Hippopotamus (Hippopotamus amphibius)

Appearance

Hippopotamuses are among the largest living land mammals, being only smaller than elephants and some rhinoceroses. Mean adult weight is around 1,500 kg and 1,300 kg for males and females respectively, very large males can reach 2,000 kg and an exceptional male weighting almost 2,700 kg has been reported. Male hippos appear to continue growing throughout their lives while females reach maximum weight at around age 25. Different from all other large land mammals, hippos are of semiaquatic habits, spending the day in lakes and rivers.: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.:259 Their barrel-shaped bodies have graviportal skeletal structures:8, adapted to carrying their enormous weight, and their specific gravity allows them to sink and move along the bottom of a river. Hippopotamuses have small legs because the water in which they live reduces the weight burden. Though they are bulky animals, hippopotamuses can gallop at 30 km/h on land but normally trot. They are incapable of jumping but do climb up steep banks. 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.:3 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 are unusual in that the vagina is ridged and two large diverticula protrude from the vulval vestibule. The function of these is unknown.:28–29

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.:259 The jaw hinge is located far back enough to allow the animal to open its mouth at almost 180°.:17 On the National Geographic Channel television program, “Dangerous Encounters with Brady Barr”, Dr. Brady Barr measured the bite force of an adult female hippo at 8,100 newtons ; 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. Hippopotamus 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 , while the canines reach up to 50 cm . 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.:259, 263 The hippo is considered to be a pseudoruminant, it has a complex three- or four-chambered stomach but does not “chew cud”.:22

Unlike most other semiaquatic animals, the hippopotamus has very little hair.:260 The skin is 15 cm thick, providing it great protection against conspecifics and predators. By contrast, its subcutaneous fat layer is thin.:3 The animals’ upper parts are purplish-gray to blue-black, while the under parts and areas around the eyes and ears can be brownish-pink.:260 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 and one orange . 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. Nevertheless, this natural sunscreen cannot prevent the animal’s skin from cracking if it stays out of water too long.

A hippo’s lifespan is typically 40–50 years.:277 Donna the Hippo was the oldest living hippo in captivity. She lived at the Mesker Park Zoo in Evansville, Indiana in the US until her death in 2012 at the age of 61. The oldest hippo ever recorded was called Tanga; she lived in Munich, Germany, and died in 1995 at the age of 61.

Naming

The word “hippopotamus” is derived 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. A group is called a pod, herd, dale, or bloat.

In Africa, the hippo is known by various names, including ”seekoei” , ”mvuvu” , ”kubu” and ”mvubu” in the south; ”kiboko” , e”nsherre” , ”tomondo” , ”nvubu” , ”ifuru” , ”emiria” , ”magawit” , ”kibei” and ”olmakau” in the Great Lakes region;:256 and ጉማርረ/”gumarre” and ”jeer” in the Horn.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 hippopotamus, likely through the process of insular dwarfism. Fossil evidence indicates 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 species of hippopotamus, the European hippopotamus 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 Mediterranean during the Pleistocene. The Pleistocene also saw a number of dwarf species evolve on several Mediterranean islands, including Crete , Cyprus , Malta , and Sicily . 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….hieroglyph snipped…

Distribution

”Hippopotamus amphibius” was widespread in North Africa and Europe during the Eemian 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. 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; the animal could still be found along the Damietta branch after the Arab Conquest in 639. 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 Gambia, and south to South Africa. They inhabit both savanna and forest areas.

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 , 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 and Tanzania possess the largest populations.

The hippo population declined most dramatically in the Democratic Republic of the Congo. The population in Virunga National Park 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 Mai-Mai rebels, poorly paid Congolese soldiers, and local militia groups. Reasons for poaching include the belief that hippos are harmful to society, as well as financial gain. The sale of hippo meat is illegal, but black-market sales are difficult for Virunga National Park officers to track. Hippo meat is considered a delicacy in some areas of central Africa and the teeth have become a valued substitute for elephant ivory.

Behavior

With the exception of eating, most of hippopotamuses’ lives – from childbirth, fighting with other hippos, to reproduction – occurs in the water. Hippos leave the water at dusk and travel inland, sometimes up to 10 km , to graze on short grasses, their main source of food. They spend four to five hours grazing and can consume 68 kg of grass each night. Like almost any herbivore, they consume other plants if presented with them, but their diets in nature consists almost entirely of grass, with only minimal consumption of aquatic plants. Hippos are born with sterile intestines, and require bacteria obtained from their mothers feces to digest vegetation. Hippos have 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.:84

Hippo defecation creates 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 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.

Adult hippos move at speeds up to 8 km/h in water; typically resurfacing to breathe every three to five minutes. The young have to breathe every two to three minutes.:4 The process of surfacing and breathing is automatic. A hippo sleeping underwater rises and breathes without waking. A hippo closes its nostrils when it submerges into the water. 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.Studying the interaction of male and female hippopotamuses has long been complicated because hippos are not sexually dimorphic; thus females and young males are almost indistinguishable in the field. 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.:49

Hippopotamuses are territorial only in water, where a bull presides over a small stretch of river, on average 250 m in length, and containing 10 females. The largest pods can contain over 100 hippos.:50 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 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.:4

Hippopotamuses appear to communicate vocally, through grunts and bellows, and they may practice echolocation, but the purpose of these vocalizations 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.

Reproduction

Female hippos reach sexual maturity at five to six years of age and have a gestation period of eight months. A study of endocrine systems revealed that female hippopotamuses may begin puberty as early as three or four years of age. Males reach maturity at around 7.5 yr. 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.:60–61 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,:63 her head emerging periodically to draw breath. Baby hippos are born underwater at a weight between 25 and 50 kg and an average length of around 127 cm , 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. Weaning starts between six and eight months after birth, and most calves are fully weaned after a year.: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 .

Evolution

Until 1909, naturalists grouped hippos with pigs, based on molar patterns. 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 cetaceans – whales, dolphins and porpoises. 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 semiaquatic ancestor that branched off from other artiodactyls around million years ago. This hypothesized ancestral group likely split into two branches around million years ago. One branch would evolve into cetaceans, possibly beginning about 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. 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 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 from Eocene and Oligocene species: ”Anthracotherium” and ”Elomeryx” to the Miocene species ”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 million years ago. 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 million years ago. 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 million years ago, 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” , may have diverged as far back as 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 ”Choeropsis” – an older and basal genus.

Cultural

A red hippo 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 and was a goddess of protection in pregnancy and childbirth, because ancient Egyptians recognized the protective nature of a female hippopotamus toward her young. The Ijo people wore masks of aquatic animals like the hippo when practicing their water spirit cults. The Behemoth from the Book of Job, 40:15–24 is thought to be based on a hippo.

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.

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 such as Huberta, which 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 which developed an intimate bond; have amused people who have bought hippo books, merchandise, and many stuffed hippo toys. 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 frames are 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. The Nintendo Company published Game Boy adventures of them in 2001 and 2007. In the game of chess, the hippopotamus lends its name to the Hippopotamus Defense, an opening system, which is generally considered weak.”The River Horse” is a popular outdoor sculpture at George Washington University, Washington, D.C.

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