Two hippo species are found in Africa. The large hippo, found in East Africa, occurs south of the Sahara. This social, group-living mammal is so numerous in some areas that “cropping” schemes are used to control populations that have become larger than the habitat can sustain. The other, much smaller (440 to 605 pounds) species of hippo is the pygmy hippopotamus (Choeropsis liberiensis). Limited to very restricted ranges in West Africa, it is a shy, solitary forest dweller, and now rare.
The large hippo is an aggressive animal; old scars and fresh, deep wounds are signs of daily fights that are accompanied by much bellowing, neighing and snorting Hippos have developed some ritualized postures the huge open-mouthed “yawn” that reveals formidable teeth is one of the most aggressive. With the long, razor-sharp incisors and tusklike canines, the hippo is well-armed and dangerous.
Hippos move easily in water, either swimming by kicking their hind legs or walking on the bottom. They are well-adapted to their aquatic life, with small ears, eyes and nostrils set at the top of the head. These senses are so keen that even submerged in water, the hippo is alert to its surroundings. By closing its ears and nostrils, the adult can stay under water for as long as six minutes.
Hippos have a flexible socials system defined by hierarchy and by feed and water conditions. Usually they are found in mixed groups of about 15 individuals, but in periods of drought large number are forced to congregate near limited pools of water. This overcrowding his overcrowding disrupts the hierarchical system, resulting in even higher levels of aggression, with the oldest and strongest males most dominant. Hippos are unpredictable. If they are encountered away from the safety of water, anything that gets between them and their refuge may be bitten or trampled.
Amazingly agile for their bulk, hippos are good climbers and often traverse rather steep banks each night to graze on grass. They exit and enter the water at the same spots and graze for four to five hours each night in loop patterns, covering one or two miles, with extended forays up to five miles. Their modest appetites are due to their sedentary life, which does not require high outputs of energy.
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.