Hippopotamus

A hippo's eyes, ears, and nostrils are all positioned in a single plane that can stay above water when the rest of the animal is submerged. Both the ears and the nostrils can close, at least partially, when in water. Hippos do not see well either on land or in the water; instead, they depend on their acute hearing to warn them of danger and their good sense of smell to find food. When alarmed, a hippo may quietly submerge or it may attack, especially if it is threatened by people in boats. Several hundred people are killed by hippos each year in Africa.

Hippos spend most of the day in groups in the water. They prefer water about 5 ft (1.5 m) deep, just deep enough to swim if they want to or to walk on the bottom. Hippos can stay completely submerged for about six minutes, but they generally rise to breathe again after only two or three minutes. They can control the rate of their rising and sinking in the water by changing the volume of air in their lungs by movements of the diaphragm. Hippos can walk on the bottom of the river or lake at a rate of about 8 mi (13 km) per hour. On land, they can run at up to 20 mi (32 km) an hour.