Hippopotamuses have barrel-shaped bodies, large mouths, short legs and four toes on each foot. They may reach a length of 4.6m, a height of 1.5m at the shoulder, and a weight of 3-4.5 tons. The skin is very thick, nearly hairless, and grayish brown above, lighter and pinkish below. The ears and nostrils protrude so that they are above water when the animal is floating with the rest of its body submerged. The mouth is enormous and can open extremely wide; the upper lip is thick and bulging; and the incisor and canine teeth are very large and grow continuously as they are worn away. The lower canines may reach a length of 30cm above the gumline. The hippopotamus's numerous skin glands release a protective, oily, pinkish secretion that has popularly, but incorrectly, been thought of as "blood sweat" because it appears red in sunlight; this pigment actually serves to protect the animal's skin in both water and dry conditions.

Hippopotamuses live in rivers, lakes, swamps, or other permanent bodies of water in groups typically numbering 7-15 animals. During the day they sleep and rest in or near the water, and at night they emerge on to the land to feed on grasses that they crop with their horny lips. In water they can swim fast, can walk along te bottom, and can submerge (with ears and nostrils closed) for between 3 to 10 minutes at a time. On land hippopotamuses are steady plodders and can gallop for short distances, but they are never found very far from water, and their grazing range extends inland for only about 3km.

A single young is born after a gestation period of about eight months and weighs 25-55kg. The baby often rides the mother's back while she swims or floats on the surface. Though gregarious, hippopotamuses tend to be aggressive, and in the breeding season males engage in sometimes fatal battles. The hippopotamus's voice is a loud grunt or bellow, and its gaping "yawn" is in fact a threat signal. Hippoptamuses are generally inoffensive to humans but sometimes make unprovoked atacks on small boats, smashing them in their jaws. Their flesh is much esteemed by Africans, and their teeth are of good quality ivory. They can live to the age of 40 years in captivity.

The rare pygmy hippopotamus (choeropsis liberiensis), the other living species of the family Hippopotamidae, is about 1.5m long, weighs about 190kg, and is greyish black in colour. It is less gregarious and less aquatic than its larger relative, and it lives alone or in pairs in lowland western Africa from Sierra Leone eastward to Nigeria, where it is found along streams and in wet forests and swamps.