How zebras find their way across the Kalahari,
‘Without trails, we would be lost,’ writes British Columbia-based journalist Robert Moor. Five muddy months spent hiking the Appalachian Trail, from Georgia to Maine, led him to contemplate the ‘deeper meaning’ of the paths that criss-cross our planet and
A sugary rain began to sift down as [biologist Nidhi] Dharithreesan and I watched the animals enact the mundane chores of being alive. An addax delicately scratched an itch on his back with one long curved horn. A baby antelope wobbled beside its mother, who bent down, stuck her nose up under the calf’s hind legs, and licked its rear. The calf looked over at us, blissfully unabashed.
Staring at these herds of striped and spotted ungulates, I could almost be fooled into believing that we were on a safari in some far-off veldt, if it weren’t for certain discordant details: the high steel fences, the cartoonish faux-wood sign reading AFRIKKA, and, most jarring of all, in the distance, the swooping steel scribbles of roller coasters. In fact, we were in an enormous outdoor zoo — reportedly “the largest drivethrough safari outside of Africa” — attached to the Six Flags Great Adventure theme park in suburban New Jersey, less than a two-hour drive down the turnpike from New York City. The safari park was introduced in 1974, alongside attractions like the world’s biggest hot-air balloon and the world’s largest teepee. It now contains over 1,200 animals from six continents, including a sizable population of African herd animals.
Dharithreesan set up a camera on a tripod on her windowsill to film the animals’ movements. She began jotting notes in a field journal: date, time, temperature, weather conditions, and any notable behaviour. She was in the preliminary stages of a multiyear campaign to tag the park’s African ungulates with GPS-enabled collars. The data would be transmitted wirelessly to a receiving station then relayed to the Swarm Lab, where it would eventually help solve the riddle of why mammals form herds.
One of the most prominent explanations, which she hoped to test, was called the “many eyes theory.” The more eyes a herd has, this theory holds, the more likely it is to detect a predator or a new source of food. By taking turns scanning the plains, more herd members are free to graze in peace. Many African ungulates — zebras, wildebeest, gazelles, antelopes — tend to live in mixed herds, perhaps because the strengths of one species make up for the deficiencies of another. Zebras, for example, are nearsighted, but have excellent hearing, while giraffes and wildebeest have keen long-range vision. By herding together, they increase their chance of spotting (or hearing) the approach of a stalking lion.
Dharithreesan planned to test this theory by installing electronic collars on all the ungulates, which would track not only each animal’s location, using GPS, but also employ gyroscopes and accelerometers to record which direction its head was pointing. Scientists have so far conducted only a few studies like this on the dynamics of mixed-species herds. The logistics were staggering, Dharithreesan told me. “You can’t really do this type of study in the wild, because there’s just too much space; we don’t have the resources,” she said. “And you can’t quite do it in a laboratory setting, because these animals are huge.” Fortunately, the owners of Six Flags Great Adventure had unwittingly built the ideal scientific testing ground.
Out in the wild, scientists often sacrifice this kind of granular data for a much broader scope. With the rise of satellite technology, humans have suddenly acquired a god’s-eye view of how animals move across vast stretches of land. Previously, to track a group of animals in the wild, scientists had to tag them with radio collars and then, using jeeps equipped with special antennas, chase after the tagged animals. Now with GPS collars, researchers can tag an animal, let it roam for months, and then download the collar’s data either manually or, increasingly, wirelessly. This new technology — paired with ever more detailed satellite imagery — is revealing how groups of mammals create and pass down migration routes from generation to generation. Some of the oldest of these migratory routes, like those of Canadian mountain sheep, likely stretch back tens of thousands of years.
Afew years ago, an ecologist named Hattie Bartlam-Brooks attached GPS collars to a group of zebras in Botswana’s Okavango Delta to track their grazing patterns. At the time, it was widely believed that the zebras never left the delta, so when a large number of the zebras disappeared from sight at the onset of the rainy season, Bartlam-Brooks assumed they had been eaten by lions. Then, six months later, the tagged zebras reappeared. When Bartlam-Brooks recovered their collars and downloaded the data, she discovered that the zebras had somehow walked halfway across the country, to feed on the sprouting grasses of the Makgadikgadi salt pan.
By reading through old hunters’ and explorers’ records, she learned that a large zebra migration had once existed along that same route, but it had been severed when the Botswanan government installed hundreds of miles of veterinary cordon fences in 1968. One of these fences blocked the zebras’ migratory route for de- cades before the government finally dismantled it in 2004. Since the fence stood for 36 years, and the average lifespan of a zebra is only 12 years, no living zebras could have possibly remembered making that trip. But then, I wondered, how could the zebras have known where to go?
When I spoke to Bartlam-Brooks on a long-distance call to Botswana, she quickly ruled out my first guess: there was no grassy runway — as I had imagined — that lured them across the country. Instead, they had to pass over hundreds of miles of dry Kalahari scrub. The study’s co-author, Pieter Beck, explained that migrations, by definition, involve not only long distances, but also high stakes: in a migration, there is always a considerable “energetic cost” to the journey. Every voyage is a gamble. (This may explain why not all the zebras ended up taking the trip. Even among zebras, there are bold and timid individuals.)
Because the cost of unsuccessful exploration is so high, successful migration routes are precious and hardwon. Older herd members teach the routes to their children, passing them down as a kind of traditional knowledge. But like all traditions, migratory routes are delicate. Once a route is disrupted, it rarely reemerges. What Bartlam-Brooks had apparently uncovered was a rare instance of a species reviving their ancestral lifeway.
But still I wondered: How? I pushed Bartlam-Brooks to venture a guess.
Her answer surprised me. She said that her hunch was that, through a series of exploratory walks, the zebras might have followed a chain of elephant trails that led them from water source to water source all the way to the salt flats.
“Elephants are obviously much more long-lived than zebras,” she said, “so when the fence went down, it’s very possible that some elephants remembered that old historical pathway that they used to take. Elephants could have easily re-created game trails, and zebras may well have just followed them.”
Of course, I thought. Elephants.