The northern section of Senegal is part of the Sahel, a transition zone between the Sahara on the north and the wetter regions to the south. Vegetation here consists largely of savanna grass with scattered clumps of trees and spiny shrubs. Farther south, in the region of the Gambia River, trees become more common. In the extreme south are mangrove swamps and dense forests of oil palm, mahogany, teak, and bamboo. Wildlife is various, but larger mammals, such as elephants, lions, cheetahs, and antelopes, are largely confined to the less populated eastern half of the nation. Hippopotamuses and crocodiles are found in the rivers. Among Senegal’s numerous varieties of snakes are the cobra and boa constrictor.