Hippos have a flexible social system defined by hierarchy and by food and water conditions.
Usually, they are found in mixed groups of about 15 individuals held by a territorial bull, but in periods of drought large numbers are forced to congregate near limited pools of water.
This overcrowding disrupts the hierarchical system, resulting in even higher levels of aggression, with the oldest and strongest males most dominant. Old scars and fresh, deep wounds are signs of daily fights that are accompanied by many vocalizations.
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 underwater for about half a minute, but adults can stay submerged for up to six minutes. Young hippos can suckle underwater 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, cause 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.
