Hipopótamo

El hipopótamo ( / ˌ h ɪ p ə p ɒ t ə m ə s / HIP -ə- POT -ə-məs ; [3] Hippopotamus amphibius ), también llamado el hipopótamo , hipopótamo común o hipopótamo río , es un grande, en su mayoría mamíferos herbívoros , semiacuáticos y ungulados nativos del África subsahariana . Es una de las dos únicas especies existentes en la familia. Hippopotamidae , el otro es el hipopótamo pigmeo ( Choeropsis liberiensis o Hexaprotodon liberiensis ). El nombre proviene del griego antiguo para "caballo de río" ( ἱπποπόταμος ).

Hipopótamo
Hipopótamo (Hippopotamus amphibius), parque nacional de Chobe, Botsuana, 2018-07-28, DD 82.jpg
Un hipopótamo y una garcilla bueyera en el Parque Nacional Chobe , Botswana
clasificación cientifica editar
Reino: Animalia
Filo: Chordata
Clase: Mammalia
Pedido: Artiodactyla
Familia: Hipopótamo
Género: Hipopótamo
Especies:
H. amphibius
Nombre binomial
Hipopótamo amphibius
Distribución de hipopótamos.gif
Mapa de distribución del hipopótamo. El rango histórico está en rojo mientras que el rango actual está en verde. [1]

Después del elefante y el rinoceronte , el hipopótamo es el tercer tipo más grande de mamífero terrestre y el artiodáctilo más pesado existente (en el sentido tradicional, no cladístico del término, sin incluir a los cetáceos). A pesar de su parecido físico con los cerdos y otros ungulados terrestres , los parientes vivos más cercanos de los Hippopotamidae son los cetáceos ( ballenas , delfines , marsopas , etc.), de los que se separaron hace unos 55 millones de años. Los hipopótamos son reconocibles por sus torsos en forma de barril, bocas de gran apertura que revelan grandes colmillos caninos , cuerpos casi sin pelo, patas columnares y gran tamaño; los adultos promedian 1.500 kg (3.310 lb) para los machos y 1.300 kg (2.870 lb) para las hembras. A pesar de su forma robusta y sus patas cortas, es capaz de correr 30 km / h (19 mph) en distancias cortas.

Los hipopótamos habitan ríos, lagos y manglares , donde los machos territoriales presiden un tramo de río y grupos de cinco a treinta hembras y jóvenes hipopótamos. Durante el día, se mantienen frescos permaneciendo en el agua o en el barro; la reproducción y el parto ocurren en el agua. Emergen al anochecer para pastar en la hierba. Mientras que los hipopótamos descansan cerca unos de otros en el agua, el pastoreo es una actividad solitaria y los hipopótamos no son territoriales en tierra. El hipopótamo se encuentra entre los animales más peligrosos del mundo debido a su naturaleza altamente agresiva e impredecible. Están amenazados por la pérdida de hábitat y la caza furtiva por su carne y sus caninos de marfil .

La palabra latina hipopótamo se deriva del griego antiguo ἱπποπόταμος , hippopótamos , de ἵππος , híppos , "caballo", y ποταμός , potamós , "río", que significa "caballo del río". [4] [5] [6] En inglés, el plural es "hippopotamuses", pero también se usa "hippopotami". [7]

Clasificación

Hippopotamus es el género tipo de la familia Hippopotamidae . El hipopótamo pigmeo pertenece a un género diferente en Hippopotamidae, ya sea Choeropsis o Hexaprotodon . Los hipopótamos a veces se conocen como hipopótamos. A veces, se utiliza la subfamilia Hippopotaminae. Además, algunos taxonomistas agrupan hipopótamos y antracoteras en la superfamilia Anthracotheroidea. [8] : 39 Los hipopótamos se clasifican junto con otros ungulados pares en el orden Artiodactyla . Otros artiodáctilos incluyen camellos , vacas , ciervos y cerdos , aunque los hipopótamos no están estrechamente relacionados con estos grupos.

Detalle de la cabeza

Se han descrito cinco subespecies de hipopótamos basadas en diferencias morfológicas en sus cráneos y diferencias geográficas: [8] : 3

  • Gran hipopótamo del norte o hipopótamo del Nilo H. a. amphibius - (la subespecie nominal) que se extendía desde Egipto , donde ahora están extintos, hacia el sur por el río Nilo hasta Tanzania y Mozambique.
  • Hipopótamo de África oriental H. a. kiboko - en Kenia en la región africana de los Grandes Lagos y en Somalia en el Cuerno de África . Nasales más anchos y región interorbitaria más ahuecada
  • Cape hipopótamo o hipopótamo sudafricano H. a. capensis - de Zambia a Sudáfrica , el cráneo más aplanado de la subespecie
  • Hipopótamo de África occidental o hipopótamo de Tchad H. a. tschadensis - en toda África occidental hasta, como su nombre indica, Chad , un rostro ligeramente más corto y ancho, con órbitas prominentes
  • Hipopótamo de Angola H. a. constrictus - en Angola , el sur de la República Democrática del Congo y Namibia , llamado así por su constricción preorbital más profunda

Las subespecies sugeridas nunca fueron ampliamente utilizadas o validadas por biólogos de campo; las diferencias morfológicas descritas eran tan pequeñas que podrían haber resultado de una simple variación en muestras no representativas. [8] : 2 Los análisis genéticos han probado la existencia de tres de estas supuestas subespecies. Un estudio que examinó el ADN mitocondrial de biopsias de piel tomadas de 13 lugares de muestreo, consideró la diversidad genética y la estructura entre las poblaciones de hipopótamos en todo el continente. Los autores encontraron una diferenciación genética baja, pero significativa, entre H. a. amphibius , H. a. capensis y H. a. kiboko . Ni H. a. tschadensis ni H. a. constrictus ha sido probado. [9] [10]

Evolución

Relaciones evolutivas entre hipopótamos y cetáceos (ballenas, delfines). [11]

Hasta 1909, los naturalistas agruparon a los hipopótamos con los cerdos, basándose en patrones molares . Varias líneas de evidencia, primero de proteínas sanguíneas, luego de sistemática molecular [12] y ADN [13] [14] y el registro fósil , muestran que sus parientes vivos más cercanos son los cetáceos ( ballenas , delfines y marsopas ). [15] El antepasado común de los hipopótamos y las ballenas se derivó de Ruminantia y el resto de los ungulados pares; los linajes de cetáceos e hipopótamos se separaron poco después. [13] [16]

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Anthracotherium magnum del Oligoceno de Europa

La teoría más reciente de los orígenes de Hippopotamidae sugiere que los hipopótamos y las ballenas compartían un ancestro semiacuático común que se ramificó de otros artiodáctilos hace unos 60 millones de años . [13] [15] Este grupo ancestral hipotético probablemente se dividió en dos ramas hace unos 54 millones de años . [12]

Una rama evolucionaría en cetáceos , posiblemente comenzando hace unos 52 millones de años , con el protowhale Pakicetus y otros ancestros de ballenas tempranos conocidos colectivamente como Archaeoceti , que eventualmente se sometieron a una adaptación acuática a los cetáceos completamente acuáticos . [16] La otra rama se convirtió en las antracoteras , una gran familia de bestias de cuatro patas, la primera de las cuales a finales del Eoceno se habría parecido a hipopótamos flacos con cabezas comparativamente pequeñas y estrechas. Todas las ramas de las antracoteras, excepto la que evolucionó a Hippopotamidae , se extinguieron durante el Plioceno sin dejar descendientes. [15]

Se puede rastrear un linaje evolutivo aproximado desde las especies del Eoceno y Oligoceno : Anthracotherium y Elomeryx hasta las especies del Mioceno Merycopotamus y Libycosaurus y las últimas antracoterias del Plioceno . [17] Se puede considerar que Merycopotamus , Libycosaurus e Hippopotamidae forman un clado , siendo Libycosaurus un pariente más cercano a los hipopótamos. Su antepasado común habría vivido en el Mioceno, hace unos 20 millones de años . Por lo tanto, los hipopótamos están profundamente anidados dentro de la familia Anthracotheriidae.

Se cree que los hipopótamos evolucionaron en África; el hipopótamo más antiguo conocido es el género Kenyapotamus , que vivió en África hace 16 a 8 millones de años . Si bien las especies de hipopótamos se extendieron por Asia y Europa, nunca se han descubierto hipopótamos en las Américas, aunque varios géneros de antracotras emigraron a América del Norte durante el Oligoceno temprano . Desde hace 7,5 a 1,8 millones de años , un antepasado del hipopótamo moderno, Archaeopotamus , vivió en África y Oriente Medio. [18]

Si bien el registro fósil de los hipopótamos aún no se comprende bien, los dos géneros modernos, Hippopotamus y Choeropsis (a veces Hexaprotodon ), pueden haber divergido desde hace 8 millones de años . Los taxónomos no están de acuerdo si el hipopótamo pigmeo moderno es o no miembro de Hexaprotodon —un género aparentemente parafilético , que también abarca muchos hipopótamos asiáticos extintos, que está más estrechamente relacionado con Hippopotamus — o de Choeropsis , un género más antiguo y basal . [17] [18]

Esqueleto de Choeropsis madagascariensis con un cráneo de hipopótamo moderno.

Especies extintas

Tres especies de hipopótamos malgaches se extinguieron durante el Holoceno en Madagascar , una de ellas en los últimos 1.000 años. Los hipopótamos malgaches eran más pequeños que el hipopótamo moderno, probablemente a través del proceso de enanismo insular . [19] La evidencia fósil indica que muchos hipopótamos malgaches fueron cazados por humanos, un factor probable en su eventual extinción. [19] Los miembros aislados de los hipopótamos malgaches pueden haber sobrevivido en lugares remotos; En 1976, los aldeanos describieron un animal vivo llamado Kilopilopitsofy , que pudo haber sido un hipopótamo malgache. [20]

Tres especies de hipopótamos, el hipopótamo europeo ( Hippopotamus antiquus ), el Hippopotamus major y el Hippopotamus gorgops , se distribuyeron por toda Europa continental y las Islas Británicas. Las tres especies se extinguieron antes de la última glaciación. Los antepasados ​​de los hipopótamos europeos llegaron a muchas islas del mar Mediterráneo durante el Pleistoceno . [21] El Pleistoceno también vio una serie de especies enanas evolucionar en varias islas del Mediterráneo, incluyendo Creta ( Hippopotamus creutzburgi ), Chipre (el hipopótamo enano de Chipre , Hippopotamus minor ), Malta ( Hippopotamus melitensis ) y Sicilia ( Hippopotamus pentlandi ). De estos, el hipopótamo enano de Chipre sobrevivió hasta el final del Pleistoceno o principios del Holoceno. La evidencia de un sitio arqueológico, Aetokremnos , continúa provocando un debate sobre si la especie fue encontrada o no, y fue llevada a la extinción por el hombre. [22] [21] En Eurasia, el hipopótamo se extinguió hace entre 50.000 y 16.000 años. [23]

Cráneo de hipopótamo , mostrando los grandes caninos e incisivos utilizados para pelear.

Los hipopótamos se encuentran entre los mamíferos terrestres vivos más grandes, siendo solo más pequeños que los elefantes y algunos rinocerontes . Entre la megafauna africana existente , detrás de las dos especies de elefantes africanos, son en promedio más pequeñas que el rinoceronte blanco, pero son más grandes en masa corporal que el rinoceronte negro y la jirafa . Los hipopótamos miden de 2,90 a 5,05 m (9,5 a 16,6 pies) de largo, incluida una cola de aproximadamente 35 a 56 cm (1,15 a 1,84 pies) de largo y de 1,30 a 1,65 m (4,3 a 5,4 pies) de altura en el hombro. [24] [25] [26] El peso medio de un adulto es de alrededor de 1500 kg (3310 lb) y 1300 kg (2870 lb) para machos y hembras respectivamente, [27] [28] los machos muy grandes pueden alcanzar los 2000 kg (4410 lb) y machos excepcionales que pesan 2.660 kg (5.860 lb), [27] 3.200 kg (7.050 lb) [29] y 4.500 kg (9.920 lb) (en cautiverio) [30] . Los hipopótamos machos parecen seguir creciendo durante toda su vida, mientras que las hembras alcanzan el peso máximo alrededor de los 25 años. [31]

Los hipopótamos tienen cuerpos en forma de barril con patas cortas y hocicos largos. [32] Sus estructuras esqueléticas son gravitacionales , [8] : 8 adaptadas para soportar su enorme peso, y su gravedad específica les permite hundirse y moverse a lo largo del fondo de un río. [33] Los hipopótamos tienen patas pequeñas (en comparación con otras megafauna ) porque el agua en la que viven reduce la carga de peso. [34] Aunque son animales voluminosos, los hipopótamos pueden galopar a 30 km / h (19 mph) en tierra, pero normalmente trotan. Son incapaces de saltar, pero trepan por pendientes empinadas. [32] A pesar de ser semiacuático y tener patas palmeadas, un hipopótamo adulto no es un nadador particularmente bueno ni puede flotar. Rara vez se encuentra en aguas profundas; cuando lo es, el animal se mueve a saltos como marsopas desde el fondo. [8] : 3 Los ojos, oídos y fosas nasales de los hipopótamos se colocan en lo alto del techo del cráneo. Esto permite que estos órganos permanezcan por encima de la superficie mientras el resto del cuerpo se sumerge. [35] : 259 Los testículos de los machos descienden sólo parcialmente y no hay escroto. Además, el pene se retrae hacia el cuerpo cuando no está erecto . Los genitales de las hembras de hipopótamo son inusuales porque la vagina está estriada y dos grandes divertículos sobresalen del vestíbulo vulvar . Se desconoce la función de estos. [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.[35]: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.[36] The bite force of an adult female hippo has been measured as 8.1 kN (1,800 lbf).[37] 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).[32] 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.[35]: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.[35]:260 The skin is 6 cm (2 in) thick,[32] 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.[35]: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.[38][39] 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.[39] Nevertheless, this natural sunscreen cannot prevent the animal's skin from cracking if it stays out of water too long.[40] The secretion does help regulate the body temperature of the hippo and acts as an antibiotic.[41]

A hippo's lifespan is typically 40–50 years.[35]: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[42][43] until her death in 2012 at the age of 61.[44] The oldest hippo 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.[45]

Hippopotamus amphibius was widespread in North Africa and Europe during the Eemian[46] 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.[47][48] 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;[49] 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.[50] 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. 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.[51] By 2005, the population in Virunga National Park had dropped to 800 or 900 from around 29,000 in the mid-1970s.[52] The decline is attributed to the disruptions caused by the Second Congo War.[52] The poachers are believed to be Mai-Mai rebels, poorly paid Congolese soldiers, and local militia groups.[52][53] Reasons for poaching include the belief that hippos are harmful to society, as well as financial gain.[54] However, as of 2016, the Virunga hippo population appears to have increased, possibly due to greater enforcement and cooperation between fishermen and park authorities.[55] The sale of hippo meat is illegal, but black-market sales are difficult for Virunga National Park officers to track.[53][54] Hippo meat is considered a delicacy in some areas of central Africa and the teeth have become a valued substitute for elephant ivory.[56]

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, 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.[57][58] 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.[58]

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.[59] 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.[60] Because of the fast-growing population, conservationists have recommended that a management plan needs to be rapidly developed.[61][62] Scientists say these hippos are breeding voraciously and are an increasing menace so they must be culled.[63]

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.[64][65] Broussard argued that the hippos would eat the invasive water hyacinth that was clogging the rivers and also produce meat to help solve the American meat crisis.[65][66] The chief collaborators and proponents of Broussard's bill were Major Frederick Russell Burnham and Captain Fritz Duquesne.[67][68] 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 the taste of hippo as "like cow bacon".[67] The "American Hippo Bill" fell just short of being passed.[65]

A grazing hippopotamus in Kruger National Park

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.[32] 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.[35]: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),[32] 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.[69]

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.[70] Hippos are born with sterile intestines, and require bacteria obtained from their mothers' feces to digest vegetation.[71] On occasion, hippos have been filmed eating carrion, usually near the water. There are other reports of meat-eating, and even cannibalism and predation.[72] 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

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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.[70] A 2015 study concluded that hippo dung provides nutrients from terrestrial material for fish and aquatic invertebrates,[73] 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.[74][75] 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.[76]

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.[77] 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.[78] The hippos spend up to 16 hours a day in water as a way to stay cool.[41]

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.[35]: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.[79] Lions occasionally prey on adults at Gorongosa National Park and calves are sometimes taken at Virunga.[80] 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.[81] 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.[82][83][84]

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.[85] 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.[86] "Yawning" serves as a threat display.[32] When fighting, males use their incisors to block each other's attacks and their large canines to inflict injuries.[35]:260 When hippos become over-populated or a habitat is reduced, males sometimes attempt infanticide, but this behaviour is not common under normal conditions.[87] 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.[88] Hippos will also express threat and alarm with exhalations.[32]

Reproduction

Female hippo with calf

Female hippos reach sexual maturity at five to six years and have a gestation period of eight months.[89] A study of endocrine systems revealed that female hippos may begin puberty as early as three or four years.[90] 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.[90]

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.[32] 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.[32] 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).[90][87]

Hippopotamus ("William"), Middle Kingdom of Egypt, 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.[91] 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.[92]

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).[49][93] The Yoruba people called the hippo erinmi, which means "elephant of the water".[94] Zulu warriors preferred to be as brave as a hippo, since even lions were not considered to match its courage.[95] 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."[96]

Attacks on humans

The hippo is considered to be extremely aggressive and has frequently been reported charging and attacking boats.[97] 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.[98] 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.[99]

In zoos

Obaysch lounging at the London Zoo in 1852

Hippos have long been popular zoo animals. The first record of hippos on captivity for display are dated to 3500 BC, in Hierakonpolis, Egypt.[100] 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".[101] 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[101][102]

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.[103] In 1987, the Toledo Zoo saw the first underwater birth by a captive hippo.[104] The exhibit was so popular, the hippos became the logo of the Toledo Zoo.[105]

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[106] and was a goddess of protection in pregnancy and childbirth, because ancient Egyptians recognised the protective nature of a female hippo toward her young.[107] The Ijaw people of the Niger Delta wore masks of aquatic animals like the hippo when practicing their water spirit cults[108] and hippo ivory was used in the divination rituals of the Yoruba.[109] The Behemoth from the Book of Job, 40:15–24 is thought to be based on a hippo.[110]

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.[111]

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.[101] Stories of hippos such as Huberta, which became a celebrity in South Africa in the 1930s for trekking across the country;[112] 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.[113][114] 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.[115][116]

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,[51] Hanna-Barbera created Peter Potamus.[117] and the Madagascar films feature a hippo named Gloria.[118] 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.[119]

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