During the day they inhabit rivers and lakes. At night, they graze on short grasslands.
The hippopotamus typically sleeps during the day and maintains activity at night. It is not, however, strictly nocturnal. Hippos may cover up to 33 km of water each night in search of food. They eat mainly the grasses along the shores of the rivers they inhabit, but they have been seen grazing up to 3.2 km from the shoreline.
Hippos are extremely graceful in the water, despite their clumsy appearance on land. Their specific gravity allows them to sink to the bottom of rivers and literally walk or run along the bottom.
Hippos may occur singly or in groups of up to 30 animals. The central core of social groups appears to be females with their dependent offspring. Adult males vie for control of these herds. Aggression between males is intense. The hippos use their long canine teeth as weapons, and death often results from fighting between males. Most adult male hides are covered with scars from injuries incurred during such fights. Losing males are often relegated to a solitary existence.
Hippos feed on grass, but have very occasionally been seen eating small animals or scavenging. They eat about 40kg of grass a night, which is only 1-1.5 per cent of their body weight - their lifestyle is energy-efficient.
A single young is born either on land or in shallow water. In water, the mother helps the newborn to the surface, later teaching it to swim. Newly born hippos are relatively small, weighing from 55 to 120 pounds, and are protected by their mothers, not only from crocodiles and lions but from male hippos that, oddly enough, do not bother them on land but attack them in water.
Young hippos can only stay under water for about half a minute, but adults can stay submerged up to six minutes. Young hippos can suckle under water by taking a deep breath, closing their nostrils and ears and wrapping their tongue tightly around the teat to suck. This procedure must be instinctive, because newborns suckle the same way on land. A young hippo begins to eat grass at 3 weeks, but its mother continues to suckle it for about a year. Newborns often climb on their mothers' backs to rest.
Compared to other animals, hippos are not very susceptible to disease, so in suitable habitats, their numbers can increase quickly. Their chief predators are people, who may hunt hippos for their meat, hides and ivory teeth.