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.