The Pygmy hippo (Choeropsis liberiensis or Hexaprotodon liberiensis) is a small hippopotamid native to the forests and swamps of West Africa. 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 hippo, it is semiaquatic and relies on water to keep its skin moist and its body temperature cool. A rare nocturnal forest creature, the Pygmy hippo is a difficult animal to study in the wild. These animals 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.
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 skin is greenish-black or brown, shading to a creamy gray on the lower body. Their skin is very similar to the Common hippos, with a thin epidermis over a dermis that is several centimeters thick. Pygmy hippos have the same unusual secretion as Common hippos, which 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 sun-screening properties. The skin of hippos dries out quickly and cracks, which is why both species spend so much time in the water. The orbits and nostrils of a Pygmy hippo are much less pronounced, an adaptation from spending less time in deep water. The feet are narrow, 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 in submerging underwater, and skin physiology is dependent on the availability of water.
The natural range of this species covers West Africa, including countries such as Liberia, Sierra Leone, Guinea, and the Ivory Coast. Within this territory, Pygmy hippos inhabit tropical lowland forests with dense cover close to rivers, streams, and swamps.
Pygmy hippos tend to lead a solitary life, avoiding conspecifics. When encountering each other in the wild, hippos usually ignore one another. Each individual defends its territory through fecal marking. These mammals socialize only during the mating season as well as when rearing their offspring. They generally exhibit nocturnal activity, being most active from the late afternoon to midnight. They spend around 6 out of 24 hours per day feeding. The daytime hours are spent resting in the water or on the river bank. Pygmy hippos sleep on the ground, typically in caves or burrows. Each site is used for only a few days since these hippos change their sleeping quarters once or twice a week. Although normally silent, they may sometimes give out snorting, grunting, hissing, and squeaking sounds.
Pygmy hippos are strictly herbivorous animals. Their diet consists of herbs, broad-leaved plants, grasses, semi-aquatic plants, herbaceous shoots, forbs, sedges, ferns as well as occasionally fallen fruits.
Pygmy hippos in captivity are known to exhibit a monogamous mating system, where each individual has only one mate. Those in the wild are thought to be polygynous since the territory of each male overlaps with these of multiple females. Some researchers suggest that Pygmy hippos are polygynandrous (promiscuous) with both males and females breeding with multiple mates. Captive Pygmy hippos do not have a specific breeding season but instead can mate at any time of the year. Individuals breed once every 7-9 months. A single baby is born after 184-210 days of gestation. The newborn hippo lives alone as the mother forages, visiting to suckle the baby 3 times a day. The calf is weaned at 6-8 months old and is reproductively mature at 3-5 years of age.
Pygmy hippos are currently threatened by a number of factors, which negatively affect the small population of this endangered species. To mention just a few, deforestation, hunting, development agriculture, and civil conflicts are among the serious threats to these animals. Although Pygmy hippos are legally protected throughout their range, the total population is decreasing due to scarcity or the absence of suitable resources.
According to the IUCN Red List, the population number of Pygmy hippos is around 2,000-2,499 mature individuals. According to the Pygmy Hippo Foundation resource, the total population of this species in the wild is about 2,000 individuals, the majority of which live in Liberia and the rest inhabit Sierra Leone, Guinea, and the Ivory Coast. Currently, Pygmy hippos are classified as Endangered (EN) and their numbers continue to decrease.
The ecological niche of this species is insufficiently explored, although Pygmy hippos are likely to impact plant communities of their range through their plant-based diet.