Migrating is not a choice but a necessity for the great migrating herds of the Serengeti region. Once the water dries from the depressions and the grass withers on the plains they must move on or starve. However, the benefits gained by migrating contribute greatly to their vast numbers. By migrating they can exploit the nutrient rich plains which only provide grazing during the wet season. They also avoid predation pressure part of the year.
Many factors contribute to the wildebeest's preference for the eastern plains of the Serengeti during the wet season. They avoid the wet, muddy ground and the parasites and flies which infest the woodlands during the wet season. Wildebeest tend to avoid thick vegetation because it limits their visibility and provides cover for predators. By migrating to the open plains, they give birth where the visibility is the greatest. The grasses of the plains also provide the highest protein and calcium content early in the wet season when wildebeest give birth.
In the absence of disease, research indicates the hiatus from predators maybe the most important contributing factor to the abundance of the migratory [G 45 / ungulates]. The wildebeest's primary predators, the [P 006 / lion] and hyena, are territorial and are limited by their young which are not able to cover long distances.
Most lion prides live in wooded areas where there are permanent sources of food and water. With the exception of nomadic lions which are non-breeders, prides are restricted to raising their young and hunting to their territories.
Most hyena dens are located on the plains. When the wildebeest leave the plains, [I 006 / hyenas] commute to the herds from their dens. However, they are limited as to how far they go and still provide for their young. When the wildebeest move into the Northern Extension and Masai Mara Game Reserve later in the dry season, few hyenas are able to follow.
