition with their descendants, until the last genus, Merycopotamus, died out in the early Pliocene of India.

There were once many species of hippopotamid, but only two survive today: Hippopotamus amphibius, and Choeropsis liberiensis. They are the last survivors of two major evolutionary lineages, the hippos proper and the pygmy hippos, respectively; these lineages could arguably be considered subfamilies, but their relationship to each other – apart from being fairly distant relatives – is not well resolved.

The enigmatic Miocene Kenyapotamus is insufficiently known to be assigned a place in the hippo phylogeny with any degree of certainty. In addition, the genus Hexaprotodon, which is now largely restricted to South Asia and Southeast Asia, formerly included many fossil hippopotamuses that are now thought to be unrelated.^[3]

Species

The systematics and taxonomy used here mostly follows a review by J.-R. Boisserie^{[9][lower-alpha 1]} and the American Society of Mammalogists.^[10]

Footnotes

References

Further reading

• "Hippopotamidae". Integrated Taxonomic Information System. Retrieved 11 August 2004.
• Petronio, C. (1995). "Note on the taxonomy of Pleistocene hippopotamuses" (PDF). Ibex. 3: 53–55. Archived from the original (PDF) on 12 September 2008.