The pygmy hippopotamus or pygmy hippo (Choeropsis liberiensis) is a small hippopotamid which is native to the forests and swamps of West Africa, primarily in Liberia, with small populations in Sierra Leone, Guinea, and Ivory Coast.

The pygmy hippopotamus displays many terrestrial adaptations, but like the common hippo, it is semiaquatic and relies on water to keep its skin moist and its body temperature cool.

The pygmy hippo is herbivorous, feeding on ferns, broad-leaved plants, grasses, and fruits it finds in the forests.

A rare nocturnal forest creature, the pygmy hippopotamus is a difficult animal to study in the wild.

[1] Pygmy hippos are primarily threatened by loss of habitat, as forests are logged and converted to farm land, and are also vulnerable to poaching, hunting for bushmeat, natural predators, and war.

[1][7][8][9] Samuel G. Morton initially classified the animal as Hippopotamus minor, but later determined it was distinct enough to warrant its own genus, and labeled it Choeropsis.

In 1977, Shirley C. Coryndon proposed that the pygmy hippopotamus was closely related to Hexaprotodon, a genus that consisted of prehistoric hippos mostly native to Asia.

[8] The Nigerian pygmy hippopotamus ranged in the Niger River Delta, especially near Port Harcourt, but no reliable reports exist after the collection of the museum specimens secured by Ian Heslop, a British colonial officer, in the early 1940s.

[15] Heslop sent four pygmy hippopotamus skulls he collected to the British Museum of Natural History in London.

Both species were long believed to be most closely related to the family Suidae (pigs and hogs) or Tayassuidae (peccaries), but research within the last 10 years has determined that pygmy hippos and hippos are most closely related to cetaceans (whales and dolphins).

Hippos and whales shared a common semi-aquatic ancestor that branched off from other artiodactyls around 60 mya.

[12] The Hippopotamidae are believed to have evolved in Africa, and while at one point the species spread across Asia and Europe, no hippopotami have ever been discovered in the Americas.

Starting 7.5 to 1.8 mya the Archaeopotamus, likely ancestors to the genus Hippopotamus and Hexaprotodon, lived in Africa and the Middle East.

[7] While the fossil record of hippos is still poorly understood, the lineages of the two modern genera, Hippopotamus and Choeropsis, may have diverged as far back as 8 mya.

Saotherium and Choeropsis are significantly more basal than Hippopotamus and Hexaprotodon, and thus more closely resemble the ancestral species of hippos.

The feet of pygmy hippos are narrower, but the toes are more spread out and have less webbing, to assist in walking on the forest floor.

The ears and nostrils of pygmy hippos have strong muscular valves to aid submerging underwater, and the skin physiology is dependent on the availability of water.

[9] While the common hippopotamus is gregarious, pygmy hippos live either alone or in small groups, typically a mated pair or a mother and calf.

The bulk of a pygmy hippo's diet consists of herbs, ferns, broad-leaved plants, herbaceous shoots, forbs, sedges and fruits that have fallen to the forest floor.

[9] The youngest reported age for giving birth is a pygmy hippo in the Zoo Basel, Switzerland which bore a calf at three years and three months.

[8] Although not considered dangerous to humans and generally docile, pygmy hippos can be highly aggressive at times.

[25] The greatest threat to the remaining pygmy hippopotamus population in the wild is loss of habitat.

As forests shrink, the populations become more fragmented, leading to less genetic diversity in the potential mating pool.

[9] The effects of West Africa's civil strife on the pygmy hippopotamus are unknown, but unlikely to be positive.

[8] C. liberiensis was identified as one of the top 10 "focal species" in 2007 by the Evolutionarily Distinct and Globally Endangered (EDGE) project.

Several skulls of the species were sent to the American natural scientist Samuel G. Morton, during his residency in Monrovia, Liberia.

The first complete specimens were collected as part of a comprehensive investigation of Liberian fauna in the 1870s and 1880s by Dr. Johann Büttikofer.

[9] The first pygmy hippo was brought to Europe in 1873 after being captured in Sierra Leone by a member of the British Colonial Service but died shortly after arrival.

[22][29] Moo Deng is a pygmy hippo living in Khao Kheow Open Zoo, in Thailand, who gained notability in September 2024 as a popular Internet meme after images of her went viral online.

Villagers sometimes believed that baby pygmy hippos do not nurse but rather lick secretions off the skin of the mother.