Hippopotamus

The hippopotamus (/ˌhɪpəˈpɒtəməs/ HIP-ə-POT-ə-məs;[3] PL: hippopotamuses or hippopotami; Hippopotamus amphibius), also called the hippo, common hippopotamus, or river hippopotamus, is a large semiaquatic mammal 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). Its name comes from the ancient Greek for "river horse" (ἱπποπόταμος).

Hippopotamus
A hippopotamus in Saadani National Park, Tanzania
CITES Appendix II (CITES)[1]
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Family: Hippopotamidae
Genus: Hippopotamus
Species:
H. amphibius
Binomial name
Hippopotamus amphibius
Range map of the hippopotamus. Historic range is in red while current range is in green.[1]

Aside from elephants and rhinos, the hippopotamus is the largest land mammal. It is also the largest extant land artiodactyl. Despite their physical resemblance to pigs and other terrestrial even-toed ungulates, the closest living relatives of the hippopotamids are cetaceans (whales, dolphins, porpoises, etc.), from which they diverged about 55 million years ago. Hippos are recognisable for their barrel-shaped torsos, wide-opening mouths with large canine tusks, nearly hairless bodies, pillar-like legs, and large size: adults average 1,500 kg (3,310 lb) for bulls (males) and 1,300 kg (2,870 lb) for cows (females). 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. Territorial bulls each preside over a stretch of water and a group of five to thirty cows and calves. Mating and birth both occur in the water. During the day, hippos remain cool by staying in water or mud, emerging at dusk to graze on grasses. While hippos rest near each other in the water, grazing is a solitary activity and hippos typically do not display territorial behaviour on land. Hippos are among the most dangerous animals in the world due to their aggressive and unpredictable nature. They are threatened by habitat loss and poaching for their meat and ivory (canine teeth).

Etymology

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]

Taxonomy and origins

Classification

The modern hippopotamus and the pygmy hippopotamus are the only living members of the family Hippopotamidae. Some taxonomists group hippos and anthracotheres in the superfamily Anthracotheroidea. Hippopotamidae are classified along with other even-toed ungulates in the order Artiodactyla.[8]: 39–40 

Detail of the head

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

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

The suggested subspecies above 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 have been tested.[9][10]

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

Until 1909, naturalists classified hippos together 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 their closest living relatives are cetaceans (whales, dolphins, and porpoises).[15][16] 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]

   Artiodactyla   
 

 Tylopoda

 
   Artiofabula   
 

 Suina    

 
   Cetruminantia   
 

 Ruminantia

 
   Whippomorpha   
 

 Hippopotamidae

 
 

 Cetacea

 
 
 
 
 
Anthracotherium magnum from the Oligocene of Europe

The most recent theory of the origins of Hippopotamidae suggests 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 again 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. This group 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][16]

A rough evolutionary lineage of the hippo can thus be traced from Eocene and Oligocene species: from Anthracotherium and Elomeryx to the Miocene species Merycopotamus and Libycosaurus and finally the very latest anthracotheres in the Pliocene.[17] These groups lived in Eurasia, and the discovery of Epirigenys in East Africa, which was likely in a sister group to Hippopotamidae, suggests hippo ancestors entered Africa around 35 million years ago and were the earliest large mammals to colonise the continent.[18][19] An early hippopotamid is the genus Kenyapotamus, which lived in Africa from 15 to 9 million years ago.[17] Hippopotamid species would spread across Africa and Eurasia, including the modern pygmy hippo. From 7.5 to 1.8 million years ago, a possible ancestor to the modern hippo, Archaeopotamus, lived in Africa and the Middle East.[20]

Choeropsis madagascariensis skeleton with a modern hippopotamus skull

Extinct species

Three species of Malagasy hippopotamus became extinct during the Holocene on Madagascar, the last of them within the past 1,000 years. The Malagasy hippos were smaller than the modern hippo, a likely result of the process of insular dwarfism.[21] Fossil evidence indicates many Malagasy hippos were hunted by humans, a factor in their eventual extinction.[21] Isolated individual 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.[22]

An extinct species, Hippopotamus antiquus, ranged throughout Europe, extending as far north as Britain during the Early and Middle Pleistocene epochs, before being replaced by the modern H. amphibius during the latter part of the Middle Pleistocene.[23] 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 the archaeological site Aetokremnos continues to cause debate on whether or not the species was driven to extinction, or even encountered, by man.[24][25] Across Eurasia, the hippopotamus became extinct between 50,000 and 16,000 years ago.[26]

Characteristics and adaptations
Hippo's skull, showing the large canines and incisors used for fighting

The hippopotamus is a megaherbivore and is exceeded in size among land animals only by elephants and some rhinoceros species. The mean adult weight is around 1,480 kg (3,260 lb) for bulls and 1,365 kg (3,010 lb) for cows. Exceptionally large males have been recorded reaching 2,660 kg (5,860 lb).[27] Male hippos appear to continue growing throughout their lives, while females reach maximum weight at around age 25.[28] Hippos measure 2.90 to 5.05 m (9.5 to 16.6 ft) long,[29] including a tail of about 35 to 56 cm (1.15 to 1.84 ft) in length and 1.30 to 1.65 m (4.3 to 5.4 ft) tall at the shoulder,[30][31] with males and females ranging 1.40 to 1.65 m (4.6 to 5.4 ft) and 1.30 to 1.45 m (4.3 to 4.8 ft) tall at the shoulder respectively.[32] The species has a typical head-body length of 3.3–3.45 m (10.8–11.3 ft) and an average standing height of 1.4 m (4.6 ft) at the shoulder.[33]

Hippos have barrel-shaped bodies with short tails and legs, and an hourglass-shaped skull with a long muzzle.[34][8]: 5, 19  Their skeletal structures are graviportal,[8]: 8  adapted to carrying their enormous weight, and their dense bones and low centre of gravity allows them to sink and move along the bottom of the water.[35] Hippopotamuses have small legs (relative to other megafauna) because the water in which they live reduces the weight burden.[36] The pelvis rests at an angle of 45 degrees.[8]: 9  Though they are bulky animals, hippos can gallop at 30 km/h (19 mph) on land, but they normally trot. They are incapable of jumping but do climb up steep banks.[34]

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 is submerged.[37]: 259  Muscular closing valves exist in the nostrils while nictitating membranes cover the eyes.[8]: 4, 116  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 off the bottom.[8]: 3  Adult hippos move at speeds of 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 hippopotamus sleeps with both hemispheres of the brain resting, as in all land mammals. Despite this, it is able to sleep while submerged, intermittently surfacing to breathe seemingly without waking. They may be able to transition between different phases of sleep more quickly than other mammals.[38]

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.[37]: 259  The jaw hinge is located far back enough to allow the animal to open its mouth at almost 180°.[8]: 17  A folded orbicularis oris muscle allows the hippo to attain an extreme gape without tearing any tissue.[39] Hippo teeth sharpen themselves as they grind together. The lower canines and lower incisors are enlarged, especially in bulls, and grow continuously. The incisors can reach 40 cm (1 ft 4 in), while the canines can grow to up to 50 cm (1 ft 8 in).[34] The canines and incisors are used for combat and play no role in feeding. Hippos rely on their broad, rough lips to grasp and pull grasses which are then ground by the molars.[37]: 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.[37]: 260  The skin is 6 cm (2 in) thick,[34] providing the hippo with great protection against predators and other hippos. 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.[37]: 260  Their skin secretes a natural, red-coloured sunscreen substance that 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 highly acidic pigments have been identified in the secretions; one red (hipposudoric acid) and one orange (norhipposudoric acid), which inhibit the growth of disease-causing bacteria and their light-absorption profile peaks in the ultraviolet range, creating a sunscreen effect.[40][41] Regardless of diet, all hippos secrete these pigments so food does not appear to be their source; rather, they may be synthesised from precursors such as the amino acid tyrosine.[41] This natural sunscreen cannot prevent the animal's skin from cracking if it stays out of water too long.[42]

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 

A hippo's lifespan is typically 40 to 50 years.[37]: 277  Donna the Hippo was one of the oldest living hippos in captivity. She lived at the Mesker Park Zoo in Evansville, Indiana, in the US[43][44] until her death in 2012 at the age of 61.[45] The oldest hippo ever recorded was called Bertha; she had lived in the Manila Zoo in the Philippines since it first opened in 1959. When she died in 2017, her age was estimated to be 65.[46]

Distribution and status

Hippopotamus amphibius was widespread in North Africa and Europe during the Eemian (130–115,000 years ago), with remains found as far north as England.[47][48] Archaeological evidence exists of its presence in the Levant, dating to less than 3,000 years ago.[49][50] The species was common in Egypt's Nile region during antiquity, but it has since been driven out. According to Pliny the Elder, in his time, the best location in Egypt for capturing this animal was in the Saite nome;[51] the animal could still be found along the Damietta branch of the Nile 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.[52] 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.[1]

Ugandan tribespeople with hippo slain for food (early 20th century)
Incised hippopotamus ivory tusk (upper canine) with four holes around top (Naqada Tomb 1419, Egypt; Naqada period)

Genetic evidence suggests 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. The species is included in Appendix II of the Convention on International Trade in Endangered Species (CITES) meaning international export/import (including in parts and derivatives) requires CITES documentation to be obtained and presented to border authorities.[1]

As of 2017, the IUCN Red List drawn up by the International Union for Conservation of Nature (IUCN) lists the species as vulnerable, with a stable population estimated between 115,000 and 130,000 animals. Zambia (40,000) and Tanzania (20,000–30,000) possess the largest populations.[1] The hippo population has declined most dramatically in the Democratic Republic of the Congo.[53] By 2005, the population in Virunga National Park had dropped to 800 or 900 from around 29,000 in the mid-1970s.[54] This decline is attributed to the disruptions caused by the Second Congo War.[54] The poachers are believed to be Mai-Mai rebels, poorly paid Congolese soldiers, and local militia groups.[54][55] Reasons for poaching include the belief hippos are harmful to society, as well as financial gain.[56] As of 2016, the Virunga hippo population appears to have increased again, possibly due to greater enforcement and cooperation between fishermen and park authorities.[57] The sale of hippo meat is illegal, but black-market sales are difficult for Virunga National Park officers to track.[55][56] Hippo meat is considered a delicacy in some areas of central Africa and the teeth have become a valued substitute for elephant ivory.[58]

Invasive potential

In the late 1980s, Pablo Escobar kept four hippos in a private menagerie at his residence in Hacienda Nápoles, 100 kilometres (62 mi) east of Medellín, Colombia, after buying them in New Orleans. They were deemed too difficult to seize and move after Escobar's death in 1993, and hence were left on the untended estate. By 2007, the animals had multiplied to 16 individuals which had taken to roaming the area for food in the nearby Magdalena River.[59][60] In 2009, two adults and one calf left their herd, attacking humans and killing cattle. One of the adults (called "Pepe") was killed by hunters under authorisation of the local authorities.[60]

When a photo of the dead hippo became public, it caused considerable controversy among animal rights groups both within the country and abroad, and plans of further culling the hippos were put on hold. Alternative methods of dealing with the hippos have been considered, but they are either unproven, too difficult, or expensive. A wild male hippo was caught, castrated, and released again, but this process cost about US$50,000.[61] As of 2020, there were no plans by the local government for managing the population, but further studies of their effect on the habitat have been initiated.[62] Because of the fast-growing population, conservationists have urged a management plan be developed quickly.[63][64] Most ecologists believe the population should be eradicated, as they are breeding rapidly and are an increasing menace to humans and the environment,[65] although some have suggested that they could represent a net positive in the ecosystem, filling the large-herbivore niche left vacant in the New World when humans arrived and wiped out almost all large species in the Pleistocene extinction.[66]

In the U.S., Representative Robert F. Broussard of Louisiana introduced the "American Hippo Bill" in 1910 to authorise the importation and release of hippopotamus into the bayous of Louisiana.[67][68] Broussard argued the hippos would eat the invasive water hyacinth that was clogging the rivers and also produce meat to help solve the American meat crisis.[68][69] The chief collaborators and proponents of Broussard's bill were Major Frederick Russell Burnham and Captain Fritz Duquesne.[70][71] Former President Theodore Roosevelt backed the plan, as did the U.S. Department of Agriculture, The Washington Post, and The New York Times, which praised hippo meat as "lake cow bacon".[72][70] Although the "American Hippo Bill" developed a broad base of support, it was never passed by the US Congress.[68]

Behaviour and ecology

Hippos differ from all other large land mammals in being semiaquatic, spending their days in lakes and rivers.[8]: 3  They require habitat with enough water to submerge themselves in and which has plenty of grass nearby.[34] When bodies of water feature firm, smooth, sloping beaches, larger numbers of hippos may congregate. Male hippos may also be found in very small numbers in rapid waters in rocky gorges.[37]: 264  Hippos mostly live in freshwater habitats; however, populations in West Africa mostly inhabit estuarine waters and may even be found out at sea.[1]

Hippos spend most the day in water to stay cool and hydrated. Just before night begins, they leave the water to feed on land.[34] Like most herbivores, hippos will consume a variety of plants if presented with them in captivity, but their diet in nature consists almost entirely of grass, with only minimal consumption of aquatic plants.[73] A hippo will eat around 40 kg (88 lb) of grass in a night while traveling 3–5 km (1.9–3.1 mi). They typically return to the water before dawn.[34] On occasion, hippos have been filmed eating carrion, usually near the water. There are other reports of meat-eating and even cannibalism and predation.[74] 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 

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, keeping the land clear of vegetation and depressing the ground. Over prolonged periods, hippos can divert the paths of swamps and channels.[75] By defecating in the water, the animals also appear to pass on microbes from their gut, affecting the biogeochemical cycle.[76]

Social interaction
Hippopotamus pod

It is challenging to study the interaction of bulls and cows because hippos are not sexually dimorphic, so cows and young bulls are almost indistinguishable in the field.[77] Although hippos lie close to one another, they are generally not social animals, and they do not seem to form social bonds except between mothers and daughters. The reason they huddle close together is unknown.[8]: 49  Hippos exhibit territorial behaviour only in water, where the dominant bull – also known as a "beachmaster" – presides over a small stretch of river, on average 250 m (270 yd) in length. The beachmaster's territory typically contains a pod of 10 cows, but pods can contain over 100 hippos.[8]: 50  Younger bachelors are allowed in a beachmaster's stretch, as long as they behave submissively toward him. Hippo territories are used to establish mating access. Within the pods, the hippos tend to segregate by sex. Bachelors lounge near other bachelors, females with other females, and the beachmaster is on his own. When hippos emerge from the water to graze, they do so individually.[8]: 4 

Male hippos fighting

Hippos engage in "muck-spreading" which involves defecating while spinning their tails to distribute the faeces over a greater area. Muck-spreading occurs both on land and in water and its function is not well understood. It is likely not territorial as the animals only establish territories in the water.[8]: 5, 51–52  "Yawning" serves as a threat display.[34] When fighting, bulls use their incisors to block each other's attacks and their large canines to inflict injuries.[37]: 260  When hippos become over-populated or a habitat shrinks, bulls sometimes attempt infanticide, but this behaviour is not common under normal conditions.[78] Incidents of hippo cannibalism have been documented, but this is believed to be the behaviour of distressed or sick hippos.[8]: 82–83 

The most common hippo vocalisation is the "wheeze honk", which can be heard over long distances.[79] This call starts as a high-pitched squeal followed by a deeper, resonant call.[8]: 5  While the purpose of these vocalisations is unknown, they appear to be used for social cohesion, and the animals can recognise the calls of other individuals. Hippos are more likely to react aggressively to the wheeze honks of strangers than those of animals they are more familiar with.[79] They may be able to utilise wheeze honks for echolocation. 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 both above and below water.[80] They will also express threat and alarm with exhalations.[34] Fighting bulls produce loud bellows.[8]: 5 

Reproduction
A pod at the Saadani National Park

Cows reach sexual maturity at five to six years of age and have a gestation period of eight months.[81] A study of endocrine systems revealed cows may begin puberty at as early as three or four years.[82] 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]: 59–61, 66  After becoming pregnant, a female hippo will typically not begin ovulation again for 17 months.[82]

Preserved hippopotamus fetus

Mating occurs in the water, with the cow submerged for most of the encounter,[8]: 63  her head emerging periodically to draw breath. Cows isolate themselves to give birth and return to the pod within 10 to 14 days.[34] Calves are born underwater weighing between 25 and 50 kg (55 and 110 lb) and at an average length of around 127 cm (4.17 ft). They must swim to the surface to take their first breaths. A mother typically gives birth to only one calf, though sometimes they give birth to twins. The young often rest on their mothers' backs when the water is too deep for them, and they swim under water to suckle. They will suckle on land as well when the mother leaves the water.[8]: 64 

Mother hippos are very protective of their young and may keep others at a distance.[34] One cow was recorded protecting a calf's carcass after it had died.[83] Calves are occasionally left in nurseries which are guarded by one or a few adults. Calves in nurseries often engage in playfights.[34] 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).[82][78]

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

Hippos coexist alongside a variety of large predators in their habitats. Nile crocodiles, lions, and spotted hyenas are known to prey on young hippos.[37]: 273 [8]: 118  Beyond these, adult hippos are not usually preyed upon by other animals due to their aggression and size. Cases where large lion prides have successfully preyed on adult hippos have been reported, but it is generally rare.[84] Lions occasionally prey on adults at Gorongosa National Park and calves are sometimes taken at Virunga.[85] 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.[86] In turn, very large Nile crocodiles have been observed preying occasionally on calves, "half-grown" hippos, and possibly also adult female hippos. Groups of crocodiles have also been observed finishing off still-living male hippos that were previously injured in mating battles with other males.[87][88]

Hippos occasionally visit cleaning stations in order to be cleaned of parasites by certain species of fishes. They signal their readiness for this service by opening their mouths wide. This is an example of mutualism, in which the hippo benefits from the cleaning while the fish receive food.[89] Hippo defecation creates allochthonous deposits of organic matter along the river beds. These deposits have an unclear ecological function.[73] A 2015 study concluded hippo dung provides nutrients from terrestrial material for fish and aquatic invertebrates,[90] 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.[91][92]

The parasitic monogenean flatworm Oculotrema hippopotami infests hippopotamus eyes, mainly the nictitating membrane. It is the only monogenean species (which normally live on fish) documented to live on a mammal.[93]

Hippos and humans
Hippopotamus ("William"), Middle Kingdom of Egypt, c. 1961–1878 BC

The earliest evidence of human interaction with hippos comes from butchery cut marks on hippo bones found at the Bouri Formation and dated to around 160,000 years ago.[94] Later rock paintings and engravings showing hippos being hunted have been found in the Tassili n'Ajjer Mountains of the central Sahara near Djanet, dated to 4,000–5,000 years ago[8]: 1  The ancient Egyptians recognised the hippo as a ferocious denizen of the Nile and representations on the tombs of nobles show the animals were hunted by humans.[95]

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).[51][96] The Yoruba people called the hippo erinmi, which means "elephant of the water".[97] Some individual hippos have achieved international fame. Huberta became a celebrity during the Great Depression for trekking across a great distance in South Africa.[98][99]: 111–112 

Attacks on humans

The hippo is considered to be extremely aggressive and has frequently been reported charging and attacking boats.[100] Small boats can easily be capsized by hippos and passengers can be injured or killed by the animals, or drown in the water. In one 2014 case in Niger, a boat was capsized by a hippo and 13 people were killed.[101] Hippos will often raid farm crops if the opportunity arises, and humans may come into conflict with them on these occasions. These encounters can be fatal to either humans or hippos.[102]

In zoos
Obaysch lounging at the London Zoo in 1852

Hippos have long been popular zoo animals. The first record of hippos taken into captivity for display is dated to 3500 BC in Hierakonpolis, Egypt.[103] 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".[104]

Hippos generally breed well in captivity; birth rates are lower than in the wild, but this can be attributed to zoos' desire to limit births, since hippos are relatively expensive to maintain.[8]: 129 [104][105] The Cincinnati Zoo began building a US$73 million exhibit for three adult hippos in 2015, featuring a 250,000 l (66,000 US gal) tank. Modern hippo enclosures also have a complex filtration system for the animal's waste, an underwater viewing platform for the visitors, and glass that may be up to 9 cm (3.5 in) thick and capable of withstanding water pressures of 31 kPa (4.5 psi).[99]: 158–159  In 1987, the Toledo Zoo saw the first underwater birth by a captive hippo.[106] The exhibit was so popular, the logo of the Toledo Zoo was updated to feature the hippos.[107]

Cultural significance
Ijaw hippopotamus masks

In Egyptian mythology, the god Set takes the form of a red hippopotamus and fights Horus for control of the land, but is defeated. The goddess Tawaret is depicted as a pregnant woman with a hippo head, representing fierce maternal love.[108] The Ijaw people of the Niger Delta wore masks of aquatic animals like the hippo when practising their water spirit cults,[109] and hippo ivory was used in the divination rituals of the Yoruba.[110] Hippo masks were also used in Nyau rituals of the Chewa of Southern Africa. These rituals were performed at funerals.[99]: 120  According to Robert Baden-Powell, Zulu warriors referred to hippos in war chants.[111][112] The Behemoth from the Book of Job, 40:15–24 is thought to be based on the hippo.[113]

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 they might eat all the fish. After begging and pleading, the hippos were finally allowed to live in the water on the condition they would eat grass instead of fish, and fling their dung so it can be inspected for fish bones. In a Ndebele tale, the hippo originally had long, beautiful hair, but it was set on fire by a jealous hare and the hippo had to jump into a nearby pool. The hippo lost most of his hair and was too embarrassed to leave the water.[114]

The "Hippopotamus Polka"

Hippopotamuses were rarely depicted in European art during the Renaissance and Baroque periods, due to less access to specimens by Europeans. One notable exception is Peter Paul Rubens' The Hippopotamus and Crocodile Hunt (1615–1616).[99]: 122–123  Ever since Obaysch inspired the "Hippopotamus Polka", hippos have been popular animals in Western culture for their rotund appearance, which many consider comical.[104] The Disney film Fantasia featured a ballerina hippo dancing to the opera La Gioconda. The film Hugo the Hippo involves the title character escaping Zanzibar to Dar es Salaam to avoid being slaughtered, where he is cared for by local children. The Madagascar films feature a hippo named Gloria.[99]: 128–129  Hippos even inspired a popular board game, Hungry Hungry Hippos.[115]

Among the most famous poems about the hippo is "The Hippopotamus" by T. S. Eliot, where he compares the animal to the Catholic Church.[99]: 128  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 popular "The Hippopotamous Song" by Flanders and Swann.[99]: 136 

A popular internet myth reports that hippos have pink milk. Biologist David Wynick states, "I think this is an Internet legend that is oft repeated but without any evidence for it that I can find... Like all mammals, hippos produce white or off-white milk for their young."[116]

See also
  • Armley Hippo
  • Owen and Mzee - hippo and tortoise who bonded

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