Genetic analyses have tested the existence of three of these putative subspecies. Three species of Malagasy hippopotamus became extinct during the Holocene on Madagascar, one of them within the past 1,000 years.
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 Malagasy hippos were smaller than the modern hippopotamus, likely through the process of insular dwarfism.
Other artiodactyls include camels, cattle, deer and pigs, although hippopotamuses are not closely related to these groups. The Hippopotamidae are believed to have evolved in Africa; the oldest known hippopotamid is the genus Kenyapotamus, which lived in Africa from 16 to .
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. While hippopotamid species spread across Asia and Europe, no hippopotamuses have ever been discovered in the Americas, although various anthracothere genera emigrated into North America during the early Oligocene. 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.
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; 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 The Gambia, and south to South Africa.
After the elephant and rhinoceros, the common 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, the closest living relatives of the Hippopotamidae are cetaceans (whales, dolphins, porpoises, etc.) from which they diverged about .
These findings have important conservation implications as hippo populations across the continent are currently threatened by loss of access to fresh water.
The common hippopotamus inhabits rivers, lakes and mangrove swamps, where territorial bulls preside over a stretch of river and groups of five to thirty females and young. 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 hippopotamuses with comparatively small and narrow heads.
During the day, they remain cool by staying in the water or mud; reproduction and childbirth both occur in water. While hippopotamuses rest near each other in the water, grazing is a solitary activity and hippos are not territorial on land. All branches of the anthracotheres, except that which evolved into Hippopotamidae, became extinct during the Pliocene without leaving any descendants.
All hippos, even those with different diets, secrete the pigments, so it does not appear that food is the source of the pigments.
Instead, the animals may synthesize the pigments from precursors such as the amino acid tyrosine.
Hippopotami are among the largest living land mammals, being only smaller than elephants and some rhinoceroses.