Hippopotamuses

(Hippopotamidae)

Class Mammalia

Order Artiodactyla

Suborder Suiformes

Family Hippopotamidae

Thumbnail description
Rotund, barrel-shaped body with short legs; large head with wide gape; eyes, ears, and nostrils high on the face; small tail flattened at the base

Size
Weight: female 500–3,000 lb (230–1,500 kg), male 600–4,000 lb (270–1,800 kg); length: female 58–106 in (150–270 cm), male 60–106 in (152–270 cm)

Number of genera, species
1 genus; 2 species

Habitat
Ponds, lakes, rivers, and wallows by day; grasslands and forests at night

Conservation status
Vulnerable: 1 species

Distribution
Sub-Saharan Africa

Evolution and systematics

The fossil record of hippos provides little evidence of their ancestry. All fossils can be readily assigned to one or other of the modern genera, except for a dwarf hippo from the Pleistocene of Cyprus, which has been placed in a separate genus, Phanourios, although it is considered to be of the genus Hippopotamus on the basis of its skull anatomy. Its limbs, however, are quite distinct and appear to be adapted for running over stony ground. It has been suggested that the differences are sufficiently great to justify its status as a separate genus.

There are numerous fossil Hexaprotodon, but none that can be equated with the modern pygmy hippo. Size is not a good criterion as there are dwarf Hippopotamus as well as full-sized Hexaprotodon. The former are usually found on islands, particularly those in the Mediterranean, and are probably examples of the phenomenon known as "island dwarfing," which occurs when herbivores are stranded without a predator on marine islands and subsequently shrink in body size. Simultaneously, they lose their abilities as saltors or cursors and evolve wide-ranging food habits as indicated, among others, by enlarging teeth to grind out what little nutrition there is from depleted and severely conditioned plant food supply. Island populations are inevitably small and, hence, are vulnerable to natural catastrophic events such as volcanic eruptions, as well as to human hunting. It is not surprising that dwarf hippos did not long survive the arrival of humans.

Most of the early fossil hippos of both genera are found in East Africa, suggesting that the family originated on that continent. But by the late Miocene, hippos had spread from Africa to most of Eurasia, though they did never reached Australia or the Americas. Some extinct species survived well into historical times. For example, Hippopotamus lemerlei was present in Madagascar as recently as a.d. 1000. As with the Mediterranean species, its extinction occurred soon after the arrival of man. In the case of Madagascar, this happened around 1,500 years ago.

The classical assumption is that hippos (Hippopotamidae) are related to pigs (Suidae) and peccaries (Tayassuidae), with the three families constituting the suborder Suiformes within the order Artiodactyla. Nine other families, containing only extinct forms, are recognized as belonging to the Suiformes, but none is obviously an ancestor to hippos. However, one superfamily, Anthracotheroidea, includes fossils that have been suggested as possible ancestors as they show some resemblance to hippos in their dentition. The anthracotherids resembled large pigs and were probably semi-aquatic. If they are ancestors, the hippo lineage branched off from them between the Oligocene and Miocene, about 25 million years ago. Hippos themselves date from the middle to late Miocene.

Some recent research points to a different evolutionary history as DNA analysis and gene sequencing suggest that the closest relatives to the hippo are whales (Cetacea). This

relationship is not evident from comparative anatomy or from the fossil record, however, the evolutionary history of whales is about as poorly known as that of hippos. It is now generally accepted that whales evolved from ungulate stock. One 1994 study suggested a link with hippos from their sequencing of the mitochondrial DNA cytochrome b gene, though other research found no support for such a relationship from the mitochondrial DNA sequencing, although a link was demonstrated between cetaceans and artiodactyls. There is, therefore, no general agreement on a hippo/whale link, though if such a link exists, it is probably a weak one.

DNA analyses have also questioned the position of the Hippopotamidae within the Suiformes. Two separate studies did not find a close relationship between pigs and peccaries on the one hand and hippos on the other. The mitochondrial sequence comparisons support the relationship between pigs and peccaries, with hippos forming a separate evolutionary line. The conclusions from these genetic studies, however, differ from those of their morphological analyses, which support the monophyly of the Suiformes. With opposite conclusions from the two techniques, the question of the ancestry of hippos remains open.

Physical characteristics

Hippos have rotund bodies and short legs. Unlike other artiodactyls, which have only two functional hooves, there are four toes on each foot with slight webbing between them. The thick skin appears hairless apart from a few bristles around the mouth and on the tail, but there is a covering of

very fine hairs at low density over the whole of the body. There are no sweat glands as such, but there are large skin glands that secrete a viscous liquid that turns pink on exposure to air. The secretion probably acts as a sunscreen; it is also thought to have antiseptic properties. The skin needs to be kept wet, and cracks appear if the hippo is prevented from entering water.

The head is large with a wide mouth. The canines and incisors are tusk-like with open roots so that the teeth grow continuously, somewhat like the teeth of rodents. The lower canines are curved and are particularly large. They are kept sharp by rubbing against the upper canines. The incisors are straight and peg-like. The lower incisors point forward, but the shorter upper incisors point downward. There are six molariform (grinding) teeth in each half-jaw, with a pattern of cusps that is diagnostic of the species. The full dental formula for the family is (I3/3, C1/1, P4/4, M3/3) × 2 = 44. The nostrils, eyes, and ears are placed high on the face. Consequently, the animal can remain submerged with very little of its body in view, while at the same time being able to breath and survey

its surroundings. The ears are small but highly moveable and wag vigorously on surfacing.

The alimentary canal shows what is known as the pseudo-ruminant condition, i.e., digestion takes place through fermentation of food in a multi-chambered stomach, as in ruminants, but there is no chewing of the cud. The post-gastric gut is typical of a large herbivore. There is no cecum, which is not typical of large herbivores, but there is a gall bladder despite statements to the contrary.

The reproductive organs show some peculiarities. Unlike most mammals, the male lacks a scrotum as the testes do not fully descend at birth. The penis is normally retracted, so it is difficult to tell the sex of a hippo. The female reproductive tract shows two marked peculiarities, one being the series of ridges in the upper vagina and, the other, two large sacs that project from the vestibule. The functions of these organs remain obscure.

Distribution

The common hippo occurs in many sub-Saharan countries, but some of the populations are extremely small, especially in West Africa. The pygmy hippo is confined to West Africa with Liberia containing the majority of the population.

Habitat

The common hippo is found in shallow freshwater aquatic habitats during the day, but at night it emerges to graze some distance from water. The pygmy hippo is a forest animal, although it too spends the day in or near water.

Behavior

Although it is highly colonial in the water, the common hippo is not a social animal and the only social bond is that between a female and her dependent calves. Males are territorial in the water and maintain mating rights over the females within a defended length of shoreline. In addition to their aerial calls, common hippos vocalize under water. Some of the sounds are amphibious and pass through the air and water simultaneously. The pygmy hippo is usually found alone; although pairs occur; the duration of the pair bond is not known. There is no evidence that males are territorial.

Feeding ecology and diet

Hippos are herbivorous, although carnivory through scavenging has been reported in the common hippo on a few occasions. Grass is the principal food of the common hippo,

while the pygmy hippo has a much wider diet that includes fruits and ferns. Both species feed by nipping off the vegetation with their muscular lips.

Reproductive biology

The common hippo mates in the water as does the pygmy hippo, which also copulates on land. Births occur in the water in both species, although the pygmy hippo sometimes calves on land. Usually, only one calf is produced at a time, though twins do occur at a low rate. Births may take place throughout the year, but there are peaks associated with increased rainfall. Hippopotamuses are polygamous.

Conservation status

The common hippo is placed on Appendix II of CITES and the pygmy hippo on Appendix I. The common hippo is not in immediate danger over much of its range in southern and east Africa, but it is vulnerable to extinction in West Africa. The conservation status of the pygmy hippo is not clear, though the IUCN lists it as Vulnerable.

Significance to humans

The common hippo impinges on human affairs as it is a dangerous animal that also raids crops, particularly rice. The pygmy hippo is not a threat to people, although it does sometimes raid crops. Both species are hunted for meat.

Species accounts

List of Species

Common hippopotamus

Hippopotamus amphibius

taxonomy

Hippopotamus amphibius Linnaeus, 1758, Nile River, Egypt. Five subspecies have been described, though it is doubtful that they are valid. No external differences between them have been noted.

other common names

English: River hippopotamus; French: Hippopotame; German: Grossflusspferd; Spanish: Hipopotamo.

physical characteristics

The common hippo has a shoulder height of 54–60 in (137–152 cm), length up to 106 in (270 cm), and weighs up to 4,000 lb (1,800 kg). It has a rotund body with disproportionately short legs. The thick purple-brown skin appears hairless, with only a few bristles around the mouth and on the tail, but there is a covering of very fine hairs at low density over the whole of the body.

The head is large with a wide mouth that opens to a gape of nearly 180°. The number of teeth is slightly reduced in the common hippo to (I2/2, C1/1, P3/3, M3/3) × 2 = 36, although sometimes the fourth milk premolar is retained in the adult jaw.

distribution

The common hippopotamus occurs in some 35 sub-Saharan countries, but many populations are extremely small, especially in West Africa, where the hippos are fragmented into isolated groups each containing a few dozen at the most. Countries with substantial populations are situated in the eastern and southern parts of the continent of Africa and include Zambia, Democratic Republic of the Congo (Zaire), and Tanzania.

habitat

During the day, the hippo is found in shallow freshwater aquatic habitats, which may be rivers, lakes, wallows, or any wetland that keeps the skin moist. Wet mud is suitable, and hippos often occur in wallows that are drying out. At night, the hippo leaves the water to graze in grasslands that may be a mile (1.6 km) or more from its daytime retreat. It reaches the grazing areas by following well-worn trails.

behavior

There have been few investigations into the behavior of the hippo as it is a difficult species to study and conclusions about its activities may not be of general applicability. The most detailed study, set in Uganda, reported that although the hippo is highly colonial in the water, it shows little in the way of a social life and is essentially solitary when grazing at night. The only social bond is that between a female and her dependent calves. Males are territorial in the water and maintain mating rights over the females within a defended length of shoreline. It is not a harem system, however, for the females do not necessarily return to the same territory every day. The territory may contain bachelor groups, which are tolerated by the territorial male provided they behave submissively on meeting. Fierce fighting may occur if a bachelor challenges the territory holder and the death of one of the combatants may follow. Hippos are not territorial on land. The spraying of bushes with feces occurs when the hippo vigorously wags its tail while defecating, thus spreading the droppings far and wide. The purpose is not clear but it may have a social function as a dominance display or possibly the deposits may serve to orientate the hippo during its nocturnal wanderings.

The bellows of hippos are well-known, though their role in communication is not clear. In addition to the aerial sounds, hippos vocalize under water, producing at least three types of call, including tonal whines, pulsed croaks, and clicks, all of which appear to be associated with communication. Some calls are amphibious and pass through the air and water simultaneously.

feeding ecology and diet

The common hippopotamus is predominantly a grazer. It plucks the grass with its lips, nipping it off close to the ground thereby creating short-grass patches known as hippo lawns. Relative to its weight, the hippo consumes less food than other herbivores, possibly reflecting its lower metabolic rate. The hippo is able to extend its grazing range in the wet season by making use of temporary wallows for daytime resting thus obviating the need to trek back to permanent water every day. However, it does not feed on aquatic vegetation to any extent. Carnivory, mainly through scavenging, has been reported several times, including a case of cannibalism.

reproductive biology

Polygamous. The hippo mates in the water with the female remaining submerged other than when she breaks surface to

breathe. The gestation period of 240 days is short for such a large animal. The female withdraws from the herd before giving birth, which usually takes place in the water. The weight of the neonate is about 110 lb (50 kg). Normally, only one calf is produced at a time, though twins do occur at a low rate, which in Uganda was found to be 0.7%. The sex ratio at birth is 1:1.

Births occur throughout the year on the equator, but there are peaks associated with rainfall, with most births taking place at the beginning of the wet season. Away from the equator, where rainfall is more seasonal, there are some dry season months when there are no births at all.

The duration of lactation is unknown; it is probably of the order of one year. The calf remains with its mother after the second calf is born, but leaves before the onset of puberty, which in Uganda was estimated to be at about 7.5 years of age in the male and nine years in the female.

conservation status

The common hippo is placed on Appendix II of CITES. It is not in immediate danger over much of its range in southern and east Africa, but the outlook in West Africa is bleak. The total number is low, and the remaining animals are scattered in small groups in which genetic problems resulting from inbreeding may be expected.

significance to humans

The hippo is a dangerous animal that is responsible for numerous human deaths each year, particularly of fishermen. It also raids crops, particularly rice. It is, in turn, hunted for meat and ivory; some of the human deaths occur during attempts to harpoon the animal. The hippo features in African folklore.

Pygmy hippopotamus

Hexaprotodon liberiensis

taxonomy

Hippopotamus minor (Morton, 1849), St. Paul's River, Liberia. Two subspecies, Hexaprotodon liberiensis liberiensis and H. l. heslopi, but the latter is most probably extinct.

other common names

French: Hippopotame nain; German: Zwergflusspferd; Spanish: Hipopotamo enano.

physical characteristics

Length 5–6 ft (1.5–1.8 m); weight 350–600 lb (159–272 kg). The pygmy hippo is similar to its larger cousin in its general body shape; its head, however, is proportionately smaller with less protuberant eyes, which, with the ears and nostrils, are not so high on the head. Its limbs and neck are relatively longer, and the back slopes forward instead of being parallel with the ground as in the common hippo. The skin is darker than that of the larger species—a blackish color. The dental formula is (I2/1, C1/1, P3/3, M3/3) × 2 = 34.

distribution

The pygmy hippo is confined to West Africa with Liberia containing the majority of the population. Other countries with pygmy hippos are Guinea, Ivory Coast, and Sierra Leone.

habitat

A forest animal that spends the day in or near water, usually a river or stream, which it leaves at night to forage. Some, at least, occupy burrows in the banks of rivers; it is not clear whether the hippo excavates the burrow itself or merely enlarges an existing hole.

behavior

Its behavior in the wild is insufficiently known. It is usually found alone, although pairs occur. It is not clear whether the species is monogamous or promiscuous. It follows game trails and spreads its feces with its tail.

feeding ecology and diet

Feeds with its lips; its diet is varied, with fruits and ferns figuring prominently. Grass is readily eaten when available.

reproductive biology

Polygamous. Copulates either on land or in the water after a consorting period; it is not known how long the pair bond persists. Estimates of the gestation period vary from 188 to 210 days. Births occur in water or on land, and singletons are the rule. Birth weights of 35 calves in Basel Zoo averaged 12.61 lb (5.73 kg). Sexual maturity in zoo specimens occurs between three and five years of age; it is probably later in the wild. The calf does not follow its mother when she is foraging, remaining in hiding and visited at intervals to be suckled.

conservation status

Listed on Appendix I of CITES, and the IUCN lists it as Vulnerable to extinction, mainly through loss of habitat. It adapts well to captivity and breeds readily so that it could, if necessary, be maintained indefinitely in zoos.

significance to humans

Not normally a threat to human beings, although it is a formidable animal and can cause injury to hunters. It occasionally raids crops. It is the subject of much folklore.

Resources

Periodicals

Barklow, W. E. "Some Underwater Sounds of the Hippopotamus (Hippopotamus amphibius)." Marine and Freshwater Behavioral Physiology 29 (1997): 237–249.

Coryndon, S. C. "The Taxonomy and Nomenclature of the Hippopotamidae (Mammalia, Ardiodactyla) and a Description of Two New Fossil Species." Proceedings of the Koninklijke Nederlandse Akademie Wetenschappen Series B 80 (1977): 61–88.

Gansberger, K., and G. Forstenpointer. "On the Existence of a Gall Bladder in the Hippopotamus." Wiener Tieraerztliche Monatsschrift 82 (1995): 157–158.

Hasegawa, M., and J. Adachi. "Phylogenetic Position of Cetaceans Relative to Artiodactyls—Reanalysis of Mitochondrial and Nuclear Sequencies." Molecular Biology and Evolution 13 (1996): 710–713.

Houtekamer, J. L., and P. Y. Sondaar. "Osteology of the Forelimb of the Pleistocene Dwarf Hippopotamus from Cyprus with Special Reference to Phylogeny and Function." Proceedings of the Koninklijke Nederlandse Akademie Wetenschappen Series B 82 (1979): 411–448.

Irwin, D. M., and U. Amason. "Cytochrome b Gene of Marine Mammals: Phylogeny and Evolution." Journal of Mammalian Evolution 2 (1994): 37–55.

Klingel, H. "The Social Organisation and Behaviour of Hippopotamus amphibius." African Wildlife: Research and Management, International Council of Scientific Unions (1991): 73–75.

Lang, E. M., M. vK. Hentschel, and W. Bulow. "Zwergflusspferde (Gattung Choeropsis)." Grzimeks Enzyklopädie: Saugetiere (1988): 62–64.

Laws, R. M., and G. Clough. "Observations on Reproduction in the Hippopotamus Hippopotamus amphibius Linn." Symposia of the Zoological Society of London 15 (1966): 117–140.

Montegelard, C., S. Ducrocq, and E. Douzery. "What is a Suiform (Artiodactyla)? Contribution of Cranioskeletal and Mitochondrial DNA Data." Molecular Phylogenetics and Evolution 9 (1998): 528–532.

Randi, E., V. Lucchini, and C. H. Diong. "Evolutionary Genetics of the Suiformes as Reconstructed Using mtDNA Sequencing." Journal of Mammalian Evolution 3 (1996): 163–194.

Stewart K. Eltringham, PhD