Hippopotamus

I

Introduction

Hippopotamus, African mammal that lives partly in water and partly on land. There are two species of hippopotamuses. The smaller species, called the pygmy hippo, stands barely waist-high to an adult human. Its bigger relative—known as the river or common hippopotamus or, simply, hippopotamus—is one of the world's largest and heaviest land animals, weighing up to about 4,000 kg (about 8,800 lb). Although strictly a plant eater, the hippopotamus has massive jaws with formidable tusks. The animal has an unpredictable temperament and is capable of biting through small boats and slicing crocodiles in two.

The word hippopotamus comes from two Greek words that mean “river horse.” While hippos do live in rivers, they differ from horses in being artiodactyls, mammals with an even number of hoofed toes on each foot. There is also very little about their shape that is horselike. Both species of hippos have barrel-shaped bodies, stocky legs, and smooth, almost hairless skin. Their heads appear outsized and their mouths look unusually broad.

Hippopotamuses are not as numerous as they once were, but they can still have a major impact on the landscape of the areas they inhabit. In some parts of Africa, river hippos trample riverbanks and lakeshores, creating a maze of waterside paths and wallows that they use to get access to the plants they eat. Hippos spend their days lazing in or near water, emerging after dark to feed.

II

Range and Habitat

Hippopotamuses have been around for over 5 million years. At their peak about 1 million years ago, hippos lived in Europe, Asia, Africa, and on the island of Madagascar. Since then the range of hippos has steadily shrunk, and the animals can now be found only in central, western, and southern Africa.

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The first hippopotamuses were probably forest dwellers, a lifestyle that the pygmy hippo still follows. The pygmy hippo lives in West Africa's forest belt, primarily in Liberia and Côte d’Ivoire. Unlike its larger and better-known relative, this animal spends more of its time near water rather than in it, and dives beneath the water surface only when in imminent danger of attack.

By contrast, the river hippopotamus inhabits more open areas. It lives in lakes and rivers near grasslands in tropical Africa south of the Sahara, usually where the water is deep and slow-flowing. During the dry season, when lakes and rivers shrink, hundreds of hippos sometimes congregate in closely packed herds, and their deep grunting calls can be heard a great distance away.

III

Physical Description

Hippos have several peculiarities that set them apart from other mammals. For example, their skin is practically hairless and much thinner than that of other mammals of comparable size. Thin skin dries out easily, which explains why river hippos wallow in mud or lounge in water during daylight hours. Most river hippos have coppery-brown skin, while pygmy hippos are greenish-black.

Hippos do not have sweat glands, but they do have pores that secrete a reddish-pink fluid. This fluid gives hippos a glossy sheen and has led to the persistent but mistaken belief that the animals sweat blood. The substance acts as a natural conditioner, preventing the animal’s skin from cracking in the tropical heat. Pygmy hippos secrete a clear liquid for the same purpose.

River hippos—unlike pygmy hippos—have a number of special adaptations to life in the water. Their eyes are on the tops of their heads and are set in sockets that bulge upward, keeping the eyes above the waterline when the body is afloat. The nostrils also face upward and can be sealed completely during a dive. River hippos usually stay underwater for less than five minutes, but they are capable of holding their breath for up to half an hour. They can walk or run along the river bottom and can even sleep while submerged.

Given that the two species have some common characteristics, the size difference between pygmy and river hippos is startling. Pygmy hippos grow to about 1.75 m (about 5 ft 9 in) long, not including their short tails, and they have a maximum weight of about 275 kg (about 600 lb). Male river hippos can be up to 5 m (more than 16 ft) long, and they can easily be a dozen times heavier than their smaller relatives. In both species the body’s weight is supported on slightly webbed feet that have four hoof-tipped toes, all equally well developed. On land, pygmy hippos can move at a trot, while river hippos normally travel at a ponderous walk. However, if danger threatens, a fully grown river hippo can run 30 km/h (about 18 mph) for a few hundred yards—about as fast as an Olympic sprinter.

Hippos have good vision and hearing but find food mainly by smell. It is believed that pygmy hippos eat a wide variety of plants, while the river hippo feeds primarily on grass. The river hippo has a specially adapted mouth over 30 cm (1 ft) wide. Like a living lawnmower, it crops grass with its lips, and then grinds it into a pulp with its large cheek teeth or molars. Most other hoofed grazers ruminate their food—that is, they regurgitate partially digested food and chew it again. Hippos, meanwhile, have a relatively inefficient digestive system and do not ruminate food. They make up for this with a slow-paced life that keeps their energy use to a minimum.

In most respects male and female hippos have similar bodies, but males are bigger and have larger tusks than females. This is most clearly demonstrated when male river hippos aggressively fight each other over territory. Both sexes have two pairs of tusks set in the lower jaw. In male river hippos, the larger pair—the canine teeth—can be up to 50 cm (20 in) long, and are sharp enough to rip open a lion or seriously wound a rival. Among pygmy hippos, the size difference between males and females is not as great or as easily visible.

Hippos live relatively long lives. In the wild, river hippos live about 40 years, but in captivity they have survived into their early 60s. The normal life span of pygmy hippos is unknown, but in captivity some pygmy hippos have lived more than 40 years.

IV

Behavior

River hippos usually live in herds of up to 15 animals, but groups ten times this size are sometimes seen. During the day hippos often crowd together to laze in a pool or wallow, seeming to enjoy the physical contact with one another. But while they are social creatures when resting, hippos like to feed in private. After dusk, as the herd fans out to graze, each animal heads for a separate area of grass and remains on its own throughout the night. After feeding for up to five hours, they return to the water just before dawn.

The distance hippos travel to feed depends on the available food supply. On average they complete a round trip of about 5 km (3 miles) a night. If food is scarce, however, they have been known to travel ten times as far. Adult hippos have no natural predators on land apart from lions, so they run a low risk of being attacked when they feed.

During their nightly food excursions, adult males scatter droppings of their dung on conspicuous piles that flank the paths. These piles may simply act as path markers—a useful aid for animals that feed at night—but the ones left near water are thought to show where territories begin and end. To add its droppings to a pile, a hippo backs up to its droppings and uses its tail as a paddle, flicking the dung through the air. This creates a pile that can become several meters wide over time.

Male river hippos start to establish territories when they are in their early 20s, and they may eventually control more than 250 m (820 ft) of the water's edge. Subordinate males come and go freely, but a rival male will spark a confrontation if it tries to intrude. The territory owner and intruder face each other open-mouthed and, if one of them does not back down, the pair attempt to gash each other with their tusks. These fights can result in serious injury, and if conditions are crowded—as they often are during the dry season—nearby females and calves may also get hurt.

Much less is known about the habits and social life of the pygmy hippo because this animal is scarce and seldom seen. Like the river hippo, the pygmy hippo uses dung to mark its territory and travels along a network of waterside paths to feed. But adult pygmy hippos seem to avoid each other except when they come together to mate.