The hippopotamus is semi-aquatic. This means that even though it usually lives on the land, it spends a great amount of time in rivers and lakes where males lead groups of 5 to 30 females and young. In the daytime, they keep cool by staying in the water or mud. They give birth to baby hippos in the water, too. At dusk, they come out to graze on grass. Hippopotamuses rest together in the water, but they like to graze by themselves.