Why do wildebeests spend their whole lives migrating in a circle?

In 2007, part of Kenya's Mara River became a mass grave for a thundering herd of wildebeest. Around 10,000 of the animals, which are also called gnu, died while attempting to cross at a bend in the river. When the first in the group reached the water, the current was too swift to ford, and hundreds behind followed in suit, not realizing the danger. That single event wiped out an estimated 1 percent of the wildebeest population in Africa [source: National Geographic News]. Although the number of casualties in that incident was particularly high, drowning at river crossings is one of the more dismal common occurrences of wildebeest life in the Serengeti.

That herd of wildebeest was traveling along its annual migration path. Like the monarch butterfly's epic 3,000-mile (4,828-kilometer) trip to Mexico every year, the wildebeest's migration is one of the most fascinating and large-scale in the world. At least one million wildebeest complete a semi-circular path from the Serengeti in Tanzania to grazing areas to the north in Kenya. The precise route and starting time change every year, but the wildebeests cross around 1,000 miles (1,609 kilometers) of land, with hundreds of thousands of zebra and Thomson's gazelles alongside them.

Wildebeests look like a cross between a moose and a bull. Growing up to 4.5 feet (1.37 meters) tall, the animals have skinny legs, a lean body and a large head with two curved horns. At first glance, you'd probably guess that they're part of the bovine family, but wildebeests are actually a relative of the antelope. Their common name comes from the phrase "wild beast" in the Afrikaans language. Despite their bulky bodies, wildebeests don't rank at the top of the food chain in their plains and savanna habitats in Africa.

Grass-grazing wildebeests -- and their calves in particular -- are targets of carnivorous cheetahs, lions and hyenas. In the first months of the year in the Serengeti when hundreds of thousands of female wildebeests give birth, the predators may pick off the newborns. That mass birthing period marks the beginning of the seemingly everlasting wildebeest migration.

Annual Wildebeest Migration

The great wildebeest migration from the Serengeti plain in Tanzania to the northern Masa Mara region in Kenya is a treacherous trip. An estimated 3 percent of the herd can't survive the clockwise circuit, whether due to predation, exhaustion or natural elements. The wildebeest move at an average pace of 6 miles (10 kilometers) per day, continuously searching for fresh land to graze on [source: Wolanksi et al]. Grass is what drives the wildebeest to migrate along the same general route their entire lives; the herds chow down on up to 4,000 tons of it daily [source: Williams].

At the beginning of the year, the wildebeest congregate en masse below the Ngorongoro Crater on the Serengeti. The rainy season from November to December nourishes the land, and acres of short grasses sprout up. Shorter grasses, which contain higher levels of protein, sodium, calcium and phosphorus, are healthier for the wildebeest. Since female wildebeest give birth at the beginning of the year, they especially benefit from the nutrient-rich food [source: Sinclair and Arcese]. The grasses usually last during a brief dry spell from January to March, and then the rains pick up in the spring. During this time, the wildebeest follow the rains and the flourishing grass southwest.

But when the dry season sets in starting in June, the grasses on the eastern Serengeti die. When that happens, it's time for the wildebeest to head north. Taller grasses in the northwestern portion of the Serengeti don't share the same nutritional quality as those in the southeast, but they're better than no grass at all. From June to October, the herds of wildebeest trek north into Kenya, curving back down south when the rainy season approaches again in November. Once they return to the southeast, the plain is lush with the desirable short grasses, and the cycle begins again.

One million wildebeest in the Serengeti plain migrate in an enormous circle each year.
Winfried Wisniewski/Getty Images

Wildebeest are anatomically well-suited for the migratory life. For one thing, wildebeests have special flaps in their nostrils to block dust kicked up by the herds [source: Estes]. If they feel threatened by a predator along the way, wildebeests are nimble on their feet, able to gallop in short bursts as fast as 40 miles per hour. Their high-set shoulders and lower hindquarters don't make them particularly attractive, but that build allows them to run for extended periods of time at a steady pace [source: Estes]. Even their calves are specially equipped for survival: The young can learn to walk and run on their spindly legs in as little as four minutes after birth [source: Williams].

In recent years, however, the numbers of wildebeest in Africa have declined. The population in the Serengeti has remained the most stable, but the expansion of farmland threatens their livelihood. Human encroachment in wildebeest migration grounds led to the extermination of a herd in Botswana in 1983. The government there erected a fence that inadvertently blocked the herd's escape from a dry, grassless area, killing 65,000 head of wildebeest [source: Flores].

Fortunately, much of the land in the Serengeti is federally protected from poaching and settlement. Otherwise, the African wildebeest's circle of life might be stamped out.

Why do wildebeests spend their whole lives migrating in a circle?

Annual Wildebeest Migration

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