At the banks of the Mara River in Tanzania's northern Serengeti, a river crossing of wildebeest and zebras raises the dust of the shore line. Often hours pass until the growing density of animals at the river bank results in the initiation of the crossing by a few leading wildebeest. Until then the heard often wanders up and down the shore line, waiting and watching their conspecifics. Every year, hundreds of thousands of animals have to overcome this obstacle while looking for greener feeding grounds. This yearly cycle in Tanzania and Kenya is known as the Serengeti's Great Migration.