Every year, across the grasslands that straddle the border of Tanzania and Kenya, close to two million large animals walk in a great circle that never ends. Wildebeest in their hundreds of thousands, mixed with zebra and gazelle, tramp a clockwise loop through the Serengeti-Mara ecosystem, following the rains and the grass, pursued by lions, hyenas and cheetahs on land and ambushed by crocodiles at the rivers. It is the largest overland migration of mammals on the planet, and it is not a journey with a destination. It is a loop, endlessly repeated, driven by rain and chemistry.
The wheel of grass and rain
The engine of the Great Migration is water, and specifically where the rain falls and makes fresh grass grow. The wildebeest are grazers with an appetite for short, nutritious new growth, and they move to wherever the recent rains have greened the plains. Broadly, the herds spend the early part of the year on the short-grass plains of the southern Serengeti, where the calving season sees a staggering pulse of births — several hundred thousand wildebeest calves dropped within a few weeks. This synchronised birthing is itself a survival strategy: by producing so many young at once, the herd swamps the predators, which can only eat so many, so the majority of calves survive the dangerous first days.
A newborn wildebeest can stand and run within minutes of birth, a necessity on a plain patrolled by lions and hyenas. As the dry season advances and the southern grass withers, the herds move north and west, eventually towards the greener Masai Mara in Kenya, then circle back as the rains return. The whole cycle turns once a year, a slow wheel of nearly two million animals grinding around the same vast landscape their ancestors have crossed for millennia.
There is a subtle chemistry beneath it. Some of the southern plains are enriched by mineral-rich volcanic soils, producing grass unusually high in the calcium and other nutrients that pregnant and nursing wildebeest need — one reason the herds return there to calve rather than simply staying wherever the grass is tallest. The migration is not just following rain; it is following nutrition.
The river of teeth
The most dramatic and most filmed episodes are the river crossings. At certain points the migrating herds must cross rivers, above all the Mara, and the banks become scenes of chaos and death. Wildebeest mass in their thousands at the water's edge, hesitating, until some trigger sends them plunging in — and then hundreds pour down the bank in a stampede, swimming across as the current sweeps some away and the crocodiles move in. Nile crocodiles, huge and ancient, wait in these rivers all year, timing their feeding to the crossings, seizing the swimming wildebeest in the water. Many animals drown or are trampled in the crush regardless of the crocodiles. The crossings are a bottleneck of terror, repeated at several rivers, and they furnish the wildlife documentaries with some of their most harrowing footage.
Yet the losses, dreadful as they look, are a small fraction of the whole, and they feed an entire economy of life. The drowned wildebeest nourish the crocodiles, and their carcasses, washing downstream and decomposing, deliver a huge annual pulse of nutrients into the river — feeding fish, birds and the whole aquatic web. Death at the crossing is, in the grim accounting of the ecosystem, another kind of harvest.
The Great Migration is really a portrait of interdependence at continental scale. The grass depends on the rain; the wildebeest depend on the grass; the predators depend on the wildebeest; the rivers are fertilised by the drowned; and the herds, by cropping and manuring the plains as they pass, keep the grassland itself healthy and productive. Remove any piece and the wheel falters. It is easy, watching a crossing, to see only the horror of the crocodile and the panicking herd. Step back, and you see something grander and older: an unbroken circle of nearly two million animals turning with the seasons, following the rain around a landscape that has shaped them, and been shaped by them, for longer than there have been people to watch. The wildebeest is nobody's idea of a noble beast — ungainly, bearded, perpetually anxious — yet on it the whole vast machinery of the Serengeti quietly turns.
Quiz nuggets
- The Great Migration involves close to two million animals — mainly wildebeest, with zebra and gazelle — circling the Serengeti-Mara ecosystem of Tanzania and Kenya.
- It is the largest overland mammal migration on Earth and follows the seasonal rains and fresh grass in a yearly loop.
- The southern Serengeti calving season produces several hundred thousand wildebeest calves within a few weeks — a predator-swamping strategy.
- The most dangerous stage is the river crossings, especially the Mara River, where Nile crocodiles ambush the herds.
- Mineral-rich volcanic soils give the southern plains nutritious grass, drawing the herds there to calve.