The Pygmy Hippopotamus according to MammalMAP
The Pygmy Hippopotamus (Choeropsis liberiensis) inhabits the cool forests and swamps of Western Africa, principally Liberia, and is an elusive creature owing to its small stature and nocturnal habits. This EDGE (Evolutionary Distinct and Globally Endangered) species stands a mere 75-100 cm tall at the shoulder and is about one fifth the weight of its larger cousin, the common hippopotamus. There are other notable differences between these two species such as the more solitary nature of the pygmy hippo compared to the common hippo as well as spending less time in water bodies (despite having webbed toes), preferring to cool off in the surrounding mud and burrows of other animals. Local legends tell of the Pygmy hippo sweating blood, but scientists believe that the red substance they secrete acts as an antibiotic and sunscreen.
This herbivorous animal can reach top speeds of 30 km/h and has a relatively long life-span of up to 40 years in the wild. However, their numbers in the wild are dwindling, predominantly as a result of habitat loss and illegal poaching, but also due to the fact that they only reach sexual maturity at 4-5 years of age and giving birth to a single calf after 7 months of gestation.
The Pygmy hippo was listed as Endangered on the IUCN Red List of Threatened Species. A formal recent assessment of the population revealed that an estimated 2000-3000 individuals remain in the wild. In 2012, a National Action Plan for the Conservation of the Pygmy Hippopotamus in Liberia was produce with the primary goal “To assess the current status of the Pygmy hippo across its range and ensure effective protection of, and connectivity between, known populations.”
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Pygmy Hippopotamuses range in weight from 160 to 275 kg (352-605 lbs.). Their skin color is dark brown on top, fading to a lighter color underneath. Large glands in the skin produce a glossy, brownish-red secretion that is referred to as “blood sweat,” which protects their sensitive skin from sun.
The range of current populations of Hexaprotodon liberiensis is limited to just four West African countries: Liberia, Ivory Coast, Sierra Leone, and Guinea. The majority of the estimated total population of 2,000 to 3,000 is concentrated in Liberia. Smaller populations occur in the other three countries in national forests and reserves. Other than its more recent habitat loss due to human development, the range of H. liberiensis is speculated to have never been much larger than it is today.
The range of H. liberiensis and that of its closest living relative, Hippopotamus amphibius, do not overlap.
The skull of a subspecies, H. l. heslopi, was described in the 1940's from the Niger Delta, Nigeria. Whether this subspecies is still in existence there is unknown, although it is highly unlikely since there is no other documented evidence of its presence.
Biogeographic Regions: ethiopian (Native )
The second subspecies, H. l. heslopi, is known only from the Niger Delta east to the vicinity of the Cross River in Nigeria (Corbet 1969). This second, isolated population in Nigeria is some 1,800 km to the east of the known populations, on the other side of the Dahomey Gap. Such a discontinuous distribution is very rare amongst forest vertebrates in West Africa and Robinson (1970) considers that there is insufficient evidence to confirm that the species ever existed in Nigeria, despite the account by Heslop (1945), who shot one near Omoku in the vicinity of the Niger Delta. This putative Pygmy Hippo subspecies was identified in 1945 based on morphological features of skulls collected in the late 1930s in Nigeria. Others have been equally skeptical, but Ritchie (1930) gave measurements of two skulls that were obtained in 1928 from the Niger Delta so there seems little doubt that the species did once occur in the country. It may be extinct, but Oates (in litt. 1993) reported that residents in the Niger Delta still knew of the species. Although the 1993 Action Plan posits that H. l. heslopi individuals may still occur in the Niger Delta, the existence of this subspecies has not been confirmed or reported. There is little possibility that this Niger delta population still survives, although it is surprising that the existence was so poorly known or documented.
Pygmy hippopotamuses range in mass from 160 to 275 kg. Body lengths are 1.5to 1.75 m and tail lengths are 0.2 m. Height is 0.7 to 1.0 m at the shoulder. Despite the name "Hexaprotodon," these hippopotamuses have only two pairs of upper and one pair of lower incisors. The dental formula is 2/1, 1/1, 3/3, 3/3; 34 teeth in all. The canines are ever-growing. The skin color is dark brown on top, fading to a lighter, fleshy color on the belly and throat. Large glands in the dermis produce a glossy, brownish-red secretion that is referred to as “blood sweat”. These secretions protect the sensitive skin from sun. It may be an adaptive replacement of sweat since the production of the blood sweat has been observed to increase when intense physical exertion such as running or mating takes place.
Hexaprotodon liberiensis is most commonly compared to its larger relative Hippopotamus amphibius. While pygmy hippopotamuses are obviously smaller in body size, they also have some rather distinct physical adaptations that distinguish them from H. amphibius individuals. Proportionally, the legs and neck of H. liberiensis are longer, and the head is smaller relative to body size. The digits of pygmy hippos are more spread out and have less webbing between digits than common hippos. In general, pygmy hippos have many adaptations that are thought to be advantageous to terrestrial mobility. Their orbits are positioned on the sides of the head rather than on top. Their backs are forward sloping, a trait thought to enhance movement through thick vegetation. A feature that they share with their larger relative is the muscular valves of the ears and nose, which are capable of closing under water.
Range mass: 160 to 275 kg.
Range length: 1.50 to 1.75 m.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: sexes alike
Hexaprotodon liberiensis is only found in low-lying, forested areas, never far from a source of water. Pygmy hippopotamuses use swamps, streams, and rivers as refuges from danger and to keep their sensitive skin moist. Open areas are completely avoided and most travel is limited to worn-down, tunnel-like paths through dense vegetation on land. Narrow waterways are also used to navigate through swampy areas. Pygmy hippopotamuses have been found in burrows deep in the sides of river banks. It is unlikely that they dig burrows themselves, but they may enlarge existing ones. Because entrances of these burrow open toward the river and are mostly submerged below the water level, they are ideal for the semi-aquatic lifestyle of H. liberiensis.
Habitat Regions: tropical ; terrestrial ; freshwater
Terrestrial Biomes: forest ; rainforest
Aquatic Biomes: lakes and ponds; rivers and streams
Other Habitat Features: caves
Habitat and Ecology
The species is exclusively vegetarian, feeding on leaves and roots of forest plants (including semi-aquatic plants and forest herbs) as well as on fallen fruit. The stomach has four chambers (Langer 1988). The first three are covered with tough keratinized epithelium, only the last containing glandular epithelial tissue. There is evidence that microbial breakdown of plant material takes place in the first three stomach chambers, no caecum being present in this species. This mode of digestion is usually considered an adaptation to a highly fibrous, generally "low-quality" vegetable diet. The droppings are poorly formed and similar to those of the common hippo.
It is not known how far individual Pygmy Hippos roam in the wild, or whether they keep to well-defined home ranges, but their habit of fecal marking implies they may be at least partly territorial in defending particular areas against incursions from other members of the species. However, during the rainy season (May–September), animals are reported by hunters to disperse over wide areas in the forest zone. The effects of predators on pygmy hippo populations are unknown, but the principal carnivore capable of attacking an animal this size is the leopard.
No accurate data on reproduction, including breeding season, have been published for the wild populations. Sexual maturity occurs at about four to five years of age. From studies of captive animals (Lang 1975; Tobler 1991), the oestrous cycle has been shown to average 35.5 days with oestrus itself being 24–48 hours long. The average gestation length is 188 days after which a singleton young is born weighing about 5.7 kg. Twins are born very rarely, the incidence being approximately one in every 200 births. The young are born on land (although can readily swim), and there is no evidence from captive births that a nest constructed. A survey of over 800 births indicates that these occur throughout the year (Tobler 1991).
Current populations of the Pygmy Hippoptamus are limited to just four West African countries: Liberia, Ivory Coast, Sierra Leone and Guinea. The majority of the total estimated population of 2,000 to 3,000 is concentrated in Liberia in patches of lowland forest near sources of water.
Pygmy hippos are strictly vegetarian, or herbivorous. They eat a wide variety of plant foods including herbs, broad-leaved plants, grasses, semi-aquatic plants, herbaceous shoots, forbs, sedges, ferns, and fallen fruit. Considered a pseudo-ruminant, pygmy hippos have a four-chambered stomach but do not ferment food or use microbes in the same way as true ruminants. They also do not chew their cud.
Plant Foods: leaves; roots and tubers; fruit
Primary Diet: herbivore (Folivore )
Specific ecosystem roles of pygmy hippos are unknown but their herbivorous diet probably has an effect on plant populations.
The main predators of pygmy hippos are leopards (Panthera pardus). Other potential predators include large African rock pythons and crocodiles. When startled, pygmy hippos flee a short distance into vegetation, where they hide.
- leopards (Panthera pardus)
Life History and Behavior
Pygmy hippos use scent marking with their feces to alert other hippos to their presence. Like other mammals, they may use scent cues to advertise reproductive status as well. Pygmy hippos are typically silent, but do make snorts, grunts, hisses, and squeaks occasionally. Otherwise, little is known about how pygmy hippos communicate.
Communication Channels: chemical
Other Communication Modes: scent marks
Perception Channels: visual ; tactile ; acoustic ; chemical
Pygmy Hippopotamuses are strictly herbivorous. They feed on a wide variety of plant foods including herbs, broad-leaved plants, grasses, semi-aquatic plants, herbaceous shoots, forbs, sedges, ferns, and fallen fruit.
The Pygmy Hippopotamus is generally solitary, scent marking with feces to alert intruders to its presence. Mothers leave their newborn calves alone while searching for food, returning about three times a day for suckling. Calves are ready to forage with their mother at about three months of age.
The lifespan of Hexaprotodon liberiensis individuals in the wild is unknown. In captivity individuals have lived up to 43 years.
Status: captivity: 43 (high) years.
Lifespan, longevity, and ageing
The mating system of H. liberiensis has only been observed in captive individuals. In captivity only monogamous mating has occurred. This is very unlikely in the wild, however, because the home range of a single male overlaps the home ranges of several females. Mating in captivity has been observed both on land and in water and can take place one to four times during the female's estrous period, which lasts one or two days.
Mating System: monogamous ; polygynous
Very little is known about the reproductive behavior of H. liberiensis in the wild. All of the information here is based on observations of captive animals. The breeding season is unknown in the wild but in captivity can occur at any time of the year. The breeding interval is between 7 and 9 months. The gestation period lasts as little as 184 days or as long as 210 days. One offspring is normally produced; the occurrence of twins is very rare. Offspring weigh 3.4 to 6.4 kg and are generally well developed. Newborn males weigh slightly more than females. Weaning occurs between 6 and 8 months and an individual reaches sexual maturity in 3 to 5 years. Births have occurred both on land and in water in captivity. Births taking place in deep water almost always result in the newborn drowning.
Breeding interval: Breeding can occur as often as every 7 to 9 months.
Breeding season: Breeding occurs year round in captivity. Seasonality in the wild is unknown.
Average number of offspring: 1.
Range gestation period: 184 to 210 days.
Range weaning age: 6 to 8 months.
Range age at sexual or reproductive maturity (female): 3 to 5 years.
Range age at sexual or reproductive maturity (male): 3 to 5 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization ; viviparous
Hexaprotodon liberiensis is considered a K-selected species, which means it produces few offspring and invests a lot of energy into offspring development. Newborn calves are left in one place while the mother searches for food, returning about three times a day for suckling. They young are usually able to feed on vegetation after three months. These behaviors have been observed both in captivity and in the wild.
Parental Investment: precocial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female)
Molecular Biology and Genetics
Barcode data: Hexaprotodon liberiensis
Below is the sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.
See the BOLD taxonomy browser for more complete information about this specimen.
Other sequences that do not yet meet barcode criteria may also be available.
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Download FASTA File
Statistics of barcoding coverage: Hexaprotodon liberiensis
Public Records: 1
Specimens with Barcodes: 1
Species With Barcodes: 1
Pygmy hippos are classified as Endangered by the IUCN and are on Appendix II of CITES. Threats to H. liberiensis populations include deforestation, hunting, agricultural land development, and civil conflicts. Pygmy hippos are legally protected in most of the regions where they are found. However, there are little or no resources available to enforce their protection and numbers in the wild continue to decrease. The subspecies H. l. heslopi is considered extinct in the wild, although its existence is still questioned because reports of individuals in Nigeria are questionable.
US Federal List: no special status
CITES: appendix ii
IUCN Red List of Threatened Species: endangered
IUCN Red List Assessment
Red List Category
Red List Criteria
- 2006Endangered(IUCN 2006)
- 1994Vulnerable(Groombridge 1994)
- 1990Vulnerable(IUCN 1990)
- 1988Vulnerable(IUCN Conservation Monitoring Centre 1988)
- 1986Vulnerable(IUCN Conservation Monitoring Centre 1986)
IUCN Status: ENDANGERED
In Liberia, where the majority of remaining Pygmy Hippos are believed to reside, legal protection is described as incomplete, and the level of enforcement of protection is described as poor. Pygmy Hippo protection has historically been most effective in the Sapo National Park. However, recent reports from Flora and Fauna International suggest that current legal protection has been suspended and Pygmy Hippos are being hunted for meat in the Park. Liberia’s Cestos-Senkwehn Rivershed, located in south-central area of the country, was believed to be home to substantial numbers of Pygmy Hippos based on a forest survey conducted in 1998. Since that time, Cestos-Senkwehn area has been largely logged and developed (Robinson and Suter 1999). In 1999, almost 190 million cubic meters of wood was exported from Liberia. The scope of the deforestation has alarmed Liberian government officials. In April 2000, Liberia's Minister of Agriculture, Roland Massaquoi, in criticizing the way logging companies were operating stated "it is evident that most of the country's natural rainforests has been depleted without reforestation". This alarming rate of deforestation has been confirmed by Friends of Liberia (FOL), Society for the Renewal of Nature Conservation in Liberia, and independent researchers.
The evidence suggests that habitat in protected areas in all resident countries is under siege. Pygmy Hippos, by nature of their habitat requirements, are extremely sensitive to this loss. The current population threats—deforestation for logging and human settlement, hunting, and regional conflicts—continue to threaten remaining Pygmy Hippo individuals. The conservation status of the Pygmy Hippo in Liberia is poor. At the present rate of habitat loss, only small insular populations will remain and, in the total absence of any regional conservation plans, effective protection or conservation actions, viability of this species should be considered extremely poor.
Threats to the Pygmy Hippopotamus include deforestation, hunting, agricultural land development, and civil conflicts. Although Pygmy Hippos are legally protected in most of the regions where they are found, there are few resources to enforce their protection and numbers in the wild continue to decrease.
A vital area for the Pygmy Hippo's protection is the Sapo forest in eastern Liberia. A 509-square-mile block of this forest was chartered as Sapo National Park in 1983, establishing its only national park. Another key area in which the species occurs is in the Tai National Park in western Côte d'Ivoire. Tai is now subject to poaching, agricultural encroachment and gold mining in the park's river beds.
As of the end of 2004, the latest edition of the International Studbook for the Pygmy Hippopotamus recorded some 303 animals (290 captive-born) from a founder population of 70, in 135 zoos in captivity (Hlavacek et al. 2005). The species breeds freely in captivity and most, if not all, of the specimens listed have been born in zoos to captive-bred parents. The world population of captive born animals has more than doubled since 1970.
Whilst the future of the species in captivity seems assured, the conditions under which it is kept need reconsideration given that most collections consist of a pair that are kept permanently together in a pool of water. Evidence from the wild suggests that Pygmy Hippopotamus come together infrequently and do not spend much time immersed in the water. The causes of death mentioned in the Studbook include many references to attacks by mates, maternal neglect and injuries inflicted by the mother. It is possible that many of these deaths are due to stress from the artificial conditions of captivity and greater attention to the way of life in the wild might help to reduce this mortality.
There are no current research projects investigating Pygmy Hippo ecology or conservation in the wild. In addition, there is little action being taken to protect Pygmy Hippo habitat or populations.
1. To ensure, as a first priority, that the species can continue to survive in the Liberian forests without further reduction or fragmentation of its range.
2. To establish more precisely the distribution and numbers of the species throughout its range but more particularly in Liberia, where the bulk of the population occurs.
3. To identify secure regions where conservation action can be concentrated.
4. To establish whether or not the isolated population reported from Nigeria still exists and if it does, to develop plans for its enhanced future protection.
(The alleged population in Guinea-Bissau is so improbable that the time and money that would be involved in an attempt to establish its existence are unlikely to be justified.)
1. Establish a reliable method for assessing the sizes of the various populations.
It is unlikely that such an elusive creature can be counted accurately and attention should be paid to developing indirect techniques that will provide an index of density, as has been done with forest elephants. These may include, for example, counts of dunging areas, trails or nest sites.
2. Identify and give special protection to areas containing adequate populations of the species and which appear free from the threat of deforestation.
This does not necessarily mean according them national park status, which might be difficult to achieve. In any case, even if new parks were created they might not be large enough to contain viable populations. As the only national park in Liberia, however, special attention should be given to Sapo National Park particularly as the species was recently recorded there.
3. Monitor the species in protected areas on a permanent basis using techniques developed for census purposes.
4. Identify potential threats to the species in each area and take steps to remove them.
Apart from the obvious problem of deforestation, attention should be paid to possible threats from meat hunting and the trophy trade. Education should play a prominent role in such projects in making local people aware of the rarity and uniqueness of the pygmy hippopotamus.
5. Mount expeditions to those regions of Nigeria where the species was last reported in order to look for evidence of its continued existence.
If it is shown to survive there, special efforts should be made to assist the development of management strategies for its enhanced future protection and to determine the taxonomic as well as the conservation status of this population.
6. Coordinate the international captive breeding effort to take advantage of recent computer programmes for analysing stud book data and to ensure that maximum use is made of the genetic potential of the existing captive population.
7. Study the behaviour of the species under a variety of captive conditions in order to generate information of benefit to their enhanced future husbandry, with particular reference to the habits of the animals in the wild.
Relevance to Humans and Ecosystems
There are no known adverse effects of Hexaprotodon liberiensis on humans.
Pygmy hippos are hunted as a source of bushmeat.
Positive Impacts: food
The pygmy hippopotamus (Choeropsis liberiensis or Hexaprotodon liberiensis) is a small hippopotamid native to the forests and swamps of West Africa, primarily in Liberia and small populations in Sierra Leone, Guinea and Ivory Coast. 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 (Hippopotamus amphibius).
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 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 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 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. 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 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.
A distinct subspecies of pygmy hippopotamus lived 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 (or Hexaprotodon liberiensis liberiensis under the old classification) 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 I. R. P. Heslop, a British colonial officer, in the early 1940s. It is probably extinct. The subspecies is separated by over 1,800 km (1,100 mi) and the Dahomey Gap, a region of savanna 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.
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 (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 .
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 (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).
Pygmy hippos 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. 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 75–100 cm (2.46–3.28 ft) high at the shoulder, are 150–175 cm (4.92–5.74 ft) in length and weigh 180–275 kg (397–606 lb). 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. The highly alkaline substance 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 hippo are much less pronounced, an adaptation from spending less time in deep water (where pronounced orbits and nostrils help the common hippo breathe and see). 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 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.
The behavior of the pygmy hippo differs from the common hippo in many ways. Much of its behavior is more similar to that of a tapir, though this is an effect of convergent evolution. While the common hippopotamus is gregarious, pygmy hippos live either alone or in small groups, typically a mated pair or a mother and calf. Pygmy hippos tend to ignore each other rather than fight when they meet. Field studies have estimated that male pygmy hippos range over 1.85 km2 (460 acres), while the range of a female is 0.4 to 0.6 km2 (99–148 acres).
Pygmy hippos spend most of the day hidden in rivers. They will rest in the same spot for several days in a row, before moving to a new spot. At least some pygmy hippos make use of dens or burrows that form in river banks. It is unknown if the pygmy hippos help create these dens, or how common it is to use them. Though a pygmy hippo has never been observed burrowing, other artiodactyls, such as warthogs, are burrowers.
Like the common hippopotamus, the pygmy hippo emerges from the water at dusk to feed. It relies on game trails to travel through dense forest vegetation. It marks trails by vigorously waving its tail while defecating to further spread its feces. The pygmy hippo spends about six hours a day foraging for food.
Pygmy hippos are herbivorous. They do not eat aquatic vegetation to a significant extent and rarely eat grass because it is uncommon in the thick forests they inhabit. The bulk of a pygmy hippo's diet consists of ferns, broad-leaved plants and fruits that have fallen to the forest floor. The wide variety of plants pygmy hippos have been observed eating suggests that they will eat any plants available. This diet is of higher quality than that of the common hippopotamus.
A study of breeding behavior in the wild has never been conducted; the artificial conditions of captivity may cause the observed behavior of pygmy hippos in zoos to differ from natural conditions. Sexual maturity for the pygmy hippopotamus occurs between three and five years of age. 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. The oestrus cycle of a female pygmy hippo lasts an average of 35.5 days, with the oestrus itself lasting between 24–48 hours.
Pygmy hippos consort for mating, but the duration of the relationship is unknown. In zoos they breed as monogamous pairs. Copulation can take place on land or in the water, and a pair will mate one to four times during an oestrus period. In captivity, pygmy hippos have been conceived and born in all months of the year. The gestation period ranges from 190–210 days, and usually a single young is born, though twins are known to occur.
The common hippopotamus gives birth and mates only in the water, but pygmy hippos mate and give birth on both land and water. Young pygmy hippos can swim almost immediately. At birth, pygmy hippos weigh 4.5–6.2 kg (9.9–13.7 lb) with males weighing about 0.25 kg (0.55 lb) more than females. Pygmy hippos are fully weaned between six and eight months of age; before weaning they do not accompany their mother when she leaves the water to forage, but instead hide in the water by themselves. The mother returns to the hiding spot about three times a day and calls out for the calf to suckle. Suckling occurs with the mother lying on her side.
The greatest threat to the remaining pygmy hippopotamus population in the wild is loss of habitat. The forests in which pygmy hippos live have been subject to logging, settling and conversion to agriculture, with little efforts made to make logging sustainable. As forests shrink, the populations become more fragmented, leading to less genetic diversity in the potential mating pool.
Pygmy hippos are among the species illegally hunted for food in Liberia. Their meat is said to be of excellent quality, like that of a wild boar; unlike those of the common hippo, the pygmy hippo's teeth have no value. The effects of West Africa's civil strife on the pygmy hippopotamus are unknown, but unlikely to be positive. The pygmy hippopotamus is capable of being killed by leopards, pythons and crocodiles. How often this occurs is unknown.
C. liberiensis was identified as one of the top-10 "focal species" in 2007 by the Evolutionarily Distinct and Globally Endangered (EDGE) project. Some populations inhabit protected areas, such as the Gola Forest Reserve in Sierra Leone.
The Zoo Basel in Switzerland holds the international studbook and coordinates the entire captive pygmy hippo population that freely breeds in zoos around the world. Between 1970 and 1991 the population of pygmy hippos born in captivity more than doubled. The survival of the species in zoos is more certain than the survival of the species in the wild. In captivity, the pygmy hippo lives from 42 to 55 years, longer than in the wild. Since 1919, only 41 percent of pygmy hippos born in zoos have been male.
History and folklore
While the common hippopotamus was known to Europeans since classical antiquity, the pygmy hippopotamus was unknown outside of its range in West Africa until the 19th century. Due to their nocturnal, forested existence, they were poorly known within their range as well. In Liberia the animal was traditionally known as a water cow.
Early field reports of the animal misidentified it as a wild hog. Several skulls of the species were sent to the American natural scientist Samuel G. Morton, during his residency in Monrovia, Liberia. Morton first described the species in 1843. The first complete specimens were collected as part of a comprehensive investigation of Liberian fauna in the 1870s and 1880s by Dr. Johann Büttikofer. The specimens were taken to the Natural History Museum in Leiden, The Netherlands.
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. Pygmy hippos were successfully introduced to Europe in 1911. They were first shipped to Germany and then to the Bronx Zoo in New York City where they also thrived.
In 1927, Harvey Firestone of Firestone Tires presented Billy the pygmy hippo to U.S. President Calvin Coolidge. Coolidge donated Billy to the National Zoo in Washington, D.C. According to the zoo, Billy is a common ancestor to most pygmy hippos in U.S. zoos today.
Several folktales have been collected about the pygmy hippopotamus. One tale says that pygmy hippos carry a shining diamond in their mouths to help travel through thick forests at night; by day the pygmy hippo has a secret hiding place for the diamond, but if a hunter catches a pygmy hippo at night the diamond can be taken. Villagers sometimes believed that baby pygmy hippos do not nurse but rather lick secretions off the skin of the mother.
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