Hippopotamus
Conservation status
Vulnerable (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Artiodactyla
Family:	Hippopotamidae
Genus:	Hippopotamus Linnaeus, 1758
Species:	H. amphibius
Binomial name
Hippopotamus amphibius Linnaeus, 1758[2]
Range map[1]

The hippopotamus (Hippopotamus amphibius), or hippo, from the ancient Greek for "river horse" (ἱπποπόταμος), is a large, mostly herbivorous mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae (the other is the Pygmy Hippopotamus.) After the elephant and rhinoceros, the hippopotamus is the third largest land mammal and the heaviest extant artiodactyl.

The hippopotamus is semi-aquatic, inhabiting rivers, lakes and mangrove swamps, 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 55 million years ago.^[3] The common ancestor of whales and hippos split from other even-toed ungulates around 60 million years ago.^[4] The earliest known hippopotamus fossils, belonging to the genus Kenyapotamus in Africa, date to around 16 million years ago.

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 3 tonnes), behind the white rhinoceros (1½ to 3½ tonnes) and the three species of elephant (3 to 9 tonnes). The hippopotamus is one of the largest quadrupeds (four legged mammals) ^[5] and despite its stocky shape and short legs, it can easily outrun a human. Hippos have been clocked at 30 km/h (19 mph) over short distances. The hippopotamus is one of the most aggressive creatures in the world and is often regarded as one of the most dangerous animals in Africa. There are an estimated 125,000 to 150,000 hippos throughout Sub-Saharan Africa; Zambia (40,000) and Tanzania (20,000–30,000) possess the largest populations.^[1] They are still threatened by habitat loss and poaching for their meat and ivory canine teeth. There is also a colony of non-zoo hippos in Colombia introduced by Pablo Escobar.

Etymology[link]

Look up hippopotamus in Wiktionary, the free dictionary.

The word "hippopotamus" is derived from the ancient Greek ἱπποπόταμος, hippopotamos, from ἵππος, hippos, "horse", and ποταμός, potamos, "river", meaning "horse of the river".^[6] In English, the plural is hippopotamuses, but hippopotami is also used;^[7] 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.

In Africa, the hippo is known by several names including Kiboko (Swahili), Ensherre (Nkore), Tomondo (Turu), Nvubu (Luganda), Ifuru (Luhya), Emiria (Ateso), Magawit (Sebei), Kibei (Kalenjin), Olmakau (Maasai) and Jir (Somali) in the east;^[8]:256 and Seekoei (Afrikaans), Mvuvu (Venda), Kubu (Lozi) and Mvubu (Xhosa, Siswati and Zulu) in the south.^[9]

Taxonomy and origins[link]

Classification[link]

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.

A pygmy hippopotamus (Hexaprotodon liberiensis)

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:^[10]

• H. a. amphibius – (the nominate subspecies) which stretched from Egypt, where they are now extinct, south up the Nile River to Tanzania and Mozambique.
• H. a. kiboko – in the Horn of Africa, in Kenya and Somalia. Broader nasals and more hollowed interorbital region.
• H. a. capensis – from Zambia to South Africa. Most flattened skull of the subspecies.
• H. a. tschadensis – throughout Western Africa to, as the name suggests, Chad. Slightly shorter and wider face, with prominent orbits.
• 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 non-representative samples.^[11]: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 have been tested.^[12][13]

Evolution[link]

Until 1909, naturalists grouped hippos with pigs, based on molar patterns. Several lines of evidence, first from blood proteins, then from molecular systematics^[14] and DNA ^[15][16] and the fossil record, show that their closest living relatives are cetaceans—whales, dolphins and porpoises.^[17][18] 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.^[15][19]

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 60 million years ago.^[15][17] This hypothesized ancestral group likely split into two branches around 54 million years ago.^[14] One branch would evolve into cetaceans, possibly beginning about 52 million years ago with the proto-whale Pakicetus and other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans.^[19]

Hippopotamus madagascariensis skeleton with a modern hippopotamus skull.

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.^[17]

A rough evolutionary lineage can be traced from Eocene and Oligocene species: Anthracotherium and Elomeryx to the Miocene Merycopotamus and Libycosaurus and the very latest anthracotheres in the Pliocene.^[20] 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 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 8 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 1.8 million years ago an ancestor to the modern hippopotamus, Archaeopotamus, lived in Africa and the Middle East.^[21]

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 Choeropsis —an older and basal genus.^[20][21]

Extinct species[link]

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.^[22] There is fossil evidence that many Malagasy Hippos were hunted by humans, a likely factor in their eventual extinction.^[22] 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.^[23]

Two 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.^[24] 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 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 was encountered, and was driven to extinction, by man.^[25][24]

Description[link]

A hippo's skull, showing the large canine teeth used for fighting

Hippopotamuses are among the largest living mammals; only elephants and some rhinoceroses and whales are heavier. They can live in the water or 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.^[11]:3

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 1,500–1,800 kg (3,300–4,000 lb). Females are smaller than their male counterparts, with average weights measuring between 1,300–1,500 kg (2,900–3,300 lb).^[11]:12 Older males can get much larger, reaching at least 3,200 kg (7,100 lb) and occasionally weighing 4,500 kg (9,900 lb).^[26][27] Male hippos appear to continue growing throughout their lives; females reach a maximum weight at around age 25.^[28]

Hippos measure 3.3 to 5.2 meters (11 to 17 ft) long, including a tail of about 56 centimeters (22 in) in length and average about 1.5 meters (5 ft) tall at the shoulder.^[29][30] 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. Despite being semi-aquatic 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.^[11]:3

An open mouth signals that the hippo feels threatened.

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. Unlike most other semi-aquatic animals, the hippopotamus has very little hair.^[8]:260 The skin is 6 in (15 cm) thick,^[31] providing it great protection against conspecifics and predators. The animals's upper parts are purplish-gray to blue-black while the under parts and areas around the eyes and ears can be brownish-pink.^[8]:260

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.^[8]:259 The animal has a distinctive gape, which can reach 150°.^[31] 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 8100 N (1821 lbf); 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.^[32] 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 (16 in) while the canines reach up to 50 cm (20 in).^[31]

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.^[33]

A hippo's lifespan is typically 40–50 years.^[8]:277 Donna the Hippo, 60, is the oldest living hippo in captivity. She lives at the Mesker Park Zoo in Evansville, Indiana, USA.^[34][35] The oldest hippo ever recorded was called Tanga; she lived in Munich, Germany, and died in 1995 at the age of 61.^[36]