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The hippopotamus (Hippopotamus amphibius) or hippo, from the ancient Greek for "river horse", is a large, mostly plant-eating mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae (the other is the Pygmy Hippopotamus). The hippopotamus is the heaviest extant artiodactyl, despite being considerably shorter than the giraffe.

The hippopotamus is semi-aquatic, inhabiting rivers and lakes where territorial bulls preside over a stretch of river and groups of 5 to 30 females and young. 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 grass. While hippopotamuses rest near each other in the water, grazing is a solitary activity and hippos are not territorial on land.

Despite their physical resemblance to pigs and other terrestrial even-toed ungulates, their closest living relatives are cetaceans (whales, porpoises, etc.) from which they diverged about . The common ancestor of whales and hippos split from other even-toed ungulates around . The earliest known hippopotamus fossils, belonging to the genus Kenyapotamus in Africa, date to around .

The hippopotamus is recognizable by its barrel-shaped torso, enormous mouth and teeth, nearly-hairless body, stubby legs and tremendous size. It is the third-largest land mammal by weight (between 1½ and 3½ tons), behind the white rhinoceros (1½ to 4 tons) and elephants (3 to 7 tons). Despite its stocky shape and short legs, it can easily outrun a human. Hippos have been clocked at over short distances. The hippopotamus is one of the most aggressive creatures in the world and is often regarded as the most ferocious animal in Africa. There are an estimated 125,000 to 150,000 hippos throughout Sub-Saharan Africa; Zambiamarker (40,000) and Tanzania (20,000–30,000) possess the largest populations. They are still threatened by habitat loss and poaching for their meat and ivory canine teeth.

Etymology

The word "hippopotamus" derives from the ancient Greek , hippopotamos, from , hippos, "horse", and , potamos, "river", meaning "horse of the river". In English, the plural is hippopotamuses, but hippopotami is also used; hippos can be used as a short plural. Hippopotamuses are gregarious, living in groups of up to 30 animals; such a group is called a pod, herd, dale or bloat. A male hippopotamus is known as a bull, a female as a cow and a baby as a calf. The species is also known as the Common Hippopotamus or the Nile Hippopotamus.

Taxonomy and origins

Classification

The 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 sub-family Hippopotaminae is used. Further, some taxonomists group hippopotamuses and anthracotheres in the super-family Anthracotheroidea or Hippopotamoidea.

Hippopotamidae are classified along with other even-toed ungulates in the order Artiodactyla. Other artiodactyls include camels, cows, deer and pigs, although hippopotamuses are not closely related to these groups.

Five subspecies of hippos have been described based on morphological differences in their skulls and geographical differences:
  • H. a. amphibius – (the nominate subspecies) which stretched from Egyptmarker, where they are now extinct, south up the Nile River to Tanzania and Mozambiquemarker.
  • H. a. kiboko – in the Horn of Africa, in Kenyamarker and Somaliamarker. Kiboko is the Swahili word for hippo. Broader nasals and more hollowed interorbital region.
  • H. a. capensis – from Zambiamarker to South Africa. Most flattened skull of the subspecies.
  • H. a. tschadensis – throughout Western Africa to, as the name suggests, Chadmarker. Slightly shorter and wider face, with prominent orbits.
  • H. a. constrictus – in Angolamarker, the southern Democratic Republic of Congomarker and Namibiamarker. 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 non-representative samples. 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.

Evolution

Until 1985, naturalists grouped hippos with pigs, based on molar patterns. However, several lines of evidence, first from blood proteins, then from molecular systematics and DNA
and the fossil record, show that their closest living relatives are cetaceanswhales, porpoises and the like. 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.


The most recent theory of the origins of Hippopotamidae suggests that hippos and whales shared a common semi-aquatic ancestor that branched off from other artiodactyls around . This hypothesized ancestral group likely split into two branches around . One branch would evolve into cetaceans, possibly beginning about with the proto-whale Pakicetus and other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans.

The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of whom in the late Eocene would have resembled skinny hippopotamuses 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.

A rough evolutionary lineage can be traced, however, from Eocene and Oligocene species: Anthracotherium and Elomeryx to the Miocene Merycopotamus and Libycosaurus and the very latest anthracotheres in the Pliocene. Merycopotamus, Libycosaurus and all hippopotamids 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 .

Hippopotamids are therefore deeply nested within the family Anthracotheriidae. The Hippopotamidae are believed to have evolved in Africa; the oldest known hippopotamid is the genus Kenyapotamus which lived in Africa from 16 to . While hippopotamid species spread across Asia and Europe, no hippopotamuses have ever been discovered in the Americas, although various anthracothere genera emigrated into North America during the early Oligocene. From 7.5 to an ancestor to the modern hippopotamus, Archaeopotamus, lived in Africa and the Middle East.

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 . 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 Choeropsis—an older and basal genus.

Extinct species

As many as three species of Malagasy Hippopotamus became extinct during the Holocene on Madagascarmarker, one of them within the past 1,000 years. The Malagasy Hippos were smaller than the modern hippopotamus, likely through the process of insular dwarfism. There is fossil evidence that many Malagasy Hippos were hunted by humans, a likely factor in their eventual extinction. Isolated members of Malagasy Hippopotamus may have survived in remote pockets; in 1976, villagers described a living animal called the Kilopilopitsofy, which may have been a Malagasy Hippopotamus.

Two separate species of Hippopotamus, the European Hippopotamus (H. antiquus) and H. gorgops ranged throughout continental Europe and the British Isles. Both species became extinct before the last glaciation. Ancestors of European Hippos found their way to many islands of the Mediterraneanmarker during the Pleistocene. Both species were larger than the modern hippopotamus, averaging about 1 meter (3.3 feet) longer.

The Pleistocene also saw a number of dwarf species evolve on several Mediterranean islands including Cretemarker (H. creutzburgi), Cyprusmarker (H. minor), Maltamarker (H. melitensis) and Sicily (H. pentlandi). Of these, the Cyprus Dwarf Hippopotamus, 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.

Description

Inside the mouth of a hippopotamus
Hippopotamuses are one of the largest mammals. They can live in the water, or go on land. Their specific gravity allows them to sink and walk or run along the bottom of a river. Hippos are considered megafauna, but unlike all other African megafauna, hippos have adapted for a semi-aquatic life in freshwater lakes and rivers.

Because of their enormous size, hippopotamuses are difficult to weigh in the wild. Most estimates of the weight come from culling operations that were carried out in the 1960s. The average weights for adult males ranged between 1500–1800 kg (3,300–4,000 lbs). Females are smaller than their male counterparts, with average weights measuring between 1300–1,500 kg (2,900–3,300 lbs). Older males can get much larger, reaching at least 3,200 kg (7,100 lbs) and occasionally weighing 4500 kg (9900 lbs). Male hippos appear to continue growing throughout their lives; females reach a maximum weight at around age 25.

On the National Geographic Channel television program, "Dangerous Encounters with Brady Barr", Dr. Brady Barr measured the bite pressure of an adult female hippo at ; Barr also attempted to measure the bite pressure of an adult male hippo, but had to abandon the attempt due to the male's aggressiveness.

Hippos measure long, including a tail of about in length, which includes a large horn and a small wisp of hair projecting out of its right side depending on the gender, and average about 1.5 meters (5 ft) tall at the shoulder. The range of hippopotamus sizes overlaps with the range of the White Rhinoceros; use of different metrics makes it unclear which is the largest land animal after elephants. Even though they are bulky animals, hippopotamuses can run faster than a human on land. Estimates of their running speed vary from 30 km/h (18 mph) to 40 km/h (25 mph), or even 50 km/h (30 mph). The hippo can maintain these higher speeds for only a few hundred meters.
A hippo's lifespan is typically 40 to 50 years. Donna the Hippo, 57, is the oldest living hippo in captivity. She lives at the Mesker Park Zoomarker in Evansville, Indianamarker. The oldest hippo ever recorded was called Tanga; she lived in Munich, Germany, and died in 1995 at the age of 61.

The eyes, ears, and nostrils of hippos are placed high on the roof of the skull. This allows them to be in the water with most of their body submerged in the waters and mud of tropical rivers to stay cool and prevent sunburn. Their skeletal structure is graviportal, adapted to carrying the animals' enormous weight. Hippopotamuses have small legs (relative to other megafauna) because the water in which they live reduces the weight burden. Like other aquatic mammals, the hippopotamus has very little hair.
Their skin secretes a natural sunscreen substance which is red-colored. The secretion is sometimes referred to as "blood sweat," but is neither blood nor sweat. This secretion is initially colorless 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. Both pigments inhibit the growth of disease-causing bacteria; as well, the light absorption of both pigments peaks in the ultraviolet range, creating a sunscreen effect. 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 synthesize the pigments from precursors such as the amino acid tyrosine.

Distribution

Hippopotamus amphibius was widespread in North Africa and Europe during the Eemian and late Pleistocene until about 30,000 years ago, and it can live in colder climates provided the water does not freeze during winter. The species was common in Egyptmarker's Nile region until historic times 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 nomemarker; the animal could still be found along the Damiettamarker branch after the Arab Conquest in 639. Hippos are still found in the rivers and lakes of Uganda, Sudanmarker, Somaliamarker, Kenyamarker, northern Democratic Republic of the Congomarker and Ethiopiamarker, west through Ghanamarker to Gambiamarker, and also in Southern Africa (Botswanamarker, Republic of South Africa, Zimbabwemarker, Zambiamarker). A separate population exists in Tanzania and Mozambiquemarker.

Conservation status

Genetic evidence suggests that common hippos in Africa experienced a marked population expansion during or after the Pleistocene Epoch, 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. Hippos are also subject to unregulated hunting and poaching. In May 2006 the hippopotamus was identified as a vulnerable species on the IUCN Red List drawn up by the World Conservation Union (IUCN), with an estimated population of between 125,000 and 150,000 hippos, a decline of between 7 percent and 20 percent since the IUCN's 1996 study.

The hippo population declined most dramatically in the Democratic Republic of the Congomarker. The population in Virunga National Parkmarker had dropped to 800 or 900 from around 29,000 in the mid 1970s. The decline is attributed to the disruptions caused by the Second Congo War. The poachers are believed to be former Hutu rebels, poorly paid Congolese soldiers, and local militia groups. Reasons for poaching include the belief that hippos are unintelligent, that they are harmful to society, and also for money. The sale of hippo meat is illegal, but black-market sales are difficult for Virunga National Parkmarker officers to track.

Invasive potential

In the late 1980s, Pablo Escobar kept four hippos in a private menagerie at his residence in Hacienda Napoles, 100 km east of Medellínmarker (Colombiamarker), after buying them in New Orleans. They were deemed too difficult to seize and move after Escobar's fall, and hence left on the untended estate. By 2007, the animals had multiplied to 16 and taken to roaming the area for food in the nearby Magdalena Rivermarker.. In 2009, 3 individuals, two adults and one calf, escaped the herd, and after attacking humans and killing cattle, one of the adults, called "Pepe," was killed by hunters under authorization of the local authorities .It is unknown what kind of effects this might have to the ecosystem in Colombia. According to experts interviewed by W Radio Colombia, the animals could survive in the Colombian jungles. However, it is believed that the lack of control from the Colombian government which is not used to deal with this species could result in human fatalities.

Behavior

An open mouth signals that the hippo feels threatened.
Hippos spend most of their days wallowing in the water or the mud, with the other members of their pod. The water serves to keep their body temperature down, and to keep their skin from drying out. With the exception of eating, most of hippopotamuses' lives—from childbirth, fighting with other hippos, and reproduction—occur in the water.

Hippos leave the water at dusk and travel inland, sometimes up to 8 kilometers (5 mi), to graze on short grass, their main source of food. They spend four to five hours grazing and can consume 68 kilograms (150 lb) of grass each night. Like almost any herbivore, they will consume many other plants if presented with them, but their diet in nature consists almost entirely of grass, with only minimal consumption of aquatic plants. Hippos have (rarely) been filmed eating carrion, usually close to the water. There are other reports of meat-eating, and even cannibalism and predation. The stomach anatomy of a hippo is not suited to carnivory, and meat-eating is likely caused by aberrant behavior or nutritional stress.

The diet of hippos consists mostly of terrestrial grasses, but they spend most of their time in the water. Most of their defecation occurs in the water, creating allochthonous deposits of organic matter along the river beds. These deposits have an unclear ecological function. Because of their size and their habit of taking the same paths to feed, hippos can have a significant impact on the land they walk across, both by keeping the land clear of vegetation and depressing the ground. Over prolonged periods hippos can divert the paths of swamps and channels.

Adult hippos cannot swim and are not buoyant. When in deep water, they usually propel themselves by leaps, pushing off from the bottom. They move at speeds up to 8 km/h (5 mph) in water. However, young hippos are buoyant and more often move by swimming—propelling themselves with kicks of their hind legs. Adult hippos typically resurface to breathe every 3–5 minutes. The young have to breathe every two to three minutes. The process of surfacing and breathing is automatic, and even a hippo sleeping underwater will rise and breathe without waking. A hippo closes its nostrils when it submerges.

Social life

Studying the interaction of male and female hippopotamuses has long been complicated by the fact that hippos are not sexually dimorphic and thus females and young males are almost indistinguishable in the field. Although hippos like to lie in close proximity to each other, they do not seem to form social bonds except between mothers and daughters, and are not social animals. The reason they huddle in close proximity is unknown.

Hippopotamuses are territorial only in water, where a bull presides over a small stretch of river, on average 250 meters in length, and containing ten females. The largest pods can contain up to 100 hippos. Other bachelors are allowed in a bull's stretch, as long as they behave submissively toward the bull. The territories of hippos exist to establish mating rights. Within the pods, the hippos tend to segregate by gender. Bachelors will lounge near other bachelors, females with other females, and the bull on his own. When hippos emerge from the water to graze, they do so individually.

Hippopotamuses appear to communicate verbally, through grunts and bellows, and it is thought that they may practice echolocation, but the purpose of these vocalizations is currently unknown. Hippos have the unique ability to hold their head partially above the water and send out a cry that travels through both water and air; hippos above and under water will respond.

Reproduction

Female hippos reach sexual maturity at five to six years of age and have a gestation period of 8 months. A study of endocrine systems revealed that female hippopotamuses may begin puberty as early as 3 or 4 years of age. Males reach maturity at around 7.5 years.

A study of hippopotamus reproductive behavior 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 estrous cycle; as with most large mammals, male hippopotamus 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. After becoming pregnant, a female hippopotamus will typically not begin ovulation again for 17 months.

Mating occurs in the water with the female submerged for most of the encounter, her head emerging periodically to draw breath. Hippos are one of the few mammals that give birth under water, along with Cetaceans and Sirenians (manatees and dugongs). Baby hippos are born underwater at a weight between 25 and 45 kg (60–110 lb) and an average length of around 127 cm (50 in) and must swim to the surface to take their first breath. A mother typically gives birth to only one hippo, although twins also occur. The young often rest on their mothers' backs when in water that is too deep for them, and they swim underwater to suckle. They also will suckle on land when the mother leaves the water. Weaning starts between six and eight months after birth and most calves are fully weaned after a year.

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 large numbers of small, poorly-developed young several times per year as is common among small mammals such as rodents).

Aggression

Hippopotami are ill-tempered animals. Adult hippos are hostile toward crocodiles, which often live in the same pools and rivers as hippos. This is especially so when hippo calves are around. Nile crocodiles, along with Lions and Spotted Hyenas, may prey on young hippos. Hippos are very aggressive towards humans and are often considered one of the most dangerous large animals in Africa. Hippos attack humans and boats.

To mark territory, hippos spin their tails while defecating to distribute their excrement over the greatest possible area. Hippos are retromingent, likely for the same reason.

Hippos rarely kill each other, even in territorial challenges. Usually a territorial bull and a challenging bachelor will stop fighting when it is clear that one hippo is stronger. When hippos become overpopulated, or when a habitat starts to shrink, bulls will sometimes attempt to kill infants; sometimes female hippos will kill the bulls to protect their infants, but neither behavior is common under normal conditions. Some incidents of hippo cannibalism have been documented, but it is believed to be the behavior of distressed or sick hippos, and not healthy behavior.

Hippos and humans

The earliest evidence of human interaction with hippos comes from butchery cut marks upon hippo bones at Bouri Formation dated around 160,000 years ago. 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 Djanetmarker in the Tassili n'Ajjer Mountainsmarker. Hippos were also well-known to the ancient Egyptians, where the hippo was recognized as a ferocious denizen of the Nile. In Egyptian mythology, the hippopotamus-headed Tawaret was a goddess of protection in pregnancy and childbirth, because ancient Egyptians recognized the protective nature of a female hippopotamus toward her young.

The hippopotamus has been known to historians since classical antiquity. The Greek historian Herodotus described the hippopotamus in The Histories (written circa 440 BC) and the Roman Historian Pliny the Elder wrote about the hippopotamus in his encyclopedia Naturalis Historia (written circa 77 AD).

Hippos in zoos

Hippopotamuses have long been popular zoo animals. The first zoo hippo in modern history was Obaysch who arrived at the London Zoomarker on May 25, 1850, where he attracted up to 10,000 visitors a day and inspired a popular song, the Hippopotamus Polka. Hippos have remained popular zoo animals since Obaysch, and generally breed well in captivity. Their birth rates are lower than in the wild, but this is attributed to zoos not wanting to breed as many hippos as possible, since hippos are large and relatively expensive animals to maintain.

Most hippos in zoos were born in captivity. There are enough hippos in the international zoo system that introducing further animals from the wild will be unnecessary if zoos cooperate to maintain the genetic diversity of the breeding stock.

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 designers increasingly designed exhibits that reflected the animals' native habitats. The best known of these, the Toledo Zoo Hippoquariummarker, features a 360,000 gallon pool for hippos. In 1987, researchers were able to tape, for the first time, an underwater birth (as in the wild) at the Toledo Zoo. The exhibit was so popular that the hippos became the logo of the Toledo Zoomarker.

Cultural depictions

The Hippopotamus was known to the Greeks and Romans as the Beast of the Nile. A red hippo also represented the Ancient Egyptian god Set; the thigh is the 'phallic leg of set' symbolic of virility. Set's consort Tawaret was also seen as part hippo. The Behemoth from the Book of Job, 40:15–24 is also thought to be based on a hippo.

Ever since Obaysch inspired the Hippopotamus Polka, hippos have been popular animals in western culture for their rotund appearance that many consider comical. Stories of hippos like Huberta who became a celebrity in South Africa in the 1930s for trekking across the country; or the tale of Owen and Mzee, a hippo and tortoise who developed an intimate bond; have amused people who have bought hippo books, merchandise, and many a stuffed hippo toy. 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.

Hippos have also been popular cartoon characters, where their rotund frame is used for humorous effect. The Disney film Fantasia featured a ballerina hippopotamus dancing to the opera, La Gioconda. Other cartoon hippos have included Hanna-Barbera's Peter Potamus, the book and TV series George and Martha, Flavio and Marita on the Animaniacs, Pat of the French duo Pat et Stanley, The Backyardigan's Tasha, and Gloria and Moto-Moto from the Madagascar franchise.

The hippopotamus characters "Happy Hippos" were created in 1988 by the French designer Andre Roche based in Munich, to be hidden in the "Kinder Surprise egg" of the Italian chocolate company Ferrero SpA. These characters were not placid like real hippos but rather cute and lively, and had such a success that they reappeared several times in different products of this company in the following years, increasing their popularity worldwide each time. The Nintendo Company published in the years 2001 and 2007 Game Boy adventures of them.

In the game of chess, the hippopotamus lends its name to the Hippopotamus Defense, a solid if unadventurous opening system.

Hippopotamus was one of the many exotic animals brought to fight gladiators in Rome by the emperor Philip I the Arab to commemorate Rome's 1000 years anniversary in 250 AD. Silver coins with hippo's image were minted that year.

References

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