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Pygmy hippopotamus
A pygmy hippopotamus at Edinburgh Zoo
CITES Appendix II [2]
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Family: Hippopotamidae
Subfamily: Hippopotaminae
Genus: Choeropsis
Leidy, 1853
Species:
C. liberiensis
Binomial name
Choeropsis liberiensis
(Morton, 1849)[3]
Subspecies
  • C. l. liberiensis
  • C. l. heslopi
Range map[1]
Synonyms[1]
  • Hexaprotodon liberiensis

The pygmy hippopotamus or pygmy hippo (Choeropsis liberiensis) 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.[1]

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 (Hippopotamus amphibius) 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.[4] 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.[1]

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

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Showing its teeth at the Lagos Zoo in Nigeria

Nomenclature of the pygmy hippopotamus reflects that of the hippopotamus; the plural form is pygmy hippopotamuses or pygmy hippopotami.[5] 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.[6]

The pygmy hippopotamus is a member of the family Hippopotamidae where it is classified as a member of the genus Choeropsis (from Ancient Greek χοῖρος (khoîros), meaning "pig", and ὄψις (ópsis), meaning "sight, resemblance"). 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.[1][7][8][9] 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.[10]

Skull

This assertion was widely accepted,[1][7][8][9] 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.[7][11][12] ITIS verifies Hexaprotodon liberiensis as the valid scientific name.[13] All agree that the modern pygmy hippopotamus, be it H. liberiensis or C. liberiensis, is the only extant member of its genus.[7][10] The American Society of Mammalogists moved it back to Choeropsis in 2021,[3] a move supported by the IUCN.[1]

Nigerian subspecies

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A distinct subspecies of pygmy hippopotamus existed in Nigeria until at least the 20th century, though the validity of this has been questioned.[1] 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.[8]

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.[1] 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 Heslop, who shot three members of it in 1935 and 1943.[14] He estimated that perhaps no more than 30 pygmy hippos remained in the region.[15]

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 Gordon Barclay Corbet [de] classified the skulls as belonging to a separate subspecies based on consistent variations in the proportions of the skulls.[16] 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.[8]

Evolution

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Main article: Hippopotamus § Evolution
Anthracotheres like Anthracotherium resembled pygmy hippos and are among their likely ancestors.

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 60 mya.[17][18]

This hypothesized ancestor likely split into two branches about six million years later.[19] 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.[11]

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.[12] 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.[7]

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.[7][12]

Extinct pygmy and dwarf hippos

[edit]

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).[20]

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.[20] There were also several species of pygmy hippopotamus on the island of Madagascar (see Malagasy hippopotamus).

Description

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Resting at Louisville Zoo. The skull of a pygmy hippopotamus has less pronounced orbits and nostrils than a common hippopotamus.

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 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).[21] Their lifespan in captivity ranges from 30 to 55 years, though it is unlikely that they live this long in the wild.[8][22]

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.[8]

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.[22]

Nuzzling couple at the Duisburg Zoo in Germany

The orbits and nostrils of a pygmy hippopotamus 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.[22]

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.[8][9]

Behavior

[edit]
(video) Bathing in the Ueno Zoo, Tokyo, Japan

The behavior of the pygmy hippopotamus 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.[9] 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 (100–150 acres).[8]

Two dive in water at Singapore Zoo, Singapore

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 hippopotamus has never been observed burrowing, other artiodactyls, such as warthogs, are burrowers.[8]

Diet

[edit]

Like the common hippopotamus, the pygmy hippopotamus 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 hippopotamus spends about six hours a day foraging for food.[8]

Eating a vegetable

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 herbs, ferns, broad-leaved plants, herbaceous shoots, forbs, sedges and fruits that have fallen to the forest floor.[23] 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.[8]

Reproduction

[edit]
Baby stands near its parent in the Jihlava Zoo, Czechia

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.[9] The youngest reported age for giving birth is a pygmy hippopotamus in the Zoo Basel, Switzerland which bore a calf at three years and three months.[8] The oestrus cycle of a female pygmy hippopotamus lasts an average of 35.5 days, with the oestrus itself lasting between 24 and 48 hours.[1][24]

Pygmy hippopotamuses 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.[9] The gestation period ranges from 190 to 210 days, and usually a single young is born, though twins are known to occur.[8]

Mother and child taking a bath at Lisbon Zoo

The common hippopotamus gives birth and mates only in the water, but pygmy hippopotamuses 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.[8]

Temperament

[edit]

Although not considered dangerous to humans and generally docile, pygmy hippopotamuses can be highly aggressive at times. Although there have been no human deaths associated with pygmy hippos, there have been several attacks - while most of these were provoked by human behaviour, several have had no apparent cause.[25]

Conservation

[edit]
Pair at the Mount Kenya Wildlife Conservancy

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.[1]

Pygmy hippos are among the species illegally hunted for food in Liberia.[26] 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.[9] The effects of West Africa's civil strife on the pygmy hippopotamus are unknown, but unlikely to be positive.[1] The pygmy hippopotamus can be killed by leopards, pythons and crocodiles. How often this occurs is unknown.[8]

C. liberiensis was identified as one of the top 10 "focal species" in 2007 by the Evolutionarily Distinct and Globally Endangered (EDGE) project.[27] Some populations inhabit protected areas, such as the Gola Forest Reserve in Sierra Leone.[28]

Basel Zoo in Switzerland holds the international studbook and coordinates the entire captive pygmy hippopotamus 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.[15][22] In captivity, the pygmy hippopotamus lives from 42 to 55 years, longer than in the wild.[8] Since 1919, only 41 percent of pygmy hippos born in zoos have been male.[24]

History and folklore

[edit]
Pair at the Mount Kenya Wildlife Conservancy

While the common hippopotamus has been known to Europeans since classical antiquity, the pygmy hippopotamus was unknown outside 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.[9]

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.[9]

The first pygmy hippopotamus 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 established in European zoos in 1911. They were first shipped to Germany and then to the Bronx Zoo in New York City where they also thrived.[8][9]

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.[22][29]

Moo Deng is a pygmy hippopotamus living in Khao Kheow Open Zoo, in Thailand, who gained notability in September 2024 as a popular Internet meme after images of her went viral online.[30] Because of the popularity of the hippo, whose name translates to "bouncy pork", the zoo saw a boosted attendance. It has been reported that some visitors to the zoo threw water and other objects at the baby hippo to get her to react.[31]

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 hippopotamus has a secret hiding place for the diamond, but if a hunter catches a pygmy hippopotamus 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.[9]

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Pygmy hippopotamus

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from Grokipedia
The pygmy hippopotamus (Choeropsis liberiensis) is a relict species in the family , distinguished by its small size, solitary nocturnal habits, and adaptation to forested riverine habitats in . Unlike the gregarious, semi-aquatic common , it exhibits greater terrestrial mobility and spends more time on land, foraging for vegetation including leaves, fruits, and roots. Adults typically weigh 160–275 kg and measure 1.5–1.75 m in length, with a height of about 75–100 cm, reflecting an ancient evolutionary lineage that diverged from other hippos millions of years ago. Classified as Endangered on the , the species persists in fragmented populations across , , , and Côte d'Ivoire, numbering fewer than 2,500 mature individuals due to habitat loss from logging and agriculture, as well as incidental snaring and hunting. Conservation efforts focus on protected areas and programs, though limited knowledge of its hinders effective interventions.

Taxonomy and Phylogeny

Classification and Nomenclature

The pygmy hippopotamus (Choeropsis liberiensis) belongs to the family within the order Artiodactyla, class Mammalia, phylum Chordata, and kingdom Animalia. Its genus Choeropsis reflects morphological similarities to suids, derived from Greek choiros (pig) and opsis (appearance), while the specific epithet liberiensis denotes its type locality in . The is monotypic in its genus, though two are recognized: the nominate C. l. liberiensis (Morton, 1844) and C. l. heslopi (Corbet, 1969), the latter potentially extinct and restricted to the region. The species was first described scientifically by American physician Samuel G. Morton in 1844, based on a skull obtained from and initially classified as Hippopotamus minor or Hippopotamus liberiensis due to its smaller cranial dimensions compared to the common hippopotamus ( amphibius). British zoologist reassigned it to the new genus Choeropsis in 1849, emphasizing dental and skeletal distinctions from both Hippopotamus and extinct hippopotamids. A synonym, Hexaprotodon liberiensis, arose from mid-20th-century classifications linking it to fossil genera with six lower incisors, but contemporary favors Choeropsis to avoid conflation with extinct Asian forms now exclusively under Hexaprotodon. This reclassification aligns with phylogenetic analyses prioritizing cranial and postcranial autapomorphies unique to the living pygmy lineage.

Subspecies Recognition

Two subspecies of the pygmy hippopotamus (Choeropsis liberiensis) are currently recognized: the nominotypical C. l. liberiensis Morton, 1849, and C. l. heslopi Corbet, 1969. This taxonomic division is based primarily on morphological analysis of cranial specimens, as genetic data remain insufficient due to limited samples from wild populations. The subspecies C. l. heslopi was described by Corbet in from four s collected by J. P. Heslop in 1945 near Omoku in Nigeria's , within the . Corbet distinguished it from C. l. liberiensis through differences in cranial proportions, including a relatively shorter and broader with specific metric variations in dimensions such as length and row measurements. These osteological traits suggest adaptive divergence possibly linked to localized pressures in the delta's estuarine forests and swamps, though sample size constraints limit definitive causal attribution. The heslopi subspecies is regarded as functionally extinct, with no verified live populations or sightings since the 1940s, attributable to intensive habitat conversion for , oil extraction, and unregulated in the . In contrast, C. l. liberiensis persists in fragmented populations across , , , and Côte d'Ivoire, though overall numbers continue to decline. Taxonomic debate persists, with Groves and Grubb (2011) elevating heslopi to species rank (Choeropsis heslopi) based on its pronounced cranial disparities and geographic isolation, arguing that the differences exceed typical intraspecific variation observed in related . However, subsequent assessments, including the IUCN evaluation by Lewison (2011) and the 2019 Mammalian Species account, retain subspecies status pending molecular corroboration, emphasizing the provisional nature of morphology-only classifications in data-poor taxa. No comprehensive phylogeographic studies have resolved this, as heslopi tissue samples are unavailable for DNA analysis.

Evolutionary Divergence

The pygmy hippopotamus (Choeropsis liberiensis) represents a basal lineage within the family , distinct from the more derived common hippopotamus (Hippopotamus amphibius). Molecular phylogenetic analyses using 26 nuclear gene sequences and Bayesian relaxed clock methods estimate the divergence between these two extant genera at approximately 4.04 million years ago (95% CI: 1.97–8.31 Ma) via RelTime-ML or 2.4 million years ago (95% CI: 1.6–3.1 Ma) via MCMCTree, placing the split in the to . These estimates suggest a closer phylogenetic relationship than previously hypothesized, challenging earlier views of a deeper separation exceeding 10 million years. Fossil records indicate that hippopotamid ancestors trace back to anthracothere , semi-aquatic herbivores from the whose body plans, such as the elongated skulls and limb structures in genera like Anthracotherium, exhibit morphological parallels to the pygmy hippopotamus. Earliest unequivocal hippopotamid remains resembling modern forms appear in the , around 8 million years ago in , with the Choeropsis lineage potentially retaining more primitive traits from these anthracothere forebears. However, the fossil record for C. liberiensis itself shows a significant gap, lacking pre-Pleistocene specimens, which complicates precise calibration of divergence timelines and supports the need for integrated molecular-morphological approaches. Post-divergence, the pygmy hippopotamus lineage appears to have undergone secondary adaptations toward a more terrestrial lifestyle compared to the semi-aquatic common hippo, potentially driven by niche partitioning in forested West African habitats. This evolutionary trajectory aligns with the family's broader origins from Eocene-Oligocene that diverged alongside cetacean ancestors around 50–55 million years ago, underscoring Hippopotamidae's position within the whale-hippo (Cetancodonta).

Physical Characteristics

Morphology and Size

The pygmy hippopotamus (Choeropsis liberiensis) possesses a compact body structure suited to navigating dense forest undergrowth and swampy terrains. Adults measure 1.5 to 1.75 meters in head-body length, with a tail length of approximately 15 to 20 centimeters, stand 75 to 100 centimeters tall at the shoulder, and weigh 160 to 275 kilograms. Males tend to be slightly larger than females, though is minimal. In comparison to the common , the pygmy species exhibits proportionally longer limbs relative to body size, a narrower and more rounded head, and a longer neck, which enhance over aquatic prowess. The feet bear four digits each, with reduced webbing and more spread-out toes compared to the fully webbed feet of its larger relative, reflecting adaptations for walking on forest floors rather than prolonged submersion. The skin is thick, nearly hairless, and dark gray to blackish, secreting a reddish, oily fluid that functions in and protection. Dentition includes 36 to 38 teeth, featuring lower incisors that project forward and less enlarged canines than in common , with molars adapted for grinding through prominent cingula and simplified cusps. The is characterized by a shorter rostrum and a less robust overall build, consistent with the ' smaller stature and dietary habits.

Sensory and Physiological Adaptations

The pygmy hippopotamus (Choeropsis liberiensis) possesses sensory adaptations that balance its semi-aquatic lifestyle with greater terrestrial mobility compared to its larger relative, the common . The eyes are positioned laterally on the head, facilitating a wider for navigating dense undergrowth and detecting threats on land, rather than the dorsal placement seen in reliant on prolonged submergence. Ears and nostrils feature muscular valves that seal during dives, preventing water entry while allowing rapid resurfacing for air. Olfactory cues are utilized for communication, primarily through scent marking with to signal presence or reproductive status, reflecting reliance on chemical signaling in low-visibility habitats. Vocalizations remain subdued, limited to occasional snorts, grunts, hisses, and squeaks, which may aid in short-range interactions without attracting predators in nocturnal environments. Physiologically, the species is adapted for and survival in humid, forested wetlands, with a geared toward minimizing expenditure and storing fat reserves to support sporadic . Its , thick yet nearly hairless and lacking conventional sebaceous or sweat glands, is prone to rapid outside water, necessitating frequent immersion for and hydration; a specialized oily , sometimes brownish-red and protective against solar damage, is produced to maintain moisture and shield sensitive . During submersion, individuals can hold their breath for 3 to 6 minutes, enabled by valvular closures and efficient lung capacity, allowing evasion of diurnal threats while resting in streams. The exhibits a reduced granular layer IV and specialized neuronal clusters akin to those in cetaceans, potentially enhancing integration of aquatic and terrestrial sensory inputs for spatial in varied media. These traits underscore a optimized for solitary, crepuscular activity rather than the gregarious, fully aquatic demands of congeners.

Distribution and Habitat

Current Geographic Range

The pygmy hippopotamus (Choeropsis liberiensis) is currently restricted to fragmented populations in the Upper Guinean lowland forests of four West African countries: Côte d'Ivoire, , , and . Liberia contains the majority of the remaining individuals, with smaller numbers persisting in the other three nations amid ongoing habitat degradation. Historically more widespread across the region, the species has been extirpated from former range areas including , leaving only these isolated remnants as of assessments through 2023. These populations occupy disjointed forest-swamp habitats, with no confirmed viable groups beyond these countries despite occasional unverified reports elsewhere. The IUCN Hippo Specialist Group notes that the nominate is endemic to this West African forest block, underscoring the narrow and contracting geographic extent.

Habitat Preferences and Requirements

The pygmy hippopotamus (Choeropsis liberiensis) primarily inhabits lowland primary and secondary forests in , favoring environments with dense canopy cover and proximity to permanent or semi-permanent water sources such as slow-moving rivers, streams, and swampy areas. These habitats typically occur at low elevations below 400 meters, where moist, tropical conditions support thick and gallery forests along watercourses. Raphia palm-dominated swamps represent a key microhabitat, providing both foraging opportunities and refuge, as the species relies on such areas for movement and evasion of predators like leopards. Unlike the common , which is largely aquatic, the pygmy hippopotamus exhibits more terrestrial habits, spending the majority of its time on or in shallow but never venturing far from aquatic features essential for and maintenance. It requires access to wallowing sites in mud or shallow pools to prevent , as its thick lacks functional sweat glands and relies on external to avoid and sunburn during daytime retreats. Nocturnal drives selection for habitats with abundant browse—such as shoots, fruits, and —under dense that offers concealment for its solitary . Observations from field studies confirm avoidance of open or fragmented areas, with individuals utilizing swamps and riverbanks for both daily submersion and nocturnal excursions. Ecological requirements emphasize undisturbed vegetation for cover and sustained water availability year-round, as seasonal drying of can limit suitable sites and force reliance on secondary with artificial water retention. Dense undergrowth facilitates minimal disturbance during rest periods, while the ' low —estimated at less than one individual per 10 square kilometers in optimal habitats—underscores the need for large, contiguous blocks exceeding 100 square kilometers to support viable home ranges of 5-20 square kilometers per adult.

Behavior and Ecology

Activity Patterns and Movement

The pygmy hippopotamus (Choeropsis liberiensis) displays primarily nocturnal and crepuscular activity patterns, with individuals resting during daylight hours in concealed locations such as swamps, , root hollows, or rivers to avoid heat and predation. studies in the wild confirm peak activity at night, though not exclusively so, as some foraging or movement may occur during daylight depending on individual variation and environmental factors like seasonal inundation. In , active browsing typically spans from late afternoon (around 4 p.m.) to evening (until 11 p.m.), followed by rest from midnight to early afternoon, reflecting adaptations to cycles where visibility and foraging efficiency favor low-light periods. Movement is predominantly solitary or in transient pairs, with individuals traversing established, meandering tunnel-like paths—up to 60 cm wide—through dense undergrowth and swamps to connect resting sites with areas, thereby minimizing energy expenditure and exposure in habitats. Home ranges vary by sex, averaging approximately 2 km² for males and 0.5 km² for females, with overlaps indicating limited territoriality; these ranges encompass streams, swampy depressions, and upland edges, and movement distances increase during rainy seasons when low-lying areas , prompting shifts to drier upland paths. Locomotion on land involves a deliberate walking gait, transitioning to a when fleeing threats, after which the animal hides until danger passes; in aquatic environments, propulsion occurs via walking along stream or swamp bottoms rather than free , facilitated by splayed toes and sturdy limbs suited for semi-aquatic traversal. These patterns underscore a low-energy, elusive lifestyle evolved for survival in predator-rich, vegetated riverine systems.

Diet and Foraging Strategies

The pygmy (Choeropsis liberiensis) is a herbivorous browser with a generalist diet comprising a wide array of vegetation, including leaves, shoots, roots, fallen fruits, ferns, tubers, and semi-aquatic plants, while grass constitutes a minor proportion compared to the common . Fecal analysis from in Côte d'Ivoire, conducted in 2021, identified consumption of diverse shrubs and grasses, confirming an intermediate feeding strategy that avoids over-reliance on any single plant type to mitigate risks from seasonal variability. Foraging occurs primarily at night, aligning with the species' nocturnal activity patterns, during which individuals traverse forested and streamside clearings to selectively crop using their broad incisors and muscular lips. This solitary minimizes competition and energy expenditure, with observations indicating less time allocated to feeding relative to resting compared to grazing , reflecting adaptations to a high-quality, nutrient-dense diet in dense habitats. Proximity to water bodies facilitates access to both terrestrial and aquatic , enhancing dietary breadth without extensive migration.

Social Interactions and Temperament

The pygmy hippopotamus (Choeropsis liberiensis) exhibits a predominantly solitary in the wild, differing markedly from the social herds of the common (Hippopotamus amphibius). Individuals are typically observed alone or in transient mother-calf pairs, with adults avoiding prolonged contact except during . This solitary nature aligns with their secretive, nocturnal habits in dense forest environments, where encounters between unrelated individuals are rare and often limited to brief territorial overlaps. Temperamentally, pygmy hippopotamuses are shy and timid, preferring evasion over confrontation when threatened, unlike the overtly aggressive displays common in their larger relatives. They lack strong territorial instincts, with males roaming larger home ranges—estimated at up to 20 square kilometers—while overlapping with those of females without frequent hostility. Natural predators like leopards pose the primary risks, prompting flight responses rather than defensive stands. However, when cornered or disturbed, they can exhibit defensive aggression, including charges or bites, though such incidents are infrequent in wild observations. In , social dynamics shift due to constraints, revealing potential for intraspecific despite their wild reticence. Zoo records indicate that while pairs may coexist peacefully for breeding, introductions between unfamiliar individuals often lead to conflicts, with males showing dominance through jaw displays or physical clashes. Successful housing typically limits groups to compatible pairs or solitary adults, minimizing stress and injury risks, as evidenced by breeding programs where separation post-mating reduces antagonism. Observations from facilities like the underscore that pygmy hippopotamuses retain a reclusive disposition, spending much time submerged or hidden, with social tolerance varying by sex and familiarity.

Reproduction and Development

Mating Behaviors and Gestation

Pygmy hippopotamuses (Choeropsis liberiensis) display a polyestrous reproductive pattern with no evident in captive populations, where the averages 30.9 to 31.8 days based on fecal metabolite from multiple females. Estrus itself lasts 24 to 48 hours, during which may occur multiple times. In , copulation takes place either on land or in water, often facilitated by managed pairings, though wild behaviors remain poorly documented due to the ' elusive and solitary . Observations suggest opportunistic encounters when a enters a male's territory, with males exhibiting territorial defense that may influence access to receptive females, but direct displays have not been reliably described. Gestation in the pygmy hippopotamus lasts an average of 203 days (range approximately 184 to 210 days), determined from breeding records of 15 captive pregnancies. Typically, a single calf is born, though twins occur rarely; births happen on land, aligning with the species' preference for forested habitats near water bodies. Pregnancy is characterized by elevated metabolites detectable non-invasively from mid-gestation onward, confirming spontaneous without induced mechanisms. is reached by males around 2.8 to 3 years and females as early as 23 months in captivity, though successful first reproductions often occur later, averaging eight years for females due to management factors. Wild reproductive rates are inferred to be low, contributing to population vulnerability, as encounters between adults are infrequent given their solitary habits and fragmented habitats.

Offspring Care and Lifespan

Female pygmy hippopotamuses give birth to a single calf, rarely twins, following a period of 6 to 7 months. Births typically occur in secluded, dense on land, where the mother provides protection by concealing the newborn to minimize predation risk. The calf, weighing 3.4 to 6.4 kilograms at birth, can stand and move shortly after delivery, enabling it to follow the mother when necessary. Maternal care is solitary, reflecting the species' generally asocial nature, with the female the calf periodically while leaving it hidden during excursions. occurs in a recumbent position, and lasts several months, with generally between 6 and 8 months of age. Calves begin to accompany the mother more frequently post- and achieve independence around one year, though they reach at 3 to 5 years. In the wild, pygmy hippopotamuses have an estimated median lifespan of 13 years, influenced by threats such as and predation. Captive individuals exhibit extended , with lifespans ranging from 35 to 50 years and maximum recorded ages up to 52 years. This disparity underscores the impact of human-related pressures on wild populations versus controlled environments in zoos.

Conservation and Threats

Population Dynamics and Decline Factors

The wild population of the pygmy hippopotamus (Choeropsis liberiensis) is estimated at 2,000–2,500 mature individuals, with a continuing decline observed across its fragmented range. This figure reflects a sharp reduction from historical estimates of around 19,000 individuals during 1982–1986, driven by habitat degradation and direct human pressures. The species persists in low densities within Upper Guinean lowland forests of four countries—primarily , which supports the largest subpopulation, followed by smaller groups in , , and Côte d'Ivoire—with no confirmed presence in since 1943. Accurate censuses remain challenging due to the animal's solitary habits, nocturnal activity, and elusive behavior in dense vegetation, limiting data on birth and mortality rates but indicating insufficient recruitment to offset losses. Habitat loss constitutes the dominant decline factor, with 17% (15,900 km²) of suitable canopy (≥50% cover) lost across the from 2000 to 2019, predominantly in and d'Ivoire. Shifting —small-scale rotational farming—drove 93.4% of this , fragmenting contiguous forests into isolated patches that reduce viable territory for the territorial, stream-dependent species. Commercial , , and expanding settlements compound this by altering and increasing exposure to , such as predation and competition. Hunting contributes additively, primarily as opportunistic bushmeat collection by local communities in degraded areas, rather than targeted commercial , though civil conflicts in range states have intensified by disrupting enforcement. The ' slow reproductive cycle—one calf every 2–3 years after a 6-month —exacerbates vulnerability, as likely depresses encounter rates and juvenile survival. Overall, these pressures have reduced the effective population size below critical thresholds for genetic viability, with projections suggesting further contraction absent intervention.

Primary Threats from Human Activities

The primary threats to the pygmy hippopotamus (Choeropsis liberiensis) from human activities center on habitat destruction and direct exploitation. Deforestation, driven predominantly by shifting agriculture, has been the leading cause of forest loss within the species' range across Liberia, Sierra Leone, Guinea, and Côte d'Ivoire from 2000 to 2019, with protected areas experiencing similar pressures despite conservation efforts. This activity fragments the dense, moist forests and swampy riparian zones essential for the animal's cover and foraging, exacerbating vulnerability to predation and reducing access to food sources. In Liberia alone, nearly 400,000 hectares of forest were lost between 2001 and 2012, contributing to the destruction of over 90% of the Upper Guinea Forest ecosystem that historically supported the species. Logging, mining, and agricultural expansion for cash crops further degrade habitats by altering hydrology and increasing soil erosion, which disrupts the pygmy hippo's preference for undisturbed, water-adjacent forests. Human encroachment, tied to population growth in West African range countries, intensifies these pressures, leading to habitat fragmentation that isolates small subpopulations and hinders gene flow. River pollution from mining and upstream activities contaminates waterways used for thermoregulation and escape, compounding habitat unsuitability. Direct hunting for and trophies poses a significant additional threat, as snares and firearms indiscriminately target the elusive, nocturnal species, with demand rising in rural communities amid economic hardship and civil conflicts. Although legally protected in all range states, enforcement is weak, resulting in ongoing that removes breeding adults and calves. These combined human-induced factors have driven a to fewer than 2,500 individuals as of recent estimates, with no evidence of stabilization.

Conservation Initiatives and Outcomes

Conservation initiatives for the pygmy hippopotamus (Choeropsis liberiensis) primarily focus on habitat protection, efforts, and programs. In , the first National Action Plan for the species was developed in collaboration with the IUCN Species Survival Commission Hippo Specialist Group, emphasizing the gazettement of key forests such as Gola, Grebo, and Wonegizi as protected areas to safeguard remaining populations. Sapo in harbors a significant subpopulation, with surveys confirming presence along the Duobe River and adjacent Grebo National Forest. Internationally, the (ZSL) and IUCN SOS initiatives support , community awareness, and a conservation roadmap to address threats in West African range states. Captive breeding programs in zoos have achieved notable success, contributing to a growing ex situ population. Facilities such as London Zoo, Houston Zoo, and Tanganyika Wildlife Park participate in breeding recommendations, with births like that of Amara in 2023 at London Zoo and a 477-pound male transfer to Houston in 2024 demonstrating program efficacy. Historical data indicate a rapid population increase in zoos from 1960 to 1980, with sustained survivorship among juveniles and adults. Despite these efforts, outcomes remain limited, with wild populations continuing to decline due to persistent and weak enforcement in protected areas. A 1993 estimate placed the global at 2,000–2,500 individuals, but numbers have likely decreased further amid forest loss from shifting between 2000 and 2019 across 87 protected areas totaling 30,669 km². IUCN assessments from 2022–2024 confirm ongoing declines driven by human activities, with no evidence of recovery or successful reintroductions. Viral popularity of captive individuals, such as Moo Deng in , has not translated into measurable conservation gains for wild populations. The species retains Endangered status on the , underscoring the insufficiency of current measures against anthropogenic pressures.

Captive Management

Breeding Programs in Zoos

Captive programs for the pygmy hippopotamus (Choeropsis liberiensis) have been established since the early to bolster the species' population amid wild declines, with the first documented introduced in at a , initiating ongoing efforts. These programs operate under coordinated frameworks like the European Association of Zoos and Aquaria (EAZA) Pygmy Hippopotamus European Endangered Species Programme (EEP) and the Association of Zoos and Aquariums (AZA) (SSP), which manage studbooks to optimize and breeding recommendations through animal transfers. Pygmy hippos reproduce successfully in , with natural encouraged over artificial methods, and females typically reaching breeding age around eight years on average, though viable have been produced as young as 23 months. Recent breeding successes underscore the programs' efficacy, including the birth of an 11-pound female calf on September 3, 2025, at John Ball Zoo in —the facility's first pygmy hippo offspring from parents Penelope and Jahari. Similarly, a male calf arrived on June 26, 2025, at Tanganyika Wildlife Park in , contributing to North American conservation goals. International movements support these efforts, such as the 2023 transfer of female Amara to for pairing and the 2025 relocation of male from to another facility under EEP guidelines. In November 2024, received a 477-pound, four-and-a-half-year-old male from per SSP recommendations to facilitate future pairings. Organizations like the (ZSL) integrate zoo breeding with field conservation, producing IUCN Small Species Specialist Group action plans while maintaining breeding at and Whipsnade Zoos. These initiatives have sustained a viable captive , with survivorship analyses indicating improved and demographic stability through managed breeding up to 2023. Despite occasional challenges like neonatal mortality, the programs demonstrate that pygmy hippos adapt well to ex situ management, providing a genetic reservoir for potential reintroduction and research.

Challenges in Captivity

Captive pygmy hippopotamuses face significant reproductive challenges, including high neonatal mortality rates, with perinatal causes such as stillbirths, , and maternal neglect accounting for 51.8% of deaths in juveniles, and parent-inflicted trauma contributing to 28% of these cases. A distorted at birth, with only 41% males compared to 59% females, exacerbates breeding difficulties by limiting male availability for pairing, further compounded by higher juvenile mortality in males. These issues persist despite monitoring and interventions like creep areas to prevent maternal crushing of calves, highlighting the species' sensitivity to captive conditions that deviate from wild solitary rearing patterns. Health problems are prevalent, with (PKD) diagnosed frequently in adults, often leading to renal insufficiency and contributing to 13% of geriatric mortalities, though it rarely impacts fertility directly. affects many adults, exceeding healthy weights of 180-250 kg due to diets high in commercial fruits and vegetables, indirectly reducing calf survival by impairing maternal mobility and increasing stereotypic behaviors like glass-pressing from enclosure boredom. Infectious diseases, including encephalomyocarditis and , cause acute deaths, particularly in regions like the , while poses high risks with opioid-only protocols linked to elevated mortality. Husbandry challenges include simulating swamp-like habitats, where low humidity below 85-90% or water temperatures under 18-25°C cause cracking, and fibrous complicate maintenance. Aggressive inter-sex interactions necessitate separate enclosures with retreat options, and prolonged use of or rubber flooring leads to foot cracks and pododermatitis. Neonatal risks arise from steep pool drop-offs, requiring gentle slopes or drainable designs, while overall population management struggles with targets due to the female-biased ratio and limited breeding success across global zoos.

Extinct Relatives and Fossil Record

Known Extinct Species

The fossil record for the genus Choeropsis is extremely limited, with no definitively identified extinct species directly attributable to it, reflecting a broader of post-Miocene fossils for the pygmy hippopotamus lineage. Phylogenetic analyses indicate an early divergence from the common hippopotamus (Hippopotamus amphibius) lineage potentially in the or early , but the absence of transitional fossils complicates precise reconstruction of its evolutionary history. Historical taxonomic placements linked C. liberiensis to the extinct genus Hexaprotodon, now restricted to Asian hippopotamids, suggesting these as potential stem relatives within . Hexaprotodon sivalensis, known from Siwalik Group sediments in northern , represents one such species, with remains dated from the (approximately 7–5 million years ago) through the (around 2.5–0.8 million years ago). This species exhibited dental features like six lower incisors, distinguishing it from modern hippos, and inhabited fluvial environments similar to those of extant forms. Its persistence until the Pleistocene underscores a prolonged Asian radiation of hippopotamids before regional extinctions linked to climatic shifts, such as weakening monsoons. Other Hexaprotodon taxa, including H. namadicus from similar Indian Pleistocene deposits, share morphological traits with early hippopotamids but differ in size and cranial robusticity from the smaller C. liberiensis, which weighs 180–275 kg compared to larger extinct congeners. These Asian forms likely migrated from via dispersals during the , representing basal branches in the family rather than direct ancestors, though ongoing debates persist regarding the exact phylogenetic position of Choeropsis relative to Hexaprotodon. Extinctions of these relatives by the correlate with habitat alterations and in .

Causes of Extinction in Relatives

The extinct dwarf hippopotamuses of , including species such as Hippopotamus laloumena, disappeared approximately 1,000 to 2,000 years ago, coinciding with colonization of the island around 2,500 years ago. Evidence from modified bones, including cut marks on at least seven specimens indicating butchery, supports overhunting by early settlers as a primary factor, though alteration from for and permanent settlements likely exacerbated the decline. Some researchers propose that interactions with , such as , further contributed to competitive exclusion and resource scarcity, rather than climate alone, as persisted briefly post-human arrival. Similarly, Pleistocene dwarf hippopotamuses on (Phanourios minor) went extinct around 13,000 to 14,000 years ago, shortly after human settlement evidenced by dated tools and faunal remains. of over 200 bones and associated artifacts indicates rapid population collapse, attributable to intensive hunting by small human groups exploiting low-density island , which lacked prior predator pressure and exhibited reducing resilience. This pattern aligns with broader Mediterranean island extinctions, where human predation outpaced natural recovery rates. In , species like and late-surviving populations persisted until about 40,000 years ago but succumbed to the onset of the , marked by rapid cooling and habitat contraction from temperate wetlands to unsuitable arid or frozen environments. Fossil assemblages from sites in and Britain show abrupt disappearance linked to decreased precipitation and vegetation shifts, independent of direct human impact, as hippo ranges retracted southward ahead of advancing sheets. Earlier Miocene-Pliocene hippopotamid relatives, such as anthracotheres, faced amid global aridification and savanna expansion, reducing aquatic refugia essential for their semi-aquatic lifestyle.

Historical Discovery and Research

Early Observations and Classification

The pygmy hippopotamus (Choeropsis liberiensis) was first scientifically described based on skull fragments collected during a expedition to in 1843. American naturalist published the initial description in 1844, naming the animal Hippopotamus minor to distinguish it from the larger common (Hippopotamus amphibius), citing its proportionally shorter rostrum, smaller molars, and overall reduced cranial size as evidence of a distinct adult form rather than a juvenile or pathological variant of the common species. In 1849, Morton provided a fuller account, formally naming it Hippopotamus liberiensis after the type locality in , emphasizing dental and skeletal traits that suggested a more primitive hippopotamid morphology. The genus Choeropsis—derived from Greek terms meaning "pig face," alluding to its narrower, less prognathic skull—was established by British zoologist in 1852 specifically for this species, separating it taxonomically from based on of available specimens. Early classifications debated its affinities, with some researchers initially viewing it as a dwarfed relative or even linking it to anthracotheres due to limb proportions and habitat preferences, but 19th-century consensus affirmed its placement within as a basal lineage. Contemporary observations were constrained by the species' nocturnal, solitary habits in dense West African forests, with initial knowledge derived almost exclusively from indigenous Liberian reports of a small, secretive "river pig" and sporadic hunter encounters, rather than direct fieldwork. Live specimens remained elusive until the late , when the first was imported to in , though it did not survive long in , underscoring early challenges in documenting beyond osteological evidence.

Modern Field Studies and Genetic Insights

Modern field studies of the Choeropsis liberiensis have employed camera traps and indirect sign surveys to overcome its nocturnal, solitary habits and preference for dense forest understory. In Sapo National Park, —a key stronghold—camera traps first documented the species in 2008, with follow-up surveys from 2009–2010 detecting it at low densities across 11 transects, estimating approximately 22 individuals based on encounter rates and signs like dung and tracks. Similarly, in , Côte d'Ivoire, the Taï Hippo Project has used camera traps to record occurrences primarily in gallery forests, revealing crepuscular activity peaks and avoidance of open areas, though detections remain infrequent due to the animal's wariness of human presence. These efforts, supplemented by sampling in southeast since 2020, confirm persistence in isolated pockets but highlight fragmentation, with no evidence of large aggregations. Population assessments from such studies inform IUCN evaluations, estimating 2,000–2,500 mature individuals across , with a declining trend attributed to conversion and incidental snaring rather than targeted . Recent IUCN Hippo Specialist Group reports (2023–2025) underscore ongoing declines in surveyed sites, where annual detection probabilities below 0.1 suggest populations below viable thresholds without intervention. Genetic analyses have clarified phylogenetic position and conservation needs. Mitochondrial and nuclear DNA studies indicate divergence from Hippopotamus amphibius in the – (circa 5.7 million years ago), later than prior estimates of 8–18 million years, based on 12S rRNA and multi-locus data that resolve Choeropsis as sister to within . Full genome sequencing in 2021 revealed adaptations like specialized skin microbiomes for secretions, evolving independently from cetacean traits despite shared ancestry. For management, restriction-site associated of wild samples yielded 1,619 high-confidence SNPs, enabling parentage testing and diversity metrics that show moderate heterozygosity but risks from isolation, informing translocations to bolster fragmented groups. Captive programs integrate these markers to avoid , as wild alleles exhibit bottlenecks from historical range contraction.

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  79. https://www.researchgate.net/publication/271631738_Hundreds_of_SNPs_for_the_Endangered_pygmy_hippopotamus_Choeropsis_liberiensis
  80. https://belgianjournalofzoology.eu/BJZ/article/view/15
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