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 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. (Eltringham, 1999; Lang, 1990; Lewison and Oliver, 2008; Oliver, 1993)
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. (Eltringham, 1999; Kingdon, 1997; Lang, 1990; Leidy, 1991; Lewison and Oliver, 2008; Oliver, 1993)
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. (Eltringham, 1999; Kingdon, 1997; Lang, 1990; Leidy, 1991; Oliver, 1993; Prothero and Foss, 2007)
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. (Eltringham, 1999; Lang, 1990)
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. (Eltringham, 1999; Kingdon, 1997; Lang, 1990; Leidy, 1991)
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. (Eltringham, 1999; Kingdon, 1997; Lang, 1990)
Unlike their social cousins, Hexaprotodon liberiensis is solitary. They are only seen in pairs when mating or with a young and are otherwise solitary. Individuals are thought to actively avoid each other by using fecal markings, accomplished by the vigorous wagging of the tail while defecating. If an encounter between two pygmy hippos does take place, they simply ignore each other.
Pygmy hippos are primarily nocturnal, being most active from the late afternoon until around midnight. About six hours a day are dedicated to feeding. During the day they will hide out in the water or rest on the bank. Sleeping quarters are on land, sometimes in burrows or caves, and are changed once or twice a week. (Eltringham, 1999; Kingdon, 1997; Lang, 1990; Leidy, 1991; Oliver, 1993; Prothero and Foss, 2007)
Pygmy hippos don’t seem to be territorial since the home ranges of males, which are 165 to 185 hectares, tend to overlap. The home ranges of females, which are 40 to 80 hectares, may also overlap, and several female ranges may be overlapped in a male’s range. (Eltringham, 1999; Kingdon, 1997; Lang, 1990; Leidy, 1991; Oliver, 1993; Prothero and Foss, 2007)
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. (Boitani and Bartoli, 1982)
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. (Eltringham, 1999; Kingdon, 1997; Lang, 1990; Leidy, 1991; Lewison and Oliver, 2008; Oliver, 1993)
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. (Boitani and Bartoli, 1982; Eltringham, 1999; Lewison and Oliver, 2008)
Specific ecosystem roles of pygmy hippos are unknown but their herbivorous diet probably has an effect on plant populations.
There are no known adverse effects of Hexaprotodon liberiensis on humans.
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. (Eltringham, 1999; Kingdon, 1997; Lang, 1990; Leidy, 1991; Lewison and Oliver, 2008; Oliver, 1993)
When the first pygmy hippo specimen was described as a species, many scientists thought it was a stunted or juvenile common hippo. The existence of this species was not confirmed scientifically until Schomburgk brought 5 live individuals to Europe in 1911. They are sometimes described as Choeropsis liberiensis. (Huffman, 2004)
Tanya Dewey (editor), Animal Diversity Web.
Daniel Fredrickson (author), University of Alaska Fairbanks, Link Olson (editor, instructor), University of Alaska Fairbanks.
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
uses sound to communicate
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
union of egg and spermatozoan
an animal that mainly eats leaves.
A substance that provides both nutrients and energy to a living thing.
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
mainly lives in water that is not salty.
An animal that eats mainly plants or parts of plants.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
Having one mate at a time.
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
having more than one female as a mate at one time
rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.
communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.
uses touch to communicate
Living on the ground.
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
uses sight to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
breeding takes place throughout the year
young are relatively well-developed when born
Boitani, L., S. Bartoli. 1982. Simon & Schuster's Guide to Mammals. New York: Fireside/Simon & Schuster, Inc.
Eltringham, S. 1999. The Hippos. San Diego, California: Academic Press.
Huffman, B. 2004. "Hexaprotodon liberiensis" (On-line). The Ultimate Ungulate. Accessed March 09, 2009 at http://www.ultimateungulate.com/Artiodactyla/Hexaprotodon_liberiensis.html.
Kingdon, J. 1997. The Kingdon Field Guide to African Mammals. San Diego, California: Academic Press.
Lang, E. 1990. Pygmy Hippoptamuses. Pp. 58-64 in B Grzimek, ed. Grzimek's Encyclopedia of Mammals, Vol. 5, 1 Edition. New York: McGraw-Hill Publishing Company.
Leidy, 1991. Pygmy Hippopotamus. Pp. 1350-1351 in R Nowak, ed. Walker's Mammals of the World, Vol. 2, 5 Edition. Baltimore Maryland: Johns Hopkins University Press.
Lewison, R., W. Oliver. 2008. "2008 IUCN Red List of Threatened Species" (On-line). IUCN 2008 Red List. Accessed September 03, 2008 at http://www.iucnredlist.org/details/10032.
Oliver, W. 1993. Status Survey and Conservation Action Plan: Pigs, Peccaries, and Hippos. Gland, Switzerland: IUCN.
Prothero, D., S. Foss. 2007. The Evolution of Artiodactyls. Baltimore, Maryland: The Johns Hopkins University Press.
Robinson, P. "River Horses and Water Cows" (On-line). Accessed December 10, 2008 at http://moray.ml.duke.edu/projects/hippos/Pygmy_Text.doc.