Hippopotamus is a species of a large, most often herbivorous mammal from the family of Hippopotamidae. Currently there are two species of hippopotamuses: so called Nile hippopotamus, and much smaller pygmy hippopotamus. This animal inhabits mostly African rivers and lakes. During the day hippopotamuses stay in water, whereas at dusk and at night they become active. Then they feed on grass and water plants. Adult hippopotamuses are very aggressive towards crocodiles that inhabit the same areas. They are particularly aggressive when the young calves are nearby. Hippopotamuses are also known to attack people. Contrary to common beliefs hippopotamuses are often considered as the most dangerous animals of Africa.