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.