“Steady on, boys,” Bradley says as a scuffle breaks out in front of our moving truck. We stop and he decodes the quadruped drama: “The one on the left is the harem stallion. He’s shepherding a young female. Maybe she’s just come on heat and he’s aggressively protecting her from other stallions.” While the 50 or so zebras in front of us appear associated, Bradley explains that the only lasting social unit is the harem, made up of a lone stallion, one to six mares and their offspring. These small, tightknit families come together by the thousands for the seasonal pilgrimages in search of grass and water.
Like a human fingerprint, a zebra’s stripe pattern is unique. There are many theories about why the stripes evolved. The dizzying lines might distort a zebra’s outline, for instance, or make the animal look bigger, confusing predators. Take away their patterns, and the zebras before me look like small horses. Their gait, mannerisms and portly shape match those of their domesticated cousins.
Nomadic and gregarious, plains zebras are not at all territorial. But stallions do fight to protect mares in their harems or abduct mares in heat. (Bradley tracks mares rather than stallions because the females are less likely to fight with each other and damage the collars.) The ties that bind a stallion and his harem are profound. Bradley once noticed a lone stallion standing for hours in the riverbed, not eating. When Bradley approached, he saw that the stallion was standing vigil over a dead mare.
The young zoologist has witnessed this single-minded devotion when he’s darted mares to collar them. “Once the tranquilizers start to take effect, some stallions bite on the females’ necks to try to keep them upright and moving,” he says. “While we’re busy with the female, the stallion moves through the herd, constantly calling, looking for his missing mare. When she wakes up and calls, the stallion heads directly to her.” Mares, too, are loyal, often remaining with a single harem for life, a period that can span 16 years.
It’s midday, the temperature is 99 degrees and Bradley still has nine mares to locate. The GPS devices on the animals’ collars have an annoying habit of failing, forcing Bradley to rely on radio signals—and instinct—to find them. He then records their position, behavior and grazing preferences.
We pass the occasional oryx antelope and ostrich pair, and every few miles a korhaan, a rooster-size bird, tumbles from the sky in a courtship display. Bradley spends an increasing amount of time on the vehicle’s roof, using the slightest rise in elevation to pick up a signal. “Come on, zebras,” he sighs. “Where are you, my girls?” We drive some more. “They’re keeping themselves hidden,” he says.
We come to an area littered with dried zebra dung and scarred by deep game trails. The grass is brittle, stubby, overgrazed. “This is where the zebras grazed in the dry season,” says Bradley, fiddling with his GPS. “Let’s see...we’re 17 miles from the Boteti River as the crow flies.” I let the information sink in—these zebras undertook 34-mile round trips every two to four days to get from water to food, to water again, on an endless journey between thirst and hunger. Bradley has calculated that the zebras travel more than 2,300 miles a year.
By tracking the zebras’ movements, Brooks and Bradley have discovered that zebras are more resilient than previously thought. Some books claim that zebras drink daily and seldom stray more than seven miles from water. Yet the Makgadikgadi researchers recorded them trekking in dry months more than 22 miles to preferred grazing lands. During such trips, the animals go without water for up to seven days. At first, the researchers believed they were forced to travel so far in part because of grazing competition from cattle. But with cattle fenced out, the zebras continue to traipse record distances. “What drives them?” Bradley wonders aloud. “I’ve seen them walk past what looks like perfectly good grass to come out here.”
The Boteti River forms a natural boundary between the Makgadikgadi Pans National Park and nearby cattle-ranching villages, and provides a crucial lifeline during the dry season, when summer rains cease and grasslands wither, and zebras, impala antelope, wildebeest and other animals seek refuge and water along the riverbanks.
But in 1989, after years of drought, the Boteti dried up, evaporating into a necklace of small stagnant pools. Herds of cattle regularly trespassed miles into the park, overwhelming the tiny water holes, trampling and overgrazing the dusty surrounds. Crowded out from water and pressured to walk long distances in search of grazing, countless zebras perished.