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Hippopotamidae
Hippopotamidae
Hippopotamidae
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For the common hippopotamid species, see Hippopotamus

Hippopotamidae
Temporal range: 7.4–0 Ma Late Miocene-Recent
Common hippopotamus
Pygmy hippopotamus
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Infraorder: Ancodonta
Superfamily: Hippopotamoidea
Family: Hippopotamidae
Gray, 1821
Subtaxa

†Trilobophorus Geze, 1985

Hippopotamidae is a family of stout, naked-skinned, and semiaquatic artiodactyl mammals, possessing three-chambered stomachs and walking on four toes on each foot. While they resemble pigs physiologically, their closest living relatives are the cetaceans. They are formally referred to as hippopotamids.

There are two living species of hippopotamid in two genera; the pygmy hippo, Choeropsis liberiensis of the forests of west Africa, and the common hippo, Hippopotamus amphibius. The term hippopotamus can also be applied to hippopotamids in general, although it is most frequently used for the common hippo and its respective genus.

Characteristics

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Hippopotamids are large mammals, with short, stumpy legs, and barrel-shaped bodies. They have large heads, with broad mouths, and nostrils placed at the top of their snouts. Like pigs, they have four toes, but unlike pigs, all of the toes are used in walking. Hippopotamids are unguligrade, although, unlike most other such animals, they have no hooves, instead using a pad of tough connective tissue on each foot. Their stomachs have three chambers, but they are not true ruminants.

The living species are smooth-skinned and lack both sebaceous glands and sweat glands. The outer epidermis is relatively thin, so hippos dehydrate rapidly in dry environments.[1]

Both the incisors and canines are large and tusk-like, although the canine tusks are by far the larger. The tusks grow throughout life. The postcanine teeth are large and complex, suited for chewing the plant matter that comprises their diets. The number of incisors varies even within the same species, but the general dental formula is given in the table below:

Dentition
2–3.1.4.3
1–3.1.4.3

Evolution

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Main article: Evolution of hippopotamids
Hippopotamus skeleton at Għar Dalam

The hippopotamids are descended from the anthracotheres, a family of semiaquatic and terrestrial artiodactyls that appeared in the late Eocene, and are thought to have resembled small- or narrow-headed hippos. The hippos split off from the anthracotheres some time during the Miocene. The oldest records of Hippopotamidae are from Afro-Arabia and date to the late Miocene, approximately 7.4 million years ago, expanding into Eurasia around 6 million years ago.[2] It has been theorised that this Late Miocene radiation of hippopotamids represents the coevolution of hippopotamids with the expansion of C4 grasslands,[3] a phenomenon known as the hippopotamine event (HE).[4] After the appearance of the hippopotamids, the remaining anthracotheres went into a decline brought about by a combination of climatic change and competition with their descendants, until the last genus, Merycopotamus, died out in the early Pliocene of India.

There were once many species of hippopotamid, but only two survive today: Hippopotamus amphibius, and Choeropsis liberiensis. They are the last survivors of two major evolutionary lineages, the hippos proper and the pygmy hippos, respectively; these lineages could arguably be considered subfamilies, but their relationship to each other – apart from being fairly distant relatives – is not well resolved.

The enigmatic Miocene Kenyapotamus is insufficiently known to be assigned a place in the hippo phylogeny with any degree of certainty. In addition, the genus Hexaprotodon, which is now largely restricted to South Asia and Southeast Asia, formerly included many fossil hippopotamuses that are now thought to be unrelated.[5]

Taxonomy

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Hippopotamidae's placement within Artiodactyla can be represented in the following cladogram:[6][7][8][9][10]

Artiodactyla

Tylopoda (camels and kin)

Artiofabula

Suina (pigs and kin)

Cetruminantia

Ruminantia (ruminants)

Whippomorpha

Hippopotamidae (hippopotamids)

Cetacea (whales)

(or Cetancodonta)

Analogous structures

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The lower canine teeth of hippopotamids are similar in function and structure to the tusks of elephants. While hippopotamids and elephants are only very distantly related within the Mammalia, the lower canine teeth of both groups are long and have a slight curve, and species of both families use this structure when fighting.


Species

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The systematics and taxonomy used here mostly follows a review by J.-R. Boisserie[11][a] and the American Society of Mammalogists.[12]

Recent species

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Fossil species

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See also

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Footnotes

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Hippopotamidae

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from Grokipedia
Hippopotamidae is a family of large, semi-aquatic mammals native to , comprising two extant : the common (Hippopotamus amphibius) and the pygmy (Choeropsis liberiensis). These even-toed ungulates are characterized by their barrel-shaped bodies, short pillar-like legs, nearly hairless skin that secretes a reddish "blood sweat" for protection, and massive heads with wide mouths featuring large canine tusks used for display and combat. Members of the family exhibit a dental formula of I 2–3/1–3, C 1/1, P 4/4, M 3/3, resulting in 38–44 teeth, with continuously growing incisors and canines forming tusk-like structures. Hippopotamids are amphibious herbivores, spending their days submerged in rivers, lakes, and swamps to regulate body and avoid sunburn, while emerging at night to graze on grasses and aquatic vegetation using a non-ruminating, multichambered . The common , the largest living , can weigh up to 4,500 kg and measure 5 m in length, inhabiting slow-moving freshwater systems across much of south of the , where herds of up to 40 individuals form complex social structures marked by vocalizations, mud wallowing, and territorial behaviors. In contrast, the smaller , weighing around 250 kg and standing about 75 cm at the shoulder, is more solitary and nocturnal, preferring forested swamps and rivers in , with a more elongated body adapted for dense vegetation. Evolutionary origins trace back to the early in , with fossils indicating diversification and migration to during the late , though both living species are now confined to ; notably, hippopotamids form the Whippomorpha with cetaceans, sharing aquatic adaptations like internal testes and echolocation-like vocalizations. Reproduction involves periods of 6–8 months, with calves born in water or on land depending on the species, and females providing extended maternal care. Ecologically, hippos play key roles in nutrient cycling by fertilizing aquatic ecosystems with dung and maintaining waterways through , but they pose risks to humans through aggressive defense of territories. Conservation challenges are significant, with the common hippopotamus classified as Vulnerable on the due to habitat loss, for meat and , and human-wildlife conflict, while the is Endangered from and , with populations estimated at fewer than 2,500 mature individuals. Both are listed under Appendix II to regulate , underscoring the need for protected areas and efforts to preserve these iconic .

Taxonomy and Classification

Etymology and nomenclature

The name Hippopotamidae is derived from the Ancient Greek words hippos (ἵππος), meaning "horse," and potamos (ποταμός), meaning "river," thus translating to "river horse family," a nomenclature that aptly reflects the semi-aquatic habits of its members. The family was formally established by British zoologist John Edward Gray in 1821, with Hippopotamus designated as the type genus. Within Hippopotamidae, the living members are classified under the subfamily Hippopotaminae (also established by Gray in 1821), which encompasses the two extant genera: and Choeropsis. The genus includes the species H. amphibius (common hippopotamus), named by in 1758, where amphibius denotes its dual terrestrial and aquatic lifestyle. The genus Choeropsis, meaning "pig-like face" from Greek choiros (hog) and opsis (appearance), contains the species C. liberiensis (), described by Samuel G. Morton in 1849 and named for its Liberian origins. Extinct subfamilies include Kenyapotaminae, proposed by Martin Pickford in 1983 to accommodate early Miocene forms like Kenyapotamus, highlighting the family's African origins.

Phylogenetic position

Hippopotamidae belongs to the order Artiodactyla, the even-toed ungulates, and is positioned within the monophyletic clade Whippomorpha, which unites it with the order Cetacea (whales, dolphins, and porpoises). This placement reflects a shared evolutionary history where hippos represent the closest living relatives to cetaceans among terrestrial mammals, supported by both molecular and morphological data. Key evidence derives from studies, particularly analyses of mitochondrial genes such as , which consistently show Hippopotamidae as the to Cetacea. These molecular phylogenies indicate that the divergence between the common ancestor of and cetaceans occurred around 55 million years ago, during the early Eocene. Morphological synapomorphies reinforcing this relationship include the even-toed foot structure characteristic of —manifested in as four toes with two primary weight-bearing digits—and a multi-chambered enabling , akin to digestion but with three chambers in . Such traits underscore the deep ties within , distinguishing it from other lineages like ruminants and suines. Historically, Hippopotamidae was misclassified as closely related to suines (pigs and peccaries) based on superficial morphological similarities in and digit reduction. This view persisted until the , when molecular studies, including pioneering work on sequences, resolved the phylogeny by demonstrating the stronger affinity with cetaceans over suines. Subsequent phylogenomic analyses have further solidified this consensus, integrating fossil and extant data to affirm Whippomorpha's position within Artiodactyla.

Living species

The family Hippopotamidae encompasses two extant genera, each containing a single living : the genus Hippopotamus, represented by the common hippopotamus (H. amphibius), and the genus Choeropsis, represented by the (C. liberiensis). These two form sister taxa within the , sharing a close phylogenetic relationship that diverged during the –. The common hippopotamus (Hippopotamus amphibius) is classified as Vulnerable on the . In contrast, the (Choeropsis liberiensis) is classified as Endangered.

Physical Description

Morphology and anatomy

Hippopotamids exhibit a distinctive suited to their semi-aquatic lifestyle, featuring a massive, barrel-shaped supported by short, sturdy pillar-like legs that end in broad, hoofed feet with four functional toes. The head is characterized by a broad, flattened muzzle housing a wide gape, while the overall form is nearly hairless in adults, with only sparse bristles on the tail, mane, and mouth edges; juveniles retain finer that is largely shed as they mature. This robust morphology emphasizes weight-bearing efficiency over agility on land. The skin of hippopotamids is exceptionally thick, with dermal layers reaching up to 6 cm in depth across much of the body, providing robust protection against abrasions, sunburn, and microbial invasion in their watery habitats; however, the outermost epidermal layer remains relatively thin and prone to cracking if not kept moist. Embedded within this skin are specialized subdermal mucous glands distributed across the body, particularly densely on the flanks and back, which secrete a viscous, fluid often called "blood sweat." This secretion contains hipposudoric acid and norhipposudoric acid, pigments that rapidly polymerize upon exposure to air, offering natural UV absorption to prevent sunburn and exhibiting activity against bacteria such as and . Skeletally, hippopotamids display graviportal adaptations, with dense, pachyosteosclerotic bones in the limbs and ribs that enhance structural support for their enormous mass and facilitate neutral buoyancy underwater by increasing overall body density. The limbs are short and columnar, with robust femora and humeri featuring pronounced muscle attachment sites to accommodate powerful propulsion both terrestrially and aquatically. Cranially, the eyes, ears, and nostrils are dorsally positioned high on the skull, enabling the animals to remain mostly submerged while maintaining vigilance and respiration; this arrangement is accentuated in the common hippopotamus, where forward-facing eyes contrast with the more laterally placed orbits in the pygmy species. The permanent dental formula for the common hippopotamus is I 2/2, C 1/1, P 3–4/3–4, M 3/3 (totaling 36–40 teeth), while the pygmy hippopotamus varies slightly as I 2/1, C 1/1, P 4/4, M 3/3 (totaling 38 teeth); both feature continuously growing tusklike canines and incisors adapted for cropping vegetation and occasional defense. Sexual dimorphism is evident in body proportions, with males typically exhibiting larger overall size and more pronounced canine tusks compared to females.

Size, weight, and sexual dimorphism

Hippopotamids exhibit significant variation in size and weight between the two living species, with the common hippopotamus (Hippopotamus amphibius) being one of the largest terrestrial mammals. Adult males typically measure 3.5–4.5 m in total length and reach a shoulder height of about 1.5 m, while females are slightly smaller at around 1.4 m in shoulder height. Males weigh 1,300–1,800 kg on average, whereas females range from 1,200–1,500 kg, reflecting moderate in body size. This dimorphism is evident in linear dimensions as well, with males averaging 3.12 m in head-body length compared to 3.0 m for females. In contrast, the (Choeropsis liberiensis) is considerably smaller and displays minimal , with sexes appearing nearly alike in size. Adults measure 1.5–1.75 m in length and 0.75–1.0 m in height, with weights ranging from 180–275 kg for both males and females. This reduced dimorphism contrasts with the common , where males are consistently larger and heavier, often by 10–20% in mass. Growth in hippopotamids is rapid during the juvenile phase, with calves gaining substantial mass in the first few months; for instance, pygmy hippo young can reach ten times their (about 4–6 kg) within five months. Both species attain near-adult size by 5–7 years, coinciding with , though common hippo males continue growing incrementally throughout life while females stabilize around age 25. Allometric scaling in hippopotamids results in a disproportionate increase in body girth relative to length, yielding a barrel-shaped form that enhances in aquatic environments despite their substantial . This supports their semi-aquatic lifestyle, with mass-length relationships emphasizing robustness over elongation.

Evolutionary History

Origins and timeline

The clade , encompassing hippopotamids and cetaceans, diverged from other cetartiodactyls approximately 59 million years ago during the Paleocene-Eocene transition, with the hippopotamid lineage separating from cetaceans around 55 million years ago in the post-Eocene period. This deep phylogenetic split positioned hippopotamids within the broader radiation, evolving from terrestrial ancestors amid the diversification of even-toed ungulates following the Cretaceous-Paleogene extinction. Hippopotamidae emerged in during the early Miocene, around 21 million years ago, originating from anthracothere ancestors within the paraphyletic family , specifically advanced selenodont forms like bothriodontines. The earliest known fossils, such as Morotochoerus ugandensis from , document this origin in East African ecosystems, marking the family's initial into two subfamilies: the more terrestrial Kenyapotaminae (spanning ~21–15 million years ago) and the later Hippopotaminae around 7.5 million years ago. evidence extends the family's record to the early , with diversification accelerating in the late Miocene as anthracothere-like traits transitioned toward the semi-aquatic adaptations seen in later forms. Major evolutionary events included the first dispersal to approximately 6–7 million years ago during the , likely via connections across the , followed by a return migration to and the eventual of non-African lineages in the , around 40,000–50,000 years ago. Recent paleogenomic studies from 2025 confirm the survival of the common hippopotamus (Hippopotamus amphibius) in during the last , with remains dated to approximately 47,000 years ago, highlighting greater adaptability to climates than previously thought. These movements coincided with climatic shifts toward cooler, drier conditions in , where expanding savannas and seasonal drove the of a semi-aquatic niche, enabling hippopotamids to exploit riverine refugia amid . This adaptation, briefly tied to shared aquatic traits with cetacean relatives, underscores the family's resilience in fluctuating paleoenvironments.

Fossil record

The fossil record of Hippopotamidae extends back to the early Miocene, with the family's emergence in Africa marking a key phase in artiodactyl evolution. Early forms include Kenyapotamus, a genus known from the middle Miocene of East Africa, particularly sites in Kenya such as those yielding dental remains dated to approximately 16 million years ago. This small-bodied taxon, estimated at 170–220 kg, exhibited more terrestrial adaptations compared to later relatives, with bunodont dentition suggesting a diet suited to forested environments. Kenyapotamus represents a transitional stage in hippopotamid evolution, bridging primitive anthracothere-like ancestors and more derived semi-aquatic forms. Advanced genera appeared during the late Miocene to Pleistocene, exemplified by Hexaprotodon, which ranged across Eurasia and Africa from roughly 7 million years ago into the late Pleistocene. Fossils of Hexaprotodon, such as H. lothagamensis from late Miocene deposits in Kenya, indicate larger body sizes—up to several tons—and enhanced aquatic traits, including broader snouts and limb modifications for wading. This genus also demonstrates island dwarfism in isolated populations, notably the Malagasy species that evolved reduced sizes, with adults weighing around 200–500 kg, likely as an adaptation to limited island resources during the Pleistocene. Late survivors among extinct hippopotamids include island-endemic species like Hippopotamus lemerlei from Madagascar, which persisted until approximately 1,000 years ago based on radiocarbon-dated subfossil remains showing human modification. These dwarf forms, confined to wetlands and rivers, highlight the vulnerability of insular populations to environmental changes and human arrival. Significant discoveries have illuminated the family's , particularly at Toros-Menalla in , a 7-million-year-old site that has produced postcranial fossils of early hippopotamids such as Archaeopotamus, revealing semi-aquatic locomotor adaptations and supporting links to cetacean ancestry through shared traits. These finds underscore Africa's role as the cradle of hippopotamid diversification.

Relationships to other artiodactyls

Hippopotamidae forms the sister group to Cetacea within the clade Whippomorpha, a relationship supported by molecular and morphological evidence indicating that cetaceans are nested within Artiodactyla as the closest living relatives of hippos. This phylogenetic linkage is reinforced by shared derived traits related to semi-aquatic lifestyles, such as streamlined body forms, multilayered skin with reduced hair follicles, and adaptations for subcutaneous fat storage, though some features like epidermal gland secretions appear to have evolved convergently rather than from a common aquatic ancestor. Unlike fully aquatic cetaceans, hippopotamids retain terrestrial mobility while exhibiting these modifications, highlighting a transitional evolutionary stage. The evolutionary origins of Hippopotamidae trace back to anthracothere lineages, a group of extinct that exhibited hippo-like dental specializations, including bunodont molars suited for processing a mix of and possibly animal matter. Transitional forms such as Bothriogenys, a late Eocene to early anthracothere from , display and cranial features foreshadowing those in modern , suggesting anthracotheres as stem hippopotamoids that migrated into aquatic niches. Fossil evidence from these ancestors supports the anthracothere-hippopotamid connection, with progressive adaptations toward larger body sizes and broader premolars observed across Miocene specimens. In contrast to other artiodactyl families like Suidae (pigs) and Camelidae (camels), hippopotamids lack advanced grazing mechanisms such as multi-chambered stomachs or specialized hypsodont teeth for abrasive forage, instead relying on a simpler digestive system that accommodates occasional omnivory alongside herbivory. Suids exhibit broader omnivorous diets with carnassial-like premolars for meat consumption, while camels, as pseudoruminants, possess foregut fermentation but elongated limbs for arid browsing, features absent in the stocky, riverine-adapted hippos. This divergence underscores hippopotamids' unique emphasis on aquatic foraging over terrestrial grazing dominance seen in suiform or tylopod artiodactyls. Molecular clock analyses estimate the divergence of the hippopotamid-cetacean lineage () from other cetartiodactyls, including , around 59-67 million years ago, aligning with post-Cretaceous radiation of even-toed ungulates. These timelines, calibrated using Bayesian relaxed clock models on genomic data, indicate that hippopotamids split from ancestors approximately 60 million years ago, preceding the Eocene diversification of anthracothere forebears.

Distribution and Habitat

Geographic range

The family Hippopotamidae, comprising the common hippopotamus (Hippopotamus amphibius) and the (Choeropsis liberiensis), is primarily distributed across today. The common hippopotamus occupies a broad range spanning 37 countries, from in the west to in the east, and south to , inhabiting rivers, lakes, and associated wetlands throughout this region. Its distribution includes key populations in countries such as , , the , , , , , , and , where it remains relatively abundant in protected areas. In contrast, the pygmy hippopotamus is restricted to a much narrower range in , with fragmented populations surviving in only four countries: Côte d'Ivoire, , , and , primarily in forested riverine habitats. Historically, hippopotamids exhibited a more extensive distribution beyond their current stronghold. Fossil evidence indicates that species within the family expanded into during the , with persistence into the Pleistocene epoch, including records of and related forms in until the , approximately 31,000 to 47,000 years ago, across regions from the to the Mediterranean and as far east as . In , endemic dwarf hippopotamids, such as , were once widespread on but became extinct around 1,000 years ago, following human colonization approximately 2,000 years ago, as evidenced by dated modified bones and associated archaeological layers. The common hippopotamus also formerly occurred in North African regions like and , from which it has been extirpated. Over the past century, hippopotamid ranges have undergone significant contractions due to , with the experiencing an estimated 7–20% over the last three generations (as of 2016), leading to isolated subpopulations increasingly confined to protected wetlands. The has faced even more severe loss, with over 90% of its original forest range in the Upper Guinean ecoregion disappearing since 1900, exacerbating its fragmented distribution. These changes have isolated populations and heightened vulnerability, though core areas in eastern and maintain the largest remaining extents for the common species.

Environmental preferences and adaptations

Hippopotamids exhibit a strong preference for semi-aquatic environments characterized by shallow, slow-moving freshwater bodies such as rivers, lakes, and swamps, which provide essential refuge during the day. These habitats are typically situated in tropical and subtropical regions of , where access to permanent or seasonal water sources is critical for and skin protection from solar radiation. Adjacent grasslands or floodplains are necessary to support their terrestrial foraging activities, ensuring a balance between aquatic safety and land-based resource availability. The common hippopotamus (Hippopotamus amphibius) thrives in open savannas and wetlands with depths of 1–2 meters, allowing full submersion to avoid overheating in climates where temperatures often exceed 30°C. In contrast, the (Choeropsis liberiensis) favors densely forested wetlands and riverine areas in West African rainforests, where shaded, moist microhabitats near streams offer cover and reduce exposure to drier conditions. Both species are highly sensitive to droughts, which can fragment habitats and limit water access, leading to stress and reduced viability in altered environments. Key aquatic adaptations enable hippopotamids to exploit these watery habitats effectively. Valvular nostrils, positioned dorsally, can seal tightly during submersion, permitting the common to remain for up to 5 minutes while resting or evading threats. Similarly positioned eyes and ears allow sensory monitoring of the surface without full exposure. Their barrel-shaped bodies, supported by dense bones rather than substantial subdermal fat, confer negative that facilitates bounding along the substrate at speeds up to 0.5 m/s, rather than true , conserving energy in shallow waters. On land, hippopotamids depend on proximity to to mitigate risks, as their thick, hairless secretes a mucous-based but lacks functional sweat glands for prolonged cooling. Foraging excursions are thus limited, with individuals typically venturing less than 2 km from aquatic refuges during nocturnal activities to prevent overheating and . This constraint underscores their vulnerability in fragmented landscapes where water bodies are isolated from suitable grazing areas.

Behavior and Ecology

Social structure and daily activities

Hippopotamuses, particularly the common hippopotamus (Hippopotamus amphibius), form gregarious social groups known as herds or pods, typically consisting of 10 to 30 individuals, though sizes can reach up to 100 in favorable habitats. These herds are polygynous and primarily comprise adult females with their offspring, supplemented by a single dominant territorial male who maintains access to the group for mating purposes. Within the herd, females often occupy the central positions in resting pools and exhibit leadership in group movements, while subadult males and non-territorial adults may associate loosely on the periphery. Unattached adult males typically live solitarily or form temporary bachelor groups away from established territories, avoiding direct competition with dominant individuals. In contrast, the (Choeropsis liberiensis) is generally solitary, though it may form temporary pairs or small family units consisting of a female and her offspring; males maintain larger territories and wander between areas occupied by females, with limited social interactions outside of . Pygmy hippos exhibit minimal territorial aggression and do not form large groups like their common relatives. Territorial behavior is prominent among adult males and is confined almost exclusively to aquatic environments, where they defend linear stretches of or lakeshore ranging from 50 to 100 meters. To demarcate boundaries, males engage in dung showering, rapidly spinning their tails while defecating to disperse along paths leading to and from the , creating distinctive middens that serve as olfactory markers. Displays of include wide-mouthed yawns that expose their large canines, up to 50 cm long, as a visual threat to rivals, often accompanied by rearing on hind legs or charging. These behaviors peak during the when water levels concentrate groups, intensifying competition for prime resting sites. Daily activities follow a distinct adapted to their semi-aquatic lifestyle, with hippopotamuses spending the daylight hours submerged in rivers, lakes, or pools for and protection from the sun, emerging only briefly to bask or reposition. At , herds exit the water along well-trodden paths, dispersing to graze nocturnally for 4 to 5 hours, covering distances of 3 to 5 km individually in search of vegetation, though this activity is largely solitary despite the social nature of daytime resting. In areas of resource scarcity, individuals may travel up to 8 km or more, returning to water before dawn to avoid and predation risks. Pygmy hippopotamuses are primarily nocturnal and secretive, spending days resting or submerged in rivers, forested swamps, or to avoid heat and predators, before emerging at night to for several hours (typically around 6 hours) on within their territories. Communication among relies on a of vocalizations, visual signals, and olfactory cues to maintain group cohesion and deter intruders over distances exceeding 1 km. Key sounds include deep grunts, wheeze-honks—loud calls reaching 115 decibels—and produced both above and below , with about 80% of vocalizations occurring submerged for efficient transmission in aquatic environments. These low-frequency calls, with fundamentals as low as 20–60 Hz, incorporate infrasonic components that propagate far through air and , enabling long-distance recognition of familiar group members versus strangers. Visual displays, such as yawning or open-mouth threats, reinforce these signals during close encounters, while dung and urine marking provide persistent chemical communication.

Diet, feeding, and interactions

Hippopotamids are strictly herbivorous, with diets dominated by plant matter that reflects their respective habitats. The common hippopotamus (Hippopotamus amphibius) primarily consumes grasses, comprising over 80% of its intake, as evidenced by stable of tusks from historical specimens. Adults forage nocturnally, consuming 35–50 kg of per night, equivalent to about 1–2% of their body mass. In contrast, the pygmy hippopotamus (Choeropsis liberiensis) maintains a more varied diet as a generalist intermediate feeder, incorporating shrubs, grasses, ferns, broad-leaved , herbaceous shoots, and fallen fruits, with minimal grass consumption compared to its larger relative. Feeding occurs primarily on during nocturnal excursions from aquatic refuges, where individuals use their broad, muscular lips to grasp and crop it close to the ground by swinging their heads side to side, aided by lower incisors for shearing. Unlike ruminants, hippopotamids exhibit minimal rumination, relying instead on in a multi-chambered where microbial activity breaks down fibrous material. This digestive strategy supports efficient processing of low-quality but results in lower digestibility rates than in true ruminants, with ingesta retention times of 24–48 hours. As large grazers, hippopotamids play a key role in shaping and vegetation through selective cropping, which prevents overgrowth and maintains short-grass landscapes favorable to other herbivores. Their dung, rich in nutrients like and , facilitates nutrient cycling by transporting terrestrial into aquatic systems, subsidizing and supporting food webs in rivers and pools—though excessive loading in high-density areas can lead to hypoxic conditions. Ecological interactions include conflicts with humans, particularly crop raiding by common hippos, which damage agricultural fields near water bodies and account for the majority of reported human-wildlife conflicts in affected regions. Predation on hippopotamids is rare due to their size and aggression, but Nile crocodiles (Crocodylus niloticus) occasionally target calves or weakened individuals in shared aquatic habitats.

Reproduction and development

Hippopotamids exhibit a polygynous , in which dominant males secure access to multiple females within their territories, influenced by social hierarchies that determine . occurs primarily in water and is not strictly seasonal, though conceptions peak during the , aligning births with the for optimal conditions. In the common hippopotamus (Hippopotamus amphibius), gestation lasts approximately 8 months (227–240 days), after which females typically give birth to a single calf in shallow water to protect against predators. The newborn calf weighs 25–55 kg and can swim immediately, though it relies on the mother for buoyancy and nursing. Females isolate themselves from the group for several days before and after birth to ensure safety, providing exclusive maternal care while males play no role in rearing. Nursing continues for about 8 months, after which calves begin grazing but remain dependent on the mother for up to 2 years. Sexual maturity is reached by females at 3–6 years and males at 6–13 years, with wild individuals living 40–50 years. The pygmy hippopotamus (Choeropsis liberiensis) shares a similar reproductive pattern but with adaptations to its more solitary, forest-dwelling . spans 6–7 months (184–204 days), resulting in litters of one calf, rarely two, born on in dense . At birth, calves weigh 3.4–6.4 kg and receive intensive maternal care, including nursing for 6–8 months, as females remain highly protective and males are uninvolved. occurs around 4–5 years for both sexes, and wild lifespan extends up to 42 years.

Conservation

Current status and threats

The common hippopotamus (Hippopotamus amphibius) is classified as Vulnerable on the , with a global estimated at 125,000–148,000 individuals as of 2025 and an overall decreasing trend. Assessments indicate a suspected decline of more than 20% over the past three generations (approximately 30 years), though recent data from the 2020s suggest range-wide annual declines of 6–8% in many regions, equating to roughly 7–20% over a in affected areas. The (Choeropsis liberiensis) is listed as Endangered, with a of 2,000–3,000 mature individuals also showing a decreasing trend, driven by ongoing and low reproductive rates. Primary threats to both species include habitat loss from agricultural expansion, dam construction, and deforestation, which reduce access to rivers, wetlands, and grazing areas essential for their semi-aquatic lifestyle. Poaching for bushmeat and ivory from their large canines persists, particularly in conflict zones, contributing to localized population crashes. Human-wildlife conflict exacerbates declines, as hippos raiding crops lead to retaliatory killings; hundreds of common hippos are shot annually across Africa due to such incidents. Diseases like anthrax outbreaks further impact populations, with recent events in 2025 killing at least 50 common hippos in Virunga National Park, Democratic Republic of Congo. Pollution from mining and agricultural runoff degrades water quality, increasing vulnerability to infections and disrupting aquatic habitats. Regional variations intensify risks, with pygmy hippo populations fragmented across and a total estimate of 2,000–3,000 individuals remaining, many in small, isolated groups due to intensive and forest clearance for cash crops. Common hippo subpopulations in the same region have declined by over 30% in the last decade from similar pressures, compounded by civil unrest facilitating . These factors fragment habitats across their sub-Saharan ranges, limiting dispersal and genetic exchange.

Protection measures and future outlook

Conservation efforts for Hippopotamidae focus on establishing and maintaining protected areas to safeguard habitats critical for both the common hippopotamus (Hippopotamus amphibius) and the (Choeropsis liberiensis). Significant portions of their ranges overlap with national parks and reserves, including in the , which supports recovering hippo populations through anti-poaching enforcement, and in , a preserving riverine habitats for the species. Both species are regulated under Appendix II, which controls international trade in hippo parts like teeth and skins to prevent overexploitation. Key initiatives include anti-poaching patrols and community-based education programs implemented across more than 20 African countries where hippos occur, aiming to reduce human-wildlife conflicts and illegal hunting. In 2025, the IUCN SSC Hippo Specialist Group published a report emphasizing the need for enhanced monitoring of declining pygmy populations and local abundance in common hippos, and a workshop developed an action plan for common hippos in West Africa. Reintroduction efforts have shown promise, particularly in South Africa, where hippos have been successfully relocated to reserves such as Amakhala Game Reserve and Botlierskop Game Reserve to bolster local populations and restore ecological roles in aquatic systems. Ongoing research emphasizes genetic studies to evaluate inbreeding risks and population connectivity, informing breeding programs in both wild and captive settings to enhance . Habitat restoration projects target rehabilitation, such as those in community-managed sanctuaries like Wechiau Hippo Sanctuary in , which integrate local involvement to protect riverine environments essential for hippo survival. Future projections indicate continued vulnerability, with common hippo declining at 6-8% annually in some regions due to ongoing threats like , potentially leading to substantial losses by 2050 without intensified interventions. However, optimistic scenarios suggest recovery is achievable through expanded protected areas, climate-resilient management, and transfrontier conservation efforts that promote connectivity across borders.

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