(Hippopotamus amphibius), from the Greek ‘ιπποπόταμος (hippopotamos, hippos
meaning "horse" and potamos meaning "river"), often shortened to "hippo", is
a large, mostly plant-eating African mammal, one of only two extant species
in the family Hippopotamidae (the other being the Pygmy Hippopotamus).|
The hippopotamus is semi-aquatic, inhabiting rivers and lakes in sub-Saharan Africa in large groups of up to 40 hippos. During the day they remain cool by staying in the water or mud; reproduction and childbirth both occur in water, where territorial bulls preside over a stretch of river. They emerge at dusk to graze on grass. While hippos rest near each other in territories 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 and the like. The common ancestor of whales and hippos split from other even-toed ungulates around 60 mya. The earliest known hippopotamus fossils, belonging to the genus Kenyapotamus in Africa, date to around 16 mya.
The hippopotamus is recognizable for its barrel-shaped torso, hairless body, stubby legs and tremendous size. It is similar in size to the White Rhinoceros; only elephants are consistently larger and despite its stocky shape and short legs, it can easily outrun a human. Despite its popularity in zoos and cuddly portrayal as gentle giants in fiction, the hippopotamus is among the most dangerous and aggressive of all mammals. There are an estimated 125,000 to 150,000 hippos remaining throughout Sub-Saharan Africa, of which Zambia (40,000) and Tanzania (20,000-30,000) have the largest populations . They are still threatened by poaching for their meat and ivory canine teeth, and by habitat loss.
Taxonomy and origins
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.
Five subspecies of hippos have been described based on morphological differences in their skulls and geographical differences:
The suggested subspecies were never widely used or validated by field biologists, the described morphological differences were small enough that they could have resulted in simple variation in non-representative samples. Genetic analyses have tested the existence of three of these putative subspecies. A study using mitochondrial DNA from skin biopsies taken from 13 sampling locations, consider genetic diversity and structure among hippo populations across the continent. They find low but significant genetic differentiation among H. a. amphibius, H. a. capensis, and H. a. kiboko. Neither H.a.constrictus nor H.a.tschadensis have been tested.
As indicated by the name, ancient Greeks considered the hippopotamus to be related to the horse. Until 1985, naturalists grouped hippos with pigs, based on molar patterns. Evidence, first from blood proteins, then from molecular systematics and DNA and the fossil record, show that their closest living relatives are cetaceans—whales, porpoises and the like. Hippopotamuses have more in common with whales than they do with other Artiodactyls (even-toed ungulates), such as pigs because the common ancestor of hippos and whales branched-off from ruminants and the rest of the even-toed ungulates. Thus, hippos are more closely related to whales than to other members of Artiodactyla. While cetaceans and hippos are each other's closest living relatives, their lineages split soon after their divergence from the rest of the even-toed ungulates.
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 without leaving any descendants.
A rough evolution can be traced, however, from Eocene and Oligocene species: Anthracotherium and Elomeryx to the Miocene anthracotheres Merycopotamus and Libycosaurus. 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 20 mya. The last species of anthracotheres became extinct during the pliocene.
Hippopotamids are therefore deeply nested within the family Anthracotheriidae. The oldest known hippopotamid is the genus Kenyapotamus which lived in Africa from 16–8 mya. The Hippopotamidae are believed to have evolved in Africa, and while at one point the species spread across Asia and Europe, no hippopotamuses have ever been discovered in the Americas. From 7.5–1.8 mya an ancestor to the modern hippopotamus, the 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 8 mya. Scientists disagree whether or not the modern Pygmy Hippopotamus is a member of Hexaprotodon—a genus of many Asian Hippopotamuses that is more-closely related to Hippopotamus; or Choeropsis—an older and basal genus.
A 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 Mediterranean, during the Pleistocene.
These Pleistocene dwarf hippos of the Mediterranean lived on Crete (H. creutzburgi), Cyprus (H. minor), Malta (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 encountered, and was driven to extinction, by man.
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). Male hippos appear to continue growing throughout their lives; females reach a maximum weight at around age 25.
Hippos average 3.5 meters (11 ft) long, 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 or yards.
A hippo's lifespan is typically 40 to 50 years. Donna the Hippo, 56, is the oldest living hippo in captivity. She lives at the Mesker Park Zoo in Evansville, Indiana. The oldest hippo ever 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 general anatomical structure is an adaptation to their riparian lifestyle. Their skeletal structure is graviportal, adapted to carrying the animals' enormous weight. hippopotamuses have legs that are small, 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 is 4 centimeters (1.5 in) thick, and accounts for 25% of their weight. For additional protection from the sun, 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. They are known as red pigment hipposudoric acid and orange pigment norhipposudoric acid. The red pigment was found to inhibit the growth of disease-causing bacteria, lending credence to the theory that the secretion has an antibiotic effect. 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.
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 individuals from around 29,000 in the mid 1970s, raising concerns about the viability of that population. The decline is attributed to the disruptions caused by the Second Congo War. 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 a harm to society, and also for money. The sale of hippo meat is illegal, but black-market sales are difficult for WWF officers to track.
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 are not generally 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. Young hippos are buoyant and more often move by swimming, propelling themselves with kicks of their back legs. Adult hippos typically resurface to breathe every 4–6 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. One hippo calf survived after being pushed out to sea during the tsunami caused by the 2004 Indian Ocean earthquake and was rescued on a nearby sandy reef.
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, but the purpose of these vocalizations is 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.
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 oestrous 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 occur at an unknown ratio. 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.
Hippos are considered K-strategists, meaning that they favor quality over quantity in their reproduction. K-selection is the norm for large animals that produce few young at each birth.
To mark territory, hippos spin their tails while defecating to distribute their excrement over the greatest possible area. Hippos also urinate backwards (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.
Hippos and humans
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
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 Hippoquarium, 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 Zoo. Hippos have been moved out of the main zoo altogether in Melbourne, transferred instead to the Werribee Open Range Zoo on the city's western outskirts.
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