Africa

Down by the river

Eric-Isselee(1)

Each year more than one million wildebeest cross the rivers to graze in Kenya’s grasslands. But not everyone completes the journey.   

They call it the Great Migration, but it is in fact nothing of the sort. Throughout the year, nearly 1.5 million wildebeest and zebras travel in a continuous cycle that stretches across the vast plains of Tanzania and southern Kenya. The climax of the journey is some of the river crossings, where hordes of wildebeest sprint across murky waters infested with large crocodiles. But before even reaching that stage, they have a long way to go.

The setting of the wildebeest’s journey is loosely defined as the ‘Serengeti ecosystem’: an area of 40,000 square kilometres (15,000 square miles) comprising of the Serengeti National Park, the Ngorongoro Conservation Area, and the Maswa Game Reserve, all located in Tanzania, as well as the Masai Mara National Reserve in Kenya. Throughout the year, the wildebeest – large muscular antelopes of up to 200kg – travel from place to place, their patterns determined by the availability of water and grass. They are joined by zebras and gazelles.

In the beginning of the year, around February, they graze in the south-eastern part of the ecosystem plain. Already here, predators are watching them closely, particularly lions, leopards and hyenas. In this period, the wildebeest give birth to around 500,000 calves. They do so within a couple of weeks, to reduce the period in which predators can chase the calves. Those born too late grow up in isolation, and are less likely to survive.

Vorobyev-Dmitry

When the rain stops, usually in May, they move to the nearest source of water. This is found in the western woodlands, near Lake Victoria. Behind them, the southern grasslands turn into a dry desert-like area, in which no wildebeest could survive. “The exodus can often be quite rapid, often within a week,” says Anthony Sinclair, professor at the University of British Columbia and author of Serengeti Story: Life and Science in the World’s Greatest Wildlife Region.They head into the woodlands to the rivers, so that they can get water, because they can’t go without water for more than a couple of days.”

As the months go by, the rain moves towards the north. The wildebeest, of course, move with it. This takes them into southern Kenya and the Masai Mara National Reserve – the wettest part of the ecosystem in the dry season. However, between Tanzania and Kenya, a series of rivers must be crossed. And as it happens, some of these, particularly the Grumeti River, are not very safe. “They are not very big rivers; perhaps only 10 metres across,” says Sinclair. “But they have lots of large crocodiles in them.”

Hedrus

The wildebeest are clearly vulnerable to the crocodiles. But they approach the crossings armed with a strategy. “The whole behaviour of wildebeest is to live in large groups,” Sinclair explains. “This is very simple: it is safety in numbers. If you are the only animal there, your chances of getting caught are very high. If you are one of 10,000, your chances of getting caught are very low. So it’s a matter of probabilities.” This tactic is used not only across rivers, but wherever the wildebeest travel, such as the vast plains, where hyenas and wild dogs circle them like vultures.

In fact, crossing the rivers is not the most dangerous aspect of the journey. Rather, it is when wildebeest wander down by the river to drink. “They have to stand at the edge of the water, spending much more time, and are therefore more likely to get caught,” says Sinclair. “And secondly, only a few can drink at a time, because there isn’t enough room for thousands at a time. This, of course, increases the probability of getting caught.“

And so not everyone manages to reach the sweet grasslands of southern Kenya. Each year, around 250,000 wildebeest are estimated to get killed on the journey. Those who do survive stay in Kenya until the end of the year, when they make their way back to Tanzania. As another year passes, they give birth to calves once again. And so it continues.

Photos: Eric Isselee, Hedrus, Vorobyev Dmitry [all via Shutterstock.com].

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