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HippopotamusThis gallery is part of a special National Geographic initiative on global freshwater resources. A hippo peers from a plant-covered pool in Kenya’s Masai Mara Game Reserve. These massive mammals keep cool by submerging their massive bodies in African ponds, rivers, and lakes for up to 16 hours a day. Though they can hold their breath for perhaps half an hour if necessary, hippos typically leave the tops of their heads above the surface. At night hippos leave the water and roam overland to graze. If caught on land too long during the heat of the day the animals can dehydrate quickly. (See more photos of aquatic species.)

Photograph by Donna Eaton