Tuesday, May 27, 2014

Longest Migration Among African Mammals Discovered (National Geographic)

Zebras migrating, Equus quagga, Chobe National Park, Botswana
Burchell's zebras (Equus quagga) migrate through Chobe National Park in Botswana.

One herd of zebras migrates more than 300 miles across Namibia and Botswana—farther than any other known African mammal.

Jennifer S. Holland
A population of zebras surprised biologists by making a more than 300-mile beeline across parts of Namibia and Botswana—the longest big-mammal migration ever documented in Africa.
In the wilds of Africa, food and water come and go with the seasons, and animals follow. The Serengeti is the site of what most consider the most dramatic migration, with giant herds of millions of animals—some 750,000 zebras and 1.2 million wildebeests as well as gazelles and eland—traveling from the Ngorongoro area in southern Tanzania to the Masai Mara in lower Kenya and returning as the rains dictate.
But when it comes to the longest hike endpoint to endpoint, Africa has a new record holder. As reported in an article published online today in the journal Oryx, the migration, which has now been observed in consecutive years, isn't on the scale of what goes down on the Serengeti—it involves just a few thousand Burchell's zebras (Equus quagga). But the animals cover more than 300 miles (500 kilometers) in a straight-line, up-and-back journey across Namibia and Botswana. (In the Serengeti the animals meander more before circling back, so their feet touch more ground, but the distance between the zebras' two destinations is greater.)
Straight Shot
"The almost unerring north-south direction was unusual," says lead author Robin Naidoo, senior conservation scientist at the World Wildlife Fund (WWF). But there was an even bigger surprise. "The distance covered by these zebra was a total shock to all of us involved in the study, as well as to people familiar with wildlife conservation in the region," he says. "Nobody knew that something of this scale, with this much ground covered, was occurring. "
Naidoo says the research team in Namibia—scientists from WWF and Namibia's Ministry of Environment and Tourism—had a suspicion that the zebras were up to something, "since animals seemed to just show up on the floodplains in Salambala communal conservancy in the dry season, where there are permanent water sources, and then disappear in the wet season."
That the animals were migrating with the rains wasn't in itself surprising. "But we didn't have any inkling they were moving such a long distance away from Namibia," Naidoo says. As it happened, the same animals his team was observing in Namibia were being satellite collared by biologists from Elephants Without Borders across the Chobe River in Botswana. When the teams joined forces, they realized the impressive extent of the animals' travels.
Another bit of intrigue: "Our preliminary investigations suggest that there were similar, alternative wet-season destinations that were closer to zebras' dry-season haunts—yet they were bypassed," says Naidoo. Why wouldn't the animals choose a more efficient trip?
Research with other migratory mammals has shown that generations of animals may stick to the same route corridors, down to the nearest meter. For example, says ungulate ecologist Mark Hebblewhite of the University of Montana, "we have evidence that pronghorn antelope in the western U.S. have migrated over the same routes for more than 6,000 years. This is probably a product of both the landscape and cultural transmission of knowledge amongst social animals." (See: "Animal overpass helps pronghorn migration.")
Naidoo says there may also be a genetic basis for such traditions. But it will take more years of monitoring the zebras to figure out what drives them, he says, "to determine whether the same endpoints and same trajectory are in fact used every year."
Taking Detours
Another long zebra migration, reported in 2011 by conservation biologist Hattie Bartlam-Brooks of the University of Bristol and colleagues, had been blocked by fences for nearly 20 years. Four years after the fences were removed, the animals resumed their old route. "They had managed to restart a historical migration and within two years were following a highly directed route between their two ranges," she says. "I think this shows that zebra can be adaptive and are able to deal with route change, although the cost to the whole population during such a period is likely to be great."
Studying migration isn't just a way to feed scientists' curiosity. "Sorting out how animals make these individual decisions has important conservation value," says Hebblewhite. "There is a global decline in migratory species. If we know what makes animals stop migrating, maybe we can reverse or maintain these movements."
Knowing their routes also gives conservationists a target when protecting land. The newly reported zebra migration takes place entirely within the boundaries of a complex of protected areas known as theKavango-Zambezi Transfrontier Conservation Area (KAZA). At about 170,000 square miles (440,000 square kilometers), it's the largest transfrontier conservation area in the world.
But other migration routes may cross territory where hunting is constant, where habitat has been destroyed, or where villages, farms, fences, and other structures are spreading. As corridors become blocked, the entire ecosystem suffers.
So just protecting blocks of land, which has often been conservationists' goal in the past, is not enough, says Jeffrey Parrish of WWF. "Wildlife like these zebra need the freedom to roam beyond protected cores, and sometimes even beyond humanity's national borders. This is particularly important as climate change causes habitats to shift, forcing migratory and nonmigratory wildlife alike to move as the habitat rug is pulled out from underneath them."
Migration as Connector
Other than giving animals a way to ride out less plentiful seasons, why do these movements matter? "Migration allows species to exist at higher densities, in different habitats, than they would be able to without the migratory corridors," says Hebblewhite. He likens the movements to the neuronal connections that power the brain. "We know that the power of biodiversity comes from the connections between species themselves, and migration is one of the most important mechanisms to maintain these connections. As we lose migratory movements, we lose links between ecosystems, species, and processes."
Naidoo's team agrees that safe travel corridors are essential to the survival of the big African mammals that use them and to the broader natural systems that support them. "Massive protected areas like KAZA are in fact quite necessary to conserve these large-scale ecological phenomena," he says. In the case of the zebras' big move, "the effective on-the-ground management of KAZA will be a key factor in determining whether it persists."
WWF's Parrish adds that it's important to understand that "protecting corridors doesn't mean locking them up. It means that we need to give wildlife the freedom to roam in how we use the land—whether that be by full protection of a migratory path or just managing it with wildlife movements in mind."
Land-management issues aside, Bartlam-Brooks says newly "discovered" migrations will no doubt continue to fascinate—in part because we don't fully understand how the animals do what they do. "Zebra find their way hundreds of kilometers through relatively featureless, arid scrubland," she says. "Pretty amazing when you think they only make that journey twice a year. We'd rely on maps, signs, GPS—and might still get lost!"
Says Naidoo, "Discoveries such as these remind us that even in this day and age, nature is full of surprises."

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