The Gathering of Animals
The coming of the short rains in late November and December is a time when optimum grazing is available in the southern short grass plains. Thousands of White-bearded Wildebeest begin to gather and fatten up. Around February they have their calves more or less simultaneously over a period of three weeks. And with so many calves around, predators abound.
Wherever they are on their travels, the drawn-out chains and thundering cavalcaded of wildebeest and zebra are attended by a host of predators and scavengers who prey on the old, the lame and the lions. Lion, spotted hyena, cheetah, leopard, three species of jackal and six kinds of vultures are aided by less conspicuous micro-organisms which recycle nutrients through this incredibly productive ecosystem.
Which animal migrate?
Wildebeest from the bulk of the migration and often follow the zebra. They prefer new shoots and short grasses but they do also eat tall grasses, especially after the zebra and buffalo ‘trimmed’ them. Zebra form the second largest group of migratory animals and follow the same route as the wildebeest, often leading the way. They congregate on the plains during the rainy season and as soon as food becomes scarce, they break up and disperse in family units to minimize grazing pressure in the low production tall grass areas. Eland browse (eat leaves) and graze (eat grass) and are well adapted to almost any environment from lowlands to mountains. Although they do not follow the same migratory routes as wildebeest, they also alternate between the plains and woodland. Thomson’s gazelle has a much shorter route than wildebeest and are first to arrive on the plains and the last to leave. They leave only on short grasses, herbs and forbs. Grant’s Gazelle does not really need to migrate as they are not dependant on water but they do move to a limited extent, mainly locally. Their route is in some cases opposite to that of migratory species, spending the rainy season in open, patches within the woodland and the dry season on the plains.
How many animals migrate?
Marching in seemingly endless columns, cantering through the dust and cavorting in green pastures, the annual movement of wildebeest and other grazing ungulates across the serengeti ecosystem is perhaps the greatest spectacle in the animal kingdom. This dramatic event is triggered by the related factors of rainfall and grass growth, which have a largely predictable seasonal pattern. Over two million herbivores partake in this great migration, with some.
During good years the wildebeest population alone may reach up to 1.6m. During the peak of rainy the short grass plain supports up to 2 million animals. Census figured during the 1990s estimated the wildebeest population at 1.4- 4.6 million, zebras at +-151,000, Eland at +-12,000, Thomson’s gazelle at +-232,000 and Grant’s gazelle at +-31,000
How far do they travel?
They travel ±1000- 2,000km annually
How much do they eat?
They consume about 4 000 tons of grass every day, that is about 1, 46 million of tons of grass per annum. Much of the energy consumed is returned to the soil by means of their droppings.
Which route do they follow?
Early Wet Season (December- April)
During this time the animals are mainly on the Short Grass Plains (from the Gol Mountains in the south-east to Seronera into the north-east, including Lake Ndutu and Moru Kopjes). If there is a dry spell in between, they move west into the Masai Mara Game Reserve and to the Mbalageti Valley. At the first signs of rain, they move back onto the serengeti plains because of the instant availability of new growth.
Late Wet Season (April- June)
During this time there is a general movement to the northwest of the plains, to Maswa Game Reserve, Moru Kopjes and along the Simiyu, Mbalageti, Seronera and Nyabogati Rivers to the Western Corridor. They reach the western corridor in about the mid-June. Part of population moves directly north through Seronera and smaller section moves north on the eastern side of the Serengeti, through the Loliondo Area. From June to July one can see the death-defying crossing of the crocodiles-infested Grumeti River.
Early dry season (July- October)
This is when the bulk of the migratory herds find itself in the Western Corridor and to the area north-east outside the serengeti. A part from the herds reaches the Mara Rivers already in early August- another dangerous obstacle to cross. During September and October a large portion of the herds spills into the Masai Mara Game Reserve.
Late Dry to Early Wet (October- December)
In the late dry season the bulk of the migratory herd starts its trek south through the Lobo area and a long the eastern boundary. Some move straight through Seronera and some move through the Western Corridor. Most of the herd will have reach the Short Grass Plains by the middle of January where they have their young. They follow local rainstorm on the plains to benefit from newly sprout grasses. When the surface water dries up they repeat the cycle again, as they have done for over a million years.
At Tanzania’s Grumeti River
By June the herds reach the Western Corridor, crossing first the Seronera River, then the Grumeti rivers, where they congregate on the plains. The migrating herbivores dominate but there plenty of other animals like warthog, giraffe, impala and buffalo. The rut is in full swing among the wildebeest. The herds build up at the edge of the river where crocodiles lurk, waiting to pounce on any unsuspecting animals.
Mara River in Kenya
The final obstacle is the Mara River at the border with Kenya. Where the crossings are deep and the current swift, the weak and feeble will be swept away into the jaws of the waiting crocodiles. By September through to November, the Maasai Mara plains in Kenya are filled with these large herds and their accompanying predators.
The Journey Back
Then when the rains clouds gather in the south, the herds begin their long track back to their breeding grounds in the serengeti plains. It is a tough journey of 600km and every year an estimated 250,000 wildebeest don’t make it.
The main course- quality or quantity?
Although the wildebeest and zebra eat a variety of grasses, they have different preferences. Wildebeest, like cows are ruminants and prefer higher quality grasses. Zebra like horses, can graze on lower quality grasses but need greater quantities. They can consume twice as much grass as a wildebeest in the same amount of time!
Why do wildebeest migrate?

The 600km trek of 1.3 million wildebeest is the largest remaining mammal migration on earth. The timing of the migration coincides with greening of the nutritious grasses on the short grass plains during the wet season. These areas are also safe because predators cab seen easily which makes it ideal for calving. However, the plains dry quickly and wildebeest are forced to move in search of greener pastures in the western corridor. The northern extension of the ecosystem has the highest rainfall but the grasses are least nutritious. This is the dry season retreat for the wildebeest at least until the south becomes green again. The result is a clockwise movement from the south, west, north and back to the south.

Generally, there is a geographical gradient present, with waters having higher salinity and higher alkalinity in the south. The waters are generally fresh and salt-free in the north and western corridor and can even be slightly acidic.  A salinity of 17 is equivalent to mixing one part of seawater and one part of fresh water, this water is undrinkable. By comparison, rice farming needs water of salinity less than 2. At the end of wet season, most of the surface waters in southern plains have salinity much higher than that, most of the waters are generally undrinkable at that time. This is also the time when animal migration of wildebeest and zebras begins.
This finding suggests that, the poor water quality, especially the excessive salinity at the end of wet season, may be what initiates the annual migration of ungulates away from southern plains at that time of the year. (An ongoing research led by Mr. Emmanuel Gereta, the SENAPA Ecologist, and Mr. Eric Wonlansky, Australia.  The program focuses on understanding the links between water and wildlife)