Hippopotamidae is afamily of stout,naked-skinned, andsemiaquaticartiodactyl mammals, possessing three-chambered stomachs and walking on four toes on each foot. While they resemblepigs physiologically, their closest living relatives are thecetaceans. They are formally referred to ashippopotamids.
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 termhippopotamus can also be applied to hippopotamids in general, although it is most frequently used for the common hippo and its respectivegenus.
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 areunguligrade, 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 trueruminants.
The living species are smooth-skinned and lack bothsebaceous glands andsweat glands. The outerepidermis is relatively thin, so hippos dehydrate rapidly in dry environments.[1]
Both theincisors andcanines 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 generaldental formula is given in the table below:
The hippopotamids are descended from theanthracotheres, a family of semiaquatic and terrestrial artiodactyls that appeared in the lateEocene, and are thought to have resembled small- or narrow-headed hippos. The hippos split off from the anthracotheres some time during theMiocene. 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] It has been theorised that thisLate Miocene radiation of hippopotamids represents the coevolution of hippopotamids with the expansion ofC4grasslands,[3] a phenomenon known as the hippopotamine event (HE).[4] 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 earlyPliocene of India.
There were once many species of hippopotamid, but only two survive today:Hippopotamus amphibius, andChoeropsis liberiensis. They are the last survivors of two majorevolutionary lineages, the hippos proper and the pygmy hippos, respectively; these lineages could arguably be consideredsubfamilies, but their relationship to each other – apart from being fairly distant relatives – is not well resolved.
The enigmaticMioceneKenyapotamus is insufficiently known to be assigned a place in the hippophylogeny with any degree of certainty. In addition, the genusHexaprotodon, which is now largely restricted to South Asia and Southeast Asia, formerly included many fossil hippopotamuses that are now thought to be unrelated.[5]
The lower canine teeth of hippopotamids are similar in function and structure to the tusks ofelephants. While hippopotamids and elephants are only very distantly related within the Mammalia, the lower canine teeth of both groups are long and have a slight curve, and species of both families use this structure when fighting.
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