Hippos have a loose, relaxed social structure. During the day, they rest in large pods made up of both sexes of various ages. Group size depends on the desirability of the habitat. Hippos prefer water that is slow-moving and deep enough to cover their bodies—5- to 6-feet (1.5- to 2.0-m) deep—along sandy, open shorelines. Preferable habitat can host gatherings of more than 200 hippos. Solitary hippos tend to be females getting ready to give birth or elderly males. Otherwise, hippos gather either in nursery schools—females and their calves—or bachelor groups. These rules are loose, though, and members of any sex or age may turn up in a group. These groups break up for evening grazing. Some adult bulls defend territories where they are the most dominant member. Dominance is loosely based on size. Bulls mark and defend their territories with urine and feces, which they fling impressive distances by waggling their tails. When a male enters another male’s territory, he becomes submissive to that male unless he intends to challenge him. When males fight for a territory, they rake their teeth along each other’s flanks. Despite their thick skin, these fights can lead to serious injury or even death.
Hippos use sound and body language to communicate. They honk—both on land and in the water. These honking calls can be nearly 115 decibels (the volume of loud thunder) and can be heard a mile (1.6 km) away. Hippos also make various noises underwater—grunts, squeaks, croaks, and whines—though the meaning of all these sounds is not understood. The most famous method of hippo communication is opening their mouths in impressively wide “yawns.” One possible explanation for this behavior is that it signals excitement.
Despite their aquatic tendencies, hippos cannot actually swim or float. Instead, they walk along the bottom of rivers and watering holes. When completely submerged, they push off from the bottom to get back to the surface. Hippos spend most of the day in and out of the water. When they get cold, they move to the banks and bask in the sun; when the sun gets too hot, they return to the water to cool down and get out of the sun. Most of the night, they graze on dry land. They tend to return to the same resting places every day. In swampy areas, the channels and trails hippos make may alter the flow of water. When moving at full tilt, hippos can reach speeds of almost 20 miles an hour (30 kph).
Hippos live all across Africa—many in protected areas—but their populations are decreasing due to drought, poaching, habitat loss, habitat fragmentation, and the damming of rivers. They are poached not only for their meat but also for their teeth. Called “ivory,” they allegedly are softer and easier to carve than elephant tusks. Hippos are also vulnerable to rinderpest, a communicable cattle disease, as well as anthrax, which is capable of killing hundreds or even thousands of hippos within an infected area. The International Union for Conservation of Nature (IUCN) lists them as a vulnerable species.