The Great Migration is the largest movement of terrestrial mammals on Earth, occurring across the plains of East Africa. This movement involves millions of animals traversing the Serengeti-Mara ecosystem, which spans northern Tanzania’s Serengeti National Park and Kenya’s Maasai Mara National Reserve. Over two million animals, primarily wildebeest, embark on this journey in response to environmental conditions. The scale of the herds is vast; columns of animals, often numbering up to 1,000 individuals per square kilometer, have been observed from space. The entire circuit maintains the ecological balance of this globally significant savanna landscape.
The Primary Driving Force
The Great Migration occurs due to the search for water and high-quality grazing. The animals are not driven by a calendar but by instinctual tracking of localized rainfall patterns. This movement responds directly to the depletion of nutrient-rich grasses in one area and the sprouting of fresh forage in another.
The plains’ ecology is defined by two main rainy seasons: the short rains (November to December) and the long rains (March to May). These seasonal downpours determine where nutritious grasses, especially those growing on the volcanic ash soils of the southern Serengeti, will flourish. When the short-grass plains dry up, the herds move on, following the “grass clock” that dictates their direction. Wildebeest instinctively move toward distant thunderstorms and the scent of rain, signaling the location of the next grazing ground.
The Annual Circuit and Timing
The migration is a continuous, clockwise circuit that flows through the Serengeti-Mara ecosystem. The cycle begins in the southern plains of the Serengeti, near the Ngorongoro Conservation Area, where the short grasses are packed with minerals. From December through March, the herds congregate here for the calving season, taking advantage of the highly nutritious forage to support their young.
As the southern plains become parched after the long rains, the herds begin their northward trek, starting around April or May. This movement guides them into the Western Corridor of the Serengeti, where they encounter the Grumeti River, typically around June. The journey continues north, and from July to October, the herds are found in the northern Serengeti and Kenya’s Maasai Mara. During this northern phase, the animals must navigate the Mara River, which is the most widely recognized feature of the circuit. The herds cross the river multiple times searching for the best grass before the short rains prompt the final leg of the journey south in late October or November. The return to the southern plains in December readies the animals to begin the cycle anew.
Key Species and Behavioral Imperatives
The migration primarily involves the blue wildebeest, but they are joined by hundreds of thousands of zebras and Thomson’s gazelles. The wildebeest, numbering over 1.5 million, form the backbone of the movement, feeding on the short, nutrient-rich grasses. Zebras, with a population of around 200,000, have a digestive system that allows them to consume the coarser, upper layers of grass stems.
This difference in grazing preference means the two species do not compete directly for the same food, allowing them to travel together efficiently. The large numbers of wildebeest also offer a “safety in numbers” advantage, particularly during the calving period. Approximately 500,000 calves are born between January and March, often in a concentrated three-week window, which temporarily overwhelms local predators. Finding high-quality sustenance for these newborns is a non-negotiable factor that keeps the herds in the southern Serengeti at the beginning of the year.
Ecological Significance
This movement of ungulates shapes the Serengeti-Mara ecosystem. The volume of animals plays a crucial role in nutrient cycling by depositing dung, which fertilizes the plains. Their constant grazing and trampling prevent the grasslands from becoming overgrown, maintaining the open savanna landscape and preventing woodland encroachment.
The migration also supports the region’s high density of predators, including lions, cheetahs, and crocodiles. These carnivores and scavengers, such as hyenas and vultures, depend on the seasonal abundance of prey, structuring the entire food chain around the herds’ movements. The migration is a self-sustaining phenomenon, ensuring the health and productivity of the ecosystem by cycling nutrients, dispersing seeds, and regulating predator-prey dynamics.