Main subject of Serengeti is the mass migration of wildebeest , which in search of nutritious grass hundreds of kilometers annually by the savannah draw. At the end of the rainy season, they break into a huge herd of over a million animals on the north. The wildebeest and other animals are joining the migration, as zebras and gazelles , continuously exposed to hazards, particularly by predators such as lions and cheetahs . A highlight of the great migration, the crossing of the Mara is: There, the wildebeest have to overcome not only meter high cliffs, they are also crocodiles delivered. With the onset of the rainy season, however, leave the wildebeest north again what had long been a mystery. The reason for this is that the grass in the north a phosphorus deficiency , and said thus wildebeest forces to retreat to the south.