"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.
"
Although the above research (see below for complete quoted reference) is considered to be the understanding (theory) of the common scientist and anthropologist world around the globe I stand on this day Wednesday, July 1, 2020 to say in print that you are on the wrong semiaquatic ancestor. The obvious 'Elephant seal' is it's predecessor as origin to only foot and mouth as shown by the delicacy of the nature and movement of the hippopotamus known today in the year of twenty-twenty (2020).
Hippopotamus
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The
hippopotamus (
HIP-ə-POT-ə-məs;
[3] Hippopotamus amphibius), also called the
hippo,
common hippopotamus or
river 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 due
to its highly 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] "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.
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]
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
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
Colombia
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 km
2 (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 km
2 (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]
United States
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.
[66][67] 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.
[67][68] The chief collaborators and proponents of Broussard's bill were Major
Frederick Russell Burnham and Captain
Fritz Duquesne.
[69][70] 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".
[69] The "American Hippo Bill" fell just short of being passed.
[67]
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.
[71]
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.
[72] Hippos are born with sterile intestines, and require bacteria obtained from their mothers' feces to digest vegetation.
[73] 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
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.
[72] A 2015 study concluded that hippo dung provides nutrients from terrestrial material for fish and aquatic invertebrates,
[75]
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.
[76][77]
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.
[78]
A grazing hippopotamus in Kruger National Park
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.
[79] 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.
[80] 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.
[81] Lions occasionally prey on adults at
Gorongosa National Park and calves are sometimes taken at Virunga.
[82] 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.
[83]
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.
[84][85][86]
Social spacing
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.
[87]
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
Hippos
mark their territory by defecation. While depositing the faeces, hippos spin their tails to distribute their excrement over a greater area.
[88] "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.
[89] 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.
[90] Hippos will also express threat and alarm with exhalations.
[28]
Reproduction
Female hippos reach sexual maturity at five to six years and have a
gestation period of eight months.
[91] A study of
endocrine systems revealed that female hippos may begin puberty as early as three or four years.
[92]
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.
[92]
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).
[92][89]
Hippos and humans
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.
[93] 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.
[94]
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][95] The
Yoruba people called the hippo
erinmi, which means "elephant of the water".
[96] Zulu warriors preferred to be as brave as a hippo, since even lions were not considered to match its courage.
[97] 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."
[98]
Attacks on humans
The hippo is considered to be extremely aggressive and has frequently been reported charging and attacking boats.
[99]
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.
[100]
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.
[101]
In zoos
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".
[102]
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[102][103]
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.
[104] In 1987, the Toledo Zoo saw the first underwater birth by a captive hippo.
[105] The exhibit was so popular, the hippos became the logo of the Toledo Zoo.
[106]
Cultural depictions
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
[107]
and was a goddess of protection in pregnancy and childbirth, because
ancient Egyptians recognised the protective nature of a female hippo
toward her young.
[108] The
Ijaw people of the
Niger Delta wore masks of aquatic animals like the hippo when practicing their
water spirit cults
[109] and hippo ivory was used in the
divination rituals of the Yoruba.
[110] The
Behemoth from the
Book of Job, 40:15–24 is thought to be based on a hippo.
[111]
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.
[112]
Ever since
Obaysch inspired the "Hippopotamus Polka", hippos have been popular animals in
Western culture for their rotund appearance that many consider comical.
[102] Stories of hippos such as
Huberta, which became a celebrity in
South Africa in the 1930s for trekking across the country;
[113] 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.
[114][115] 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.
[116][117]
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.
[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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