Pygmy hippopotamus
The pygmy hippopotamus or pygmy hippo 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. It has been extirpated from Nigeria.
The pygmy hippopotamus is reclusive and nocturnal. It is one of only two extant species in the family Hippopotamidae, the other being its much larger relative, the common hippopotamus or Nile hippopotamus. The pygmy hippopotamus displays many terrestrial adaptations, but like the common hippopotamus, it is semiaquatic and relies on water to keep its skin moist and its body temperature cool. Behaviors such as mating and giving birth may occur in water or on land. The pygmy hippopotamus 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.
Pygmy hippos were unknown outside 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; in a 2015 assessment, the International Union for Conservation of Nature estimated that fewer than 2,500 pygmy hippos remain in the wild.
Pygmy hippos are primarily threatened by loss of habitat, as the forests are logged and converted to farm land, and are also vulnerable to poaching, hunting for bushmeat, natural predators and war. Pygmy hippos are among the species illegally hunted for food in Liberia.
Taxonomy and origins
Nomenclature of the pygmy hippopotamus reflects that of the hippopotamus; the plural form is pygmy hippopotamuses or pygmy hippopotami. A male pygmy hippopotamus is known as a bull, a female as a cow, and a baby as a calf. A group of hippopotami is known as a herd or a bloat.The pygmy hippopotamus is a member of the family Hippopotamidae where it is classified as a member of the genus Choeropsis, meaning "pig", and ὄψις. 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.
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, Shirley C. 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. ITIS verifies Hexaprotodon liberiensis as the valid scientific name. All agree that the modern pygmy hippopotamus, be it H. liberiensis or C. liberiensis, is the only extant member of its genus. The American Society of Mammalogists moved it back to Choeropsis in 2021, a move supported by the IUCN.
Nigerian subspecies
A distinct subspecies of pygmy hippopotamus existed in Nigeria until at least the 20th century, though the validity of this has been questioned. The existence of the subspecies, makes Choeropsis liberiensis liberiensis the full trinomial nomenclature for the Liberian pygmy hippopotamus. The Nigerian pygmy hippopotamus 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 Ian Heslop, a British colonial officer, in the early 1940s. It is probably extinct. The subspecies is separated by over and the Dahomey Gap, a region of savanna that divides the forest regions of West Africa. The subspecies is named after Heslop, who shot three members of it in 1935 and 1943. He estimated that perhaps no more than 30 pygmy hippos remained in the region.
Heslop 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 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 humans are aware that the species once existed, its history in the region is poorly documented.
Evolution
The 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 or Tayassuidae, but later research near the end of the 20th century determined that hippos are most closely related to cetaceans. Hippos and whales shared a common semi-aquatic ancestor that branched off from other artiodactyls around.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. 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 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. 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, the Sicilian hippopotamus, the Maltese hippopotamus and the Cyprus dwarf hippopotamus.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 hippopotamus on the island of Madagascar.
Description
Pygmy hippopotamuses share the same general form as a hippopotamus. They have a graviportal skeleton, with four stubby legs and four toes on each foot, supporting a portly frame. Yet, the pygmy 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 high at the shoulder, are in length and weigh. Their lifespan in captivity ranges from 30 to 55 years, though it is unlikely that they live this long in the wild.The skin is greenish-black or brown, shading to a creamy gray on the lower body. Their skin is very similar to the common hippo's, with a thin epidermis over a dermis that is several centimeters thick. Pygmy hippos have the same unusual secretion as common hippos, that gives a pinkish tinge to their bodies, and is sometimes described as "blood sweat" though the secretion is neither sweat nor blood. This substance, hipposudoric acid, is believed to have antiseptic and sunscreening properties. The skin of hippos dries out quickly and cracks, which is why both species spend so much time in water.
The skeleton of C. liberiensis is more gracile than that of the common hippopotamus, meaning their bones are proportionally thinner. The common hippo's spine is parallel with the ground; the pygmy hippo's back slopes forward, a likely adaptation to pass more easily through dense forest vegetation. Proportionally, the pygmy hippo's legs and neck are longer and its head smaller.
The orbits and nostrils of a pygmy hippopotamus are much less pronounced, an adaptation from spending less time in deep water. 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.
Despite adaptations to a more terrestrial life than the common hippopotamus, pygmy hippos are still more aquatic than all other terrestrial even-toed ungulates. 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.