Watch a hippo on land or in the water, and you’ll soon discover that this roly-poly mammal is one of the animal world’s great characters. It is the most rotund land mammal and spends its daytime hours in lakes, pools, mudholes, or in the slow-moving waters of rivers. The name hippopotamus comes from a Greek word meaning “water or river horse.” There are two species of the hippopotamus: the river, or common, hippo, and the much smaller pygmy hippo. Hippos have unique skin that needs to be kept wet for a good part of the day. They don’t have true sweat glands; instead, hippos secrete a thick, red substance from their pores known as "blood sweat,” which creates a layer on the hippo's skin to protect it from sunburn and infection.