The pygmy hippopotamus ( Choeropsis liberiensis or Hexaprotodon liberiensis ) is a large mammal native to the forests and swamps of western Africa (the scientific species classification means "of Liberia", as this is where the vast majority live). The pygmy hippo is reclusive and nocturnal. It is one of only two extant species in the Hippopotamidae family, the other being its much larger cousin the common hippopotamus.
The pygmy hippopotamus displays many terrestrial adaptations, but like its larger cousin, it is semi-aquatic and relies on proximity to water to keep its skin moisturized and its body temperature cool. Behaviors such as mating and giving birth may occur in water or on land. The pygmy hippo is herbivorous, feeding on whatever 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. Pygmy hippos were unknown outside of West Africa until the 19th century. Introduced to zoos in the early 20th century, they breed well in captivity and the vast majority of research is derived from zoo specimens. The survival of the species in captivity is more assured than in the wild: the World Conservation Union estimates that there are fewer than 3,000 pygmy hippos remaining in the wild. 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, natural predators and war.
Taxonomy and origins
Nomenclature of the pygmy hippopotamus reflects that of the hippopotamus. The plural form is pygmy hippopotami ( hippopotamuses is also accepted as a plural form by the OED, or pygmy hippos for short). A male pygmy hippopotamus is known as a bull , a female as a cow , and a baby as a calf .
The pygmy hippopotamus is a member of the family Hippopotamidae where it is classified as a member of either the genus Choeropsis ("resembling a hog") or, the genus Hexaprotodon ("six front teeth"). Members of Hippopotamidae are sometimes known as hippopotamids . Sometimes the sub-family Hippopotaminae is used. Further, some taxonomists group hippopotami and anthracotheres in the superfamily Anthracotheroidea or Hippopotamoidea .
A sister species of the pygmy hippopotamus may have been the little-studied Malagasy pygmy hippopotamus ( Hexaprotodon madagascariensis or Hippopotamus madagascariensis ), one of three recently extinct species from Madagascar. C. madagascariensis was the same size as C. liberiensis and shared its terrestrial behavior, inhabiting the forested highlands of Madagascar, rather than open rivers. It is believed to have gone extinct within the last 500 years.
The taxonomy of the genus of the pygmy hippopotamus has changed as understanding of the animal has developed. 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, Coryndon proposed that the pygmy hippopotamus was closely related to Hexaprotodon , a genus that consisted of prehistoric hippos mostly native to Asia. This assertion was widely accepted, until Boisserie asserted in 2005 that the pygmy hippopotamus was not a member of Hexaprotodon , after a thorough examination of the phylogeny of Hippopotamidae; he suggested instead that the pygmy hippopotamus was a distinct genus, and returned the animal to Choeropsis . All agree that the modern pygmy hippopotamus, be it H. liberiensis or C. liberiensis , is the only extant member of its genus.
Nigerian subspecies
A distinct subspecies of pygmy hippopotamus lived in Nigeria until at least the 20th century. The existence of the subspecies, makes Choeropsis liberiensis liberiensis (or Hexaprotodon liberiensis liberiensis under the old classification) the full trinomial nomenclature for the Liberian pygmy hippopotamus. The Nigerian pygmy hippopotamus subspecies was never studied in the wild and never captured. All research and all zoo specimens are the Liberian subspecies. The Nigerian subspecies is classified as C. liberiensis heslopi .
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 I. R. P. Heslop, a British colonial officer, in the early 1940s. It is believed to be extinct. The subspecies was separated by over 1,800 kilometres (1,100 mi) and the Dahomey Gap, a region of desert that divides the forest regions of West Africa. The subspecies is named after I. R. P. Heslop, who claimed in 1945 to have shot a pygmy hippo in the Niger Delta region and collected several skulls. He estimated that perhaps no more than 30 pygmy hippos remained in the region.
Heslop reportedly sent four pygmy hippopotamus skulls he collected to the British Museum of Natural History in London. These specimens were not subjected to taxonomic evaluation, however, until 1969 when G. B. Corbet classified the skulls as belonging to a separate subspecies based on consistent variations in the proportions of the skulls. The Nigerian pygmy hippos were seen or shot in Rivers State, Imo State and Bayelsa State, Nigeria. While some local populations are aware that the species once existed, its history in the region is poorly documented.
Evolution
Main article: Hippopotamus#EvolutionThe evolution of the pygmy hippopotamus is most often studied in the context of its larger cousin. 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 . This hypothesized ancestor likely split into two branches about six million years later. One branch would evolve into cetaceans, the other branch became the anthracotheres, a large family of four-legged beasts, whose earliest member, from the Late Eocene, would have resembled narrow hippopotami with comparatively small and thin heads.
Hippopotamids are deeply nested within the family Anthracotheriidae. The oldest known hippopotamid is the genus Kenyapotamus , which lived in Africa from 16 to 8 mya . Kenyapotamus is known only through fragmentary fossils, but was similar in size to C. liberiensis . 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.
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 . The ancestral form of the pygmy hippopotamus may be the genus Saotherium . Saotherium and Choeropsis are significantly more basal than Hippopotamus and Hexaprotodon , and thus more closely resemble the ancestral species of hippos.
Extinct pygmy and dwarf hippos
Several species of small hippopotamids have also become extinct in the Mediterranean in the late Pleistocene or early Holocene. Though these species are sometimes known as "pygmy hippopotami" they are not believed to be closely related to C. liberiensis . These include the Cretan dwarf hippopotamus ( Hippopotamus creutzburgi ), the Sicilian hippopotamus ( Hippopotamus pentlandi ), the Maltese hippopotamus ( Hippopotamus melitensis ) and the Cyprus dwarf hippopotamus ( Hippopotamus minor ).
These species, though comparable in size to the pygmy hippopotamus, are considered dwarf hippopotamuses, rather than pygmies. They are likely descended from a full-sized species of European hippopotamus, and reached their small size through the evolutionary process of insular dwarfism which is common on islands; the ancestors of pygmy hippopotami were also small and thus there was never a dwarfing process. There were also several species of pygmy hippo on the island of Madagascar (see Malagasy hippopotamus).
Description
Pygmy hippos share the same general form as a hippopotamus. They have a graviportal skeleton, with four short legs and four toes on each foot, supporting a portly frame. The pygmy hippo, however, is only half as tall as the hippopotamus and weighs less than 1/4 as much as its larger cousin. Adult pygmy hippos stand about 75–83 cm (30–32 inches) high at the shoulder, are 150–177 cm (59–70 inches) in length and weigh 180–275 kilograms (400–600 pounds). Their lifespan in captivity ranges from 30 to 55 years, though it is unli
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