Hippopotamus
A hippopotamus in Chobe National Park, Botswana
Conservation status
Vulnerable (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Artiodactyla
Family:	Hippopotamidae
Genus:	Hippopotamus
Species:	H. amphibius
Binomial name
Hippopotamus amphibius Linnaeus, 1758[2]
Range map of the hippopotamus. Historic range is in red while current range is in green.[1]

The hippopotamus (/ˌhɪpəˈpɒtəməs/ HIP-ə-POT-ə-məs;^[3] Hippopotamus amphibius), also called the common hippopotamus is a large, mostly herbivorous, semiaquatic mammal and ungulate native to sub-Saharan Africa. It is one of only two extant species in the family Hippopotamidae, the other being the pygmy hippopotamus (Choeropsis liberiensis or Hexaprotodon liberiensis). The name comes from the ancient Greek for “river horse” (ἱπποπόταμος). After the elephant and rhinoceros, both of which are found in Africa, the hippopotamus is the third-largest type of land mammal and the heaviest extant artiodactyl. Despite their physical resemblance to pigs and other terrestrial even-toed ungulates, the closest living relatives of the Hippopotamidae are cetaceans (whales, dolphins, porpoises, etc.) from which they diverged about 55 million years ago.

Hippos are recognisable by their barrel-shaped torsos, wide-opening mouths revealing large canine tusks, nearly hairless bodies, columnar legs and large size; adults average 1,500 kg (3,310 lb) and 1,300 kg (2,870 lb) for males and females respectively. Despite its stocky shape and short legs, it is capable of running 30 km/h (19 mph) over short distances.

Hippos inhabit rivers, lakes, and mangrove swamps, where territorial males preside over a stretch of river and groups of five to thirty females and young hippos. During the day, they remain cool by staying in the water or mud; reproduction and childbirth both occur in water. They emerge at dusk to graze on grasses. While hippos rest near each other in the water, grazing is a solitary activity and hippos are not territorial on land. The hippo is among the most dangerous animals in the world as it is highly aggressive and unpredictable. They are threatened by habitat loss and poaching for their meat and ivory canine teeth.

The Latin word “hippopotamus” is derived from the ancient Greek ἱπποπόταμος, hippopótamos, from ἵππος, híppos, “horse”, and ποταμός, potamós, “river”, meaning “horse of the river”.^[4][5][6] In English, the plural is “hippopotamuses”, but “hippopotami” is also used;^[7] “hippos” can be used as a short plural. Hippos are gregarious, living in groups of up to thirty animals. A group is called a pod, herd, dale, or bloat.

Taxonomy and origins

Classification

Hippopotamus is the type genus of the family Hippopotamidae. The pygmy hippopotamus belongs to a different genus in Hippopotamidae, either Choeropsis or Hexaprotodon. Hippopotamidae are sometimes known as hippopotamids. Sometimes, the subfamily Hippopotaminae is used. Further, some taxonomists group hippos and anthracotheres in the superfamily Anthracotheroidea.^[8]:39 Hippopotamidae are classified along with other even-toed ungulates in the order Artiodactyla. Other artiodactyls include camels, cattle, deer and pigs, although hippos are not closely related to these groups.

Detail of the head

Five subspecies of hippos have been described based on morphological differences in their skulls and geographical differences:^[8]:3

• Great northern hippopotamus or Nile hippopotamus H. a. amphibius – (the nominate subspecies) which stretched from Egypt, where they are now extinct, south up the Nile River to Tanzania and Mozambique
• East African hippopotamus H. a. kiboko – in Kenya in the African Great Lakes region, and in Somalia in the Horn of Africa. Broader nasals and more hollowed interorbital region
• Cape hippopotamus or South African hippopotamus H. a. capensis – from Zambia to South Africa, most flattened skull of the subspecies
• West African hippopotamus or Tchad hippopotamus H. a. tschadensis – throughout Western Africa to, as the name suggests, Chad, slightly shorter and wider face, with prominent orbits
• Angola hippopotamus H. a. constrictus – in Angola, the southern Democratic Republic of Congo and Namibia, named for its deeper preorbital constriction

The suggested subspecies were never widely used or validated by field biologists; the described morphological differences were small enough that they could have resulted from simple variation in nonrepresentative samples.^[8]:2 Genetic analyses have tested the existence of three of these putative subspecies. A study examining mitochondrial DNA from skin biopsies taken from 13 sampling locations, considered genetic diversity and structure among hippo populations across the continent. The authors found low, but significant, genetic differentiation among H. a. amphibius, H. a. capensis, and H. a. kiboko. Neither H. a. tschadensis nor H. a. constrictus has been tested.^[9][10]

Evolution

Evolutionary relationships among hippo and Cetacea (whales, dolphins).^[11]

Until 1909, naturalists grouped hippos with pigs, based on molar patterns. Several lines of evidence, first from blood proteins, then from molecular systematics^[12] and DNA ^[13][14] and the fossil record, show that their closest living relatives are cetaceans – whales, dolphins and porpoises.^[15] The common ancestor of hippos and whales branched off from Ruminantia and the rest of the even-toed ungulates; the cetacean and hippo lineages split soon afterwards.^[13][16]

Anthracotherium magnum from the Oligocene of Europe

The most recent theory of the origins of Hippopotamidae suggests that hippos and whales shared a common semiaquatic ancestor that branched off from other artiodactyls around 60 million years ago.^[13][15] This hypothesised ancestral group likely split into two branches around 54 million years ago.^[12]

One branch would evolve into cetaceans, possibly beginning about 52 million years ago, with the protowhale Pakicetus and other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans.^[16] The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of which in the late Eocene would have resembled skinny hippos with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved into Hippopotamidae, became extinct during the Pliocene without leaving any descendants.^[15]

A rough evolutionary lineage can be traced from Eocene and Oligocene species: Anthracotherium and Elomeryx to the Miocene species Merycopotamus and Libycosaurus and the very latest anthracotheres in the Pliocene.^[17] Merycopotamus, Libycosaurus and Hippopotamidae can be considered to form a clade, with Libycosaurus being more closely related to hippos. Their common ancestor would have lived in the Miocene, about 20 million years ago. Hippopotamidae are therefore deeply nested within the family Anthracotheriidae.

Hippopotamidae are believed to have evolved in Africa; the oldest known hippopotamid is the genus Kenyapotamus, which lived in Africa from 16 to 8 million years ago. While hippopotamid species spread across Asia and Europe, no hippos have ever been discovered in the Americas, although various anthracothere genera emigrated into North America during the early Oligocene. From 7.5 to 1.8 million years ago, an ancestor to the modern hippo, Archaeopotamus, lived in Africa and the Middle East.^[18]

While the fossil record of hippos is still poorly understood, the two modern genera, Hippopotamus and Choeropsis (sometimes Hexaprotodon), may have diverged as far back as 8 million years ago. Taxonomists disagree whether or not the modern pygmy hippopotamus is a member of Hexaprotodon – an apparently paraphyletic genus, also embracing many extinct Asian hippopotamuses, that is more closely related to Hippopotamus – or of Choeropsis, an older and basal genus.^[17][18]

Choeropsis madagascariensis skeleton with a modern hippopotamus skull.

Extinct species

Three species of Malagasy hippopotamus became extinct during the Holocene on Madagascar, one of them within the past 1,000 years. The Malagasy hippos were smaller than the modern hippo, likely through the process of insular dwarfism.^[19] Fossil evidence indicates many Malagasy hippos were hunted by humans, a likely factor in their eventual extinction.^[19] Isolated members of Malagasy hippos may have survived in remote pockets; in 1976, villagers described a living animal called the kilopilopitsofy, which may have been a Malagasy hippo.^[20]

Three species of hippopotamus, the European hippopotamus (Hippopotamus antiquus), Hippopotamus major and Hippopotamus gorgops, ranged throughout continental Europe and the British Isles. All three species became extinct before the last glaciation. Ancestors of European hippos found their way to many islands of the Mediterranean Sea during the Pleistocene.^[21] The Pleistocene also saw a number of dwarf species evolve on several Mediterranean islands, including Crete (Hippopotamus creutzburgi), Cyprus (the Cyprus dwarf hippopotamus, Hippopotamus minor), Malta (Hippopotamus melitensis), and Sicily (Hippopotamus pentlandi). Of these, the Cyprus dwarf hippo survived until the end of the Pleistocene or early Holocene. Evidence from an archaeological site, Aetokremnos, continues to cause debate on whether or not the species was encountered, and was driven to extinction, by man.^[22][21]

Physical attributes

Hippo’s skull, showing the large canines and incisors used for fighting

Hippos are among the largest living land mammals, being only smaller than elephants and some rhinoceroses. Among the extant African megafauna, behind the two African elephant species, they average smaller than the white rhinoceros but are larger by body mass than the black rhinoceros and the giraffe. Mean adult weight is around 1,500 kg (3,310 lb) and 1,300 kg (2,870 lb) for males and females respectively,^[23][24] very large males can reach 2,000 kg (4,410 lb) and exceptional males weighing 2,660 kg (5,860 lb)^[23], 3,200 kg (7,050 lb)^[25] and 4,500 kg (9,920 lb) (in captivity)^[26] have been reported. Male hippos appear to continue growing throughout their lives while females reach maximum weight at around age 25.^[27]

Mostly submerged hippo with exposed eyes, ears, and nostrils

Hippos have barrel-shaped bodies with short legs and long muzzles.^[28] Their skeletal structures are graviportal,^[8]:8 adapted to carrying their enormous weight, and their specific gravity allows them to sink and move along the bottom of a river.^[29] Hippopotamuses have small legs (relative to other megafauna) because the water in which they live reduces the weight burden.^[30] Though they are bulky animals, hippos can gallop at 30 km/h (19 mph) on land but normally trot. They are incapable of jumping but do climb up steep banks.^[28] Despite being semiaquatic and having webbed feet, an adult hippo is not a particularly good swimmer nor can it float. It is rarely found in deep water; when it is, the animal moves by porpoise-like leaps from the bottom.^[8]:3 The eyes, ears, and nostrils of hippos are placed high on the roof of their skulls. This allows these organs to remain above the surface while the rest of the body submerges.^[31]:259 The testes of the males descend only partially and a scrotum is not present. In addition, the penis retracts into the body when not erect. The genitals of the female hippos are unusual in that the vagina is ridged and two large diverticula protrude from the vulval vestibule. The function of these is unknown.^[8]:28–29

Characteristic “yawn” of a hippo

The hippo’s jaw is powered by a large masseter and a well-developed digastric; the latter loops up behind the former to the hyoid.^[31]:259 The jaw hinge is located far back enough to allow the animal to open its mouth at almost 180°.^[8]:17 A moderate folding of the orbicularis oris muscle allows the hippo to achieve such a gape without tearing any tissue.^[32] The bite force of an adult female hippo has been measured as 8,100 newtons (1,800 lbf).^[33] Hippo teeth sharpen themselves as they grind together. The lower canines and lower incisors are enlarged, especially in males, and grow continuously. The incisors can reach 40 cm (1 ft 4 in), while the canines reach up to 50 cm (1 ft 8 in).^[28] The canines and incisors are used for combat and play no role in feeding. Hippos rely on their broad horny lips to grasp and pull grasses which are then ground by the molars.^{[31]:259, 263} The hippo is considered to be a pseudoruminant; it has a complex three-chambered stomach but does not “chew cud“.^[8]:22

Completely submerged hippo (San Diego Zoo)

Unlike most other semiaquatic animals, hippos have very little hair.^[31]:260 The skin is 6 cm (2 in) thick,^[28] providing it great protection against conspecifics and predators. By contrast, its subcutaneous fat layer is thin.^[8]:3 The animals’ upper parts are purplish-grey to blue-black, while the under parts and areas around the eyes and ears can be brownish-pink.^[31]:260 Their skin secretes a natural sunscreen substance which is red-coloured. The secretion is sometimes referred to as “blood sweat”, but is neither blood nor sweat. This secretion is initially colourless and turns red-orange within minutes, eventually becoming brown. Two distinct pigments have been identified in the secretions, one red (hipposudoric acid) and one orange (norhipposudoric acid). The two pigments are highly acidic compounds. They inhibit the growth of disease-causing bacteria, and their light absorption peaks in the ultraviolet range, creating a sunscreen effect.^[34][35] All hippos, even those with different diets, secrete the pigments, so it does not appear that food is the source of the pigments. Instead, the animals may synthesise the pigments from precursors such as the amino acid tyrosine.^[35] Nevertheless, this natural sunscreen cannot prevent the animal’s skin from cracking if it stays out of water too long.^[36] The secretion does help regulate the body temperature of the hippo and acts an antibiotic source.^[37]

A hippo’s lifespan is typically 40–50 years.^[31]:277 Donna the Hippo was the oldest living hippo in captivity. She lived at the Mesker Park Zoo in Evansville, Indiana in the US^[38][39] until her death in 2012 at the age of 61.^[40]

Distribution and status

Hippopotamus amphibius was widespread in North Africa and Europe during the Eemian^[41] and late Pleistocene until about 30,000 years ago. Archaeological evidence exists of its presence in the Levant, dating to less than 3,000 years ago.^[42][43] The species was common in Egypt‘s Nile region during antiquity, but has since been extirpated. Pliny the Elder writes that, in his time, the best location in Egypt for capturing this animal was in the Saite nome;^[44] the animal could still be found along the Damietta branch after the Arab Conquest in 639. Reports of the slaughter of the last hippo in Natal Province were made at the end of the 19th century.^[45] Hippos are still found in the rivers and lakes of the northern Democratic Republic of the Congo, Uganda, Tanzania and Kenya, north through to Ethiopia, Somalia and Sudan, west to The Gambia, and south to South Africa.

Ugandan tribespeople with hippo slain for food (early 20th century)

Incised hippopotamus ivory tusk, upper canine. Four holes around top (Naqada Tomb 1419, Egypt; Naqada period)

Genetic evidence suggests that common hippos in Africa experienced a marked population expansion during or after the Pleistocene, attributed to an increase in water bodies at the end of the era. These findings have important conservation implications as hippo populations across the continent are currently threatened by loss of access to fresh water.^[9] Hippos are also subject to unregulated hunting and poaching. In May 2006, the hippo was identified as a vulnerable species on the IUCN Red List drawn up by the International Union for Conservation of Nature (IUCN), with an estimated population of between 125,000 and 150,000 hippos, a decline of between 7% and 20% since the IUCN’s 1996 study. Zambia (40,000) and Tanzania (20,000–30,000) possess the largest populations.^[1]

The hippo population declined most dramatically in the Democratic Republic of the Congo.^[46] By 2005, the population in Virunga National Park had dropped to 800 or 900 from around 29,000 in the mid-1970s.^[47] The decline is attributed to the disruptions caused by the Second Congo War.^[47] The poachers are believed to be Mai-Mai rebels, poorly paid Congolese soldiers, and local militia groups.^[47][48] Reasons for poaching include the belief that hippos are harmful to society, as well as financial gain.^[49] However, as of 2016, the Virunga hippo population appears to have increased, possibly due to greater enforcement and cooperation between fishermen and park authorities.^[50] The sale of hippo meat is illegal, but black-market sales are difficult for Virunga National Park officers to track.^[48][49] Hippo meat is considered a delicacy in some areas of central Africa and the teeth have become a valued substitute for elephant ivory.^[51]

As an introduced species

In the late 1980s, Colombian drug lord Pablo Escobar kept four hippos in a private menagerie at his residence in Hacienda Nápoles, 100 kilometres (62 mi) east of Medellín, Colombia. They were deemed too difficult to seize and move after Escobar’s death, and hence left on the untended estate. By 2007, the animals had multiplied to 16 and had taken to roaming the area for food in the nearby Magdalena River.^[52][53] In 2013, the National Geographic Channel produced a documentary about them titled Cocaine Hippos.^[54] As of early 2014, 40 hippos were reported to exist in Puerto Triunfo, Antioquia from the original four belonging to Escobar,^[55] and in 2018 the growing population was estimated at 50–70.^[56] In December 2019, it was estimated that there were 65–80 individuals with their range covering almost 2,000 km² (770 sq mi) in Antioquia, Bolívar, Boyacá, Cundinamarca and Santander; it is expected that the population will increase to 150 individuals within a decade and that the range eventually could cover more than 13,500 km² (5,200 sq mi).^[57] Population projections estimate that there could be thousands within a few decades.^[58]

Being non-native introductions, most conservationists considered them problematic and invasive in Colombia, as they have the potential to change the ecosystems, feeding heavily on plants and displacing native species like the West Indian manatee, Neotropical otter, spectacled caiman and turtles.^[57][59][60] The critically endangered Dahl’s toad-headed turtle and Magdalena River turtle are largely restricted to the Magdalena River basin,^[61] as are many threatened fish.^[62] In 2020, a study showed that there was an increase in the nutrient levels and cyanobacteria in Colombian lakes inhabited by hippos. Cyanobacteria can cause toxic algae blooms and die-offs of aquatic fauna. Despite the limited magnitude of the observed change, it was noticeable since the species’ population was still quite small.^[59][60] Additionally, they can represent a serious threat to fishers and other locals. There have been attacks on humans, but as of 2017 nobody had been killed or seriously injured by the Colombian hippos.^[63]

In contrast to the opposition by most conservationists, some ecologists have argued that they should remain and might even have a positive effect on the local environment. It has been suggested that the nutrients they introduce to the water and the occasional fish kills caused by them are overall positive,^[59] but this was based on a study in their native Africa.^[64] Alternatively, the introduced hippos could be a form of Pleistocene rewilding project, replacing species like Toxodon that became extinct in prehistoric times,^[59] but Pleistocene rewilding itself is highly controversial.^[65] Others have argued that the Colombian hippos should be regarded as a safe population, isolated from the threats faced by African hippos, and that they could be benefitial to the local ecotourism industry.^[59]

In 2009, two adults and one calf left their herd and, after attacking humans and killing cattle, one of the adults (called “Pepe”) was killed by hunters under authorisation of the local authorities.^[53] When a photo of the dead hippo became public, it caused considerable controversy among animal rights groups both within the country and abroad, and further plans of culling ceased. Alternative methods have been considered, but they are unproven, or difficult and expensive. A wild male hippo was caught, castrated and released again, but it cost about US$50,000.^[63] As of 2020, there were no plans by the local government on managing the population, but further studies on their effect on the habitat have been initiated.^[59] Because of the fast-growing population, conservationists have recommended that a management plan needs to be rapidly developed.^[57][58]

Behaviour and life history

Hippopotamus out of the water after sunrise

Hippos differ from all other large land mammals, being of semiaquatic habits, and spending their days in lakes and rivers.^[8]:3 They can be found in both savannah and forest areas.^[1] Proper habitat requires enough water to submerge in and grass nearby.^[28] Larger densities of the animals inhabit quiet waters with mostly firm, smooth sloping beaches. Male hippos may be found in very small numbers in rapid waters in rocky gorges.^[31]:264 Hippos mostly live in freshwater habitats, however populations in West Africa mostly inhabit estuarine waters and may even be found at sea.^[1] With the exception of eating, most of a hippo’s life occurs in the water. Hippos leave the water at dusk and travel inland, sometimes up to 15 km (9 mi),^[28] to graze on short grasses, their main source of food. They spend four to five hours grazing and can consume 68 kg (150 lb) of grass each night.^[66]

Like most herbivores, hippos consume other plants if presented with them, but their diet in nature consists almost entirely of grass, with only minimal consumption of aquatic plants.^[67] Hippos are born with sterile intestines, and require bacteria obtained from their mothers’ feces to digest vegetation.^[68] On occasion, hippos have been filmed eating carrion, usually near the water. There are other reports of meat-eating, and even cannibalism and predation.^[69] The stomach anatomy of a hippo is not suited to carnivory, and meat-eating is likely caused by aberrant behaviour or nutritional stress.^[8]:84

Video of hippos in the wild

Hippo defecation creates allochthonous deposits of organic matter along the river beds. These deposits have an unclear ecological function.^[67] A 2015 study concluded that hippo dung provides nutrients from terrestrial material for fish and aquatic invertebrates,^[70] while a 2018 study found that their dung can be toxic to aquatic life in large quantities, due to absorption of dissolved oxygen in water bodies.^[71][72] Because of their size and their habit of taking the same paths to feed, hippos can have a significant impact on the land across which they walk, both by keeping the land clear of vegetation and depressing the ground. Over prolonged periods, hippos can divert the paths of swamps and channels.^[73]

A hippopotamus walking on the grass land in Serengeti National Park in the morning

Adult hippos move at speeds up to 8 km/h (5 mph) in water; typically resurfacing to breathe every three to five minutes. The young have to breathe every two to three minutes.^[8]:4 The process of surfacing and breathing is subconscious: a hippo sleeping underwater will rise and breathe without waking up. A hippo closes its nostrils when it submerges into the water.^[74] As with fish and turtles on a coral reef, hippos occasionally visit cleaning stations and signal, by opening their mouths wide, their readiness for being cleaned of parasites by certain species of fishes. This is an example of mutualism, in which the hippo benefits from the cleaning while the fish receive food.^[75] The hippos spend up to 16 hours a day in water as a way to stay cool.^[37]

A hippopotamus and Nile crocodile side by side in Kruger National Park

Hippos coexist with a variety of large predators. Nile crocodiles, lions and spotted hyenas are known to prey on young hippos.^{[31]:273[8]:118} However, due to their aggression and size, adult hippos are not usually preyed upon by other animals. Cases where large lion prides have successfully preyed on adult hippos have been reported; however, this predation is generally rare.^[76] Lions occasionally prey on adults at Gorongosa National Park and calves are sometimes taken at Virunga.^[77] Crocodiles are frequent targets of hippo aggression, probably because they often inhabit the same riparian habitats; crocodiles may be either aggressively displaced or killed by hippos.^[78] In turn, beyond cases of killing the seldom unguarded hippo calf, very large Nile crocodiles have been verified to occasionally prey on “half-grown” hippos and anecdotally perhaps adult female hippos. Aggregations of crocodiles have also been seen to dispatch still-living male hippos that have been previously injured in mating battles with other males.^[79][80][81]

Social spacing

Hippopotamus pod

Studying the interaction of males and females has long been complicated because hippos are not sexually dimorphic; thus females and young males are almost indistinguishable in the field.^[82] Although hippos lie close to each other, they do not seem to form social bonds except between mothers and daughters, and they are not social animals. The reason they huddle close together is unknown.^[8]:49 Hippos are territorial only in water, where a male presides over a small stretch of river, on average 250 m (270 yd) in length, and containing 10 females. The largest pods can contain over 100 hippos.^[8]:50 Younger bachelors are allowed in a male’s stretch, as long as they behave submissively toward the male. The territories of hippos exist to establish mating rights. Within the pods, the hippos tend to segregate by gender. Bachelors lounge near other bachelors, females with other females, and the male on his own. When hippos emerge from the water to graze, they do so individually.^[8]:4

Male hippos fighting

Hippos mark their territory by defecation. While depositing the faeces, hippos spin their tails to distribute their excrement over a greater area.^[83] “Yawning” serves as a threat display.^[28] When fighting, males use their incisors to block each other’s attacks and their large canines to inflict injuries.^[31]:260 When hippos become over-populated or a habitat is reduced, males sometimes attempt infanticide, but this behaviour is not common under normal conditions.^[84] Incidents of hippo cannibalism have been documented, but this is believed to be the behaviour of distressed or sick hippos.^[8]:82–83

Hippos appear to communicate vocally, through grunts and bellows, and they may practice echolocation, but the purpose of these vocalisations is currently unknown. Hippos have the unique ability to hold their heads partially above the water and send out a cry that travels through both water and air; individuals respond above and under water.^[85] Hippos will also express threat and alarm with exhalations.^[28]

Reproduction

Female hippo with calf

Female hippos reach sexual maturity at five to six years and have a gestation period of eight months.^[86] A study of endocrine systems revealed that female hippos may begin puberty as early as three or four years.^[87] Males reach maturity at around 7.5 years. A study of hippo reproductive behaviour in Uganda showed that peak conceptions occurred during the end of the wet season in the summer, and peak births occurred toward the beginning of the wet season in late winter. This is because of the female’s oestrous cycle; as with most large mammals, male hippo spermatozoa is active year-round. Studies of hippos in Zambia and South Africa also showed evidence of births occurring at the start of the wet season.^[8]:60–61 After becoming pregnant, a female hippo will typically not begin ovulation again for 17 months.^[87]

Preserved hippopotamus fetus

Mating occurs in the water, with the female submerged for most of the encounter,^[8]:63 her head emerging periodically to draw breath. Female hippos isolate themselves to give birth and return within 10–14 days.^[28] Calves are born underwater at a weight between 25 and 50 kg (55 and 110 lb) and an average length of around 127 cm (4.17 ft), and must swim to the surface to take their first breaths. A mother typically gives birth to only one calf, although twins also occur. The young often rest on their mothers’ backs when the water is too deep for them, and they swim under water to suckle. They suckle on land when the mother leaves the water.^[8]:64

Mother hippos are very protective of their young and may keep others at a distance. However, calves are occasionally left in nurseries which are guarded by one or a few adults. Calves in nurseries engage in playfights.^[28] Weaning starts between six and eight months after birth, and most calves are fully weaned after a year.^[8]:64 Like many other large mammals, hippos are described as K-strategists, in this case typically producing just one large, well-developed infant every couple of years (rather than many small, poorly developed young several times per year as is common among small mammals such as rodents).^[87][84]

Hippos and humans

Hippopotamus (“William”), Egyptian, Middle Kingdom, ca. 1961–1878 B.C.

The earliest evidence of human interaction with hippos comes from butchery cut marks on hippo bones at Bouri Formation dated around 160,000 years ago.^[88] Later rock paintings and engravings showing hippos being hunted have been found in the mountains of the central Sahara dated 4,000–5,000 years ago near Djanet in the Tassili n’Ajjer Mountains.^[8]:1 The ancient Egyptians recognised the hippo as a ferocious denizen of the Nile and representations on the tombs of nobles show that the animals were hunted.^[89]

The hippo was also known to the Greeks and Romans. The Greek historian Herodotus described the hippo in The Histories (written circa 440 BC) and the Roman naturalist Pliny the Elder wrote about the hippo in his encyclopedia Naturalis Historia (written circa 77 AD).^[44][90] The Yoruba people called the hippo erinmi, which means “elephant of the water”.^[91] Zulu warriors preferred to be as brave as a hippo, since even lions were not considered to match its courage.^[92] They would chant their chief: “Een-gonyama Gonyama! Invooboo! Yah-bo! Yah-bo! Invooboo!” which translates as “He is a lion. Yes, he is better than a lion – he is a hippopotamus.”^[93]

Attacks on humans

The Hippopotamus and Crocodile Hunt (1617), by Peter Paul Rubens

The hippo is considered to be extremely aggressive and has frequently been reported charging and attacking boats.^[94] Small boats can easily be capsized by hippos and passengers can be injured or killed by the animals or drown. In one 2014 case in Niger, a boat was capsized by a hippo and 13 people were killed.^[95] As hippos will often engage in raiding nearby crops if the opportunity arises, humans may also come in conflict with them on these occasions, with potential for fatalities on both sides.^[96]

In zoos

Obaysch lounging at the London Zoo in 1852

Hippos have long been popular zoo animals. The first zoo hippo in modern history was Obaysch, who arrived at the London Zoo on 25 May 1850, where he attracted up to 10,000 visitors a day and inspired a popular song, the “Hippopotamus Polka”.^[97] Hippos generally breed well in captivity; birth rates are lower than in the wild, but this is attributed to zoos wanting to limit births, since hippos are relatively expensive to maintain.^{[8]:129[97][98]}

Like many zoo animals, hippos were traditionally displayed in concrete exhibits. In the case of hippos, they usually had a pool of water and patch of grass. In the 1980s, zoo exhibits increasingly reflected native habitats. For example, the Toledo Zoo Hippoquarium features a 360,000-US-gallon (1,400,000 l) pool.^[99] In 1987, the Toledo Zoo saw the first underwater birth by a captive hippo.^[100] The exhibit was so popular, the hippos became the logo of the Toledo Zoo.^[101]

Cultural depictions

Ijaw hippopotamus masks

A red hippo represented the Ancient Egyptian god Set; the thigh is the “phallic leg of Set”, symbolising virility. Set’s consort Tawaret was also seen as part hippo^[102] and was a goddess of protection in pregnancy and childbirth, because ancient Egyptians recognised the protective nature of a female hippo toward her young.^[103] The Ijaw people of the Niger Delta wore masks of aquatic animals like the hippo when practicing their water spirit cults^[104] and hippo ivory was used in the divination rituals of the Yoruba.^[105] The Behemoth from the Book of Job, 40:15–24 is thought to be based on a hippo.^[106]

Hippos have been the subjects of various African folktales. According to a San story; when the Creator assigned each animal its place in nature, the hippos wanted to live in the water, but were refused out of fear that they might eat all the fish. After begging and pleading, the hippos were finally allowed to live in the water on the conditions that they would eat grass instead of fish and would fling their dung so that it can be inspected for fish bones. In a Ndebele tale, the hippo originally had long, beautiful hair, but was set on fire by a jealous hare and had to jump into a nearby pool. The hippo lost most of his hair and was too embarrassed to leave the water.^[107]

The Hippopotamus Polka

Ever since Obaysch inspired the “Hippopotamus Polka”, hippos have been popular animals in Western culture for their rotund appearance that many consider comical.^[97] Stories of hippos such as Huberta, which became a celebrity in South Africa in the 1930s for trekking across the country;^[108] or the tale of Owen and Mzee, a hippo and tortoise which developed an intimate bond; have amused people who have bought hippo books, merchandise, and many stuffed hippo toys.^[109][110] Hippos were mentioned in the novelty Christmas song “I Want a Hippopotamus for Christmas” that became a hit for child star Gayla Peevey in 1953. They also feature in the songs “The Hippopotamus” and “Hippo Encore” by Flanders and Swann, with the famous refrain “Mud, Mud, Glorious Mud”. They even inspired a popular board game, Hungry Hungry Hippos.^[111][112]

Hippos have also been popular cartoon characters, where their rotund frames are used for humorous effect. For example, the Disney film Fantasia featured a ballerina hippo dancing to the opera La Gioconda,^[46] and Hanna-Barbera created Peter Potamus.^[113]

The “Happy Hippos” characters were created in 1987 by the French designer André Roche to be hidden in the “Kinder Surprise egg” of the Italian chocolate company Ferrero SpA.^[114]