The Great Migration: Science of the World's Most Spectacular Wildlife Event

Every year, approximately 1.5 million wildebeest (Connochaetes taurinus), 200,000 zebras, and 500,000 gazelles undertake a continuous circuit of the Serengeti-Masai Mara ecosystem in Tanzania and Kenya — the largest overland animal migration on Earth. The migration is not a simple seasonal movement between fixed locations: it is a dynamic, continuously adjusted tracking of rainfall and grass growth across a landscape of more than 30,000 square kilometres. The wildebeest follow the rains — and the fresh, nutritious grass that the rains produce — in a circuit that has been refined by millions of years of natural selection and is now acutely sensitive to the rainfall variability that is increasing under climate change.

Following the Rains

The wildebeest migration is driven by spatial and temporal variation in rainfall and grass growth across the Serengeti-Mara ecosystem. The southern Serengeti receives rain during the long and short rains of November-May, producing the short, nutritious grass that wildebeest prefer for calving — which occurs synchronously in February, with approximately 500,000 calves born within a 3-week window. As the southern plains dry out in May-June, the herds move northwest and north to the taller, wetter grass of the northern Serengeti and Masai Mara, where they spend the dry season before the rains return them south.

Predator Dynamics

The wildebeest migration sustains one of the world's most diverse large predator communities. Lions, cheetahs, leopards, hyenas, wild dogs, and crocodiles all exploit different phases of the migration in different ways. The synchronised calving season is a feast for predators — but the sheer numbers of wildebeest mean that the predator population is quickly satiated and individual calves' survival probability increases rapidly as the calving peak passes. The Mara River crossings — where wildebeest must cross water inhabited by enormous Nile crocodiles and then climb steep banks under pressure from lions — are among the most dramatic predator-prey interactions in the natural world.

Population Dynamics of the Serengeti Wildebeest

The Serengeti wildebeest population — currently approximately 1.5 million individuals — is one of the best-studied large mammal populations in the world and provides a compelling demonstration of bottom-up (food-driven) population regulation. The population nearly quadrupled between the 1960s and 1977, when it stabilised at approximately 1.3-1.5 million — not through top-down predator regulation (predators kill a relatively constant proportion of the population regardless of its size) but through food limitation: as the population grew, the quantity of dry-season forage available per individual declined until mortality from starvation balanced reproduction. The key insight, confirmed by long-term monitoring, was that the wildebeest population was regulated by the abundance of dry-season grass in the Serengeti woodlands — and that the dramatic population growth in the 1960s-70s was triggered by the control of rinderpest (a viral disease that had been killing large numbers of wildebeest annually since its introduction in 1890) through veterinary vaccination campaigns for domestic cattle.

Threats to the Migration — Fences and Infrastructure

The Serengeti-Mara wildebeest migration — which has existed in essentially its current form for at least 10,000 years — faces unprecedented threats from the expansion of human settlement, agriculture, and infrastructure in and around the ecosystem. The most acute threat has been the proposed Serengeti highway — a paved road that would bisect the northern Serengeti, severing the migration corridor between Tanzania and Kenya. Research modelling the impact of a paved road with vehicle traffic found that road mortality alone could reduce wildebeest numbers by 35% within 50 years, with cascading effects through the entire ecosystem. Following intense international scientific opposition, the Tanzanian government redirected the road to the south of the ecosystem, though a gravel road through the park boundary remains in use and continues to pose collision risks during migration.

The "fencing trap" — the practice of erecting veterinary or agricultural fences that inadvertently sever wildlife migration routes — has devastated savanna wildlife populations across eastern and southern Africa. In Botswana, veterinary cordon fences erected to prevent foot-and-mouth disease transmission from wildlife to cattle have blocked the migrations of wildebeest and hartebeest, leading to mass mortality events when drought concentrates animals against fence lines. An estimated 50,000-70,000 wildebeest died in a single drought event in the 1980s when the Kuke veterinary fence prevented the animals from reaching water. In the Karamoja ecosystem of Uganda, fencing of agricultural land for cultivation has progressively constricted the movement of elephant and buffalo herds, increasing human-wildlife conflict as animals confined to shrinking habitat range increasingly damage crops. These case studies illustrate that migratory ecosystems require landscape-scale planning that transcends the boundaries of individual protected areas and national jurisdictions.