One of the world's longest migrations² of zebras occurs in the African nation of Botswana, but predicting when and where zebras will move has not been possible until now. Using NASA rain and vegetation data, researchers can track when and where arid³ lands begin to green, and for the first time anticipate if zebras will make the trek⁴ or, if the animals find poor conditions en route, understand why they will turn back. Covering an area of approximately 8,500 square miles (22,000 square kilometers), Botswana's Okavango Delta⁵ is one end of the second-longest zebra migration¹ on Earth, a 360-mile (580-kilometer) round trip to the Makgadikgadi Salt Pans -- the largest salt pan system on the planet. Zebras walk an unmarked route that takes them to the next best place for grazing, while overhead thundering cloudbursts of late October rains drive new plant growth, filling pockmarks（麻子，凹坑） across this largest inland delta（三角洲） in the world. In a matter of weeks, the flooded landscape could yield ecosystems⁶ flush with forage⁷ for the muscled movers.

 

High above, Earth-orbiting satellites capture images of the zebras' movements on this epic⁸ trek, as well as the daily change in environmental conditions. Zebras don't need data to know when it's time to find better forage: The surge of rain-coaxed grasses greening is their prompt to depart. But now, researchers are able to take that data and predict when the zebras will move.

 

Pieter Beck, research associate with the Woods Hole Research Center in Falmouth, Mass., and three collaborators studied animal migration in a novel way, which they described in a paper published in the Journal of Geophysical Research--Biogeoscences, a publication of the American Geophysical Union. While tracking animal movement with satellites has been accomplished⁹ many times, Beck said, he and his team combined that information with in-depth use of environmental satellite data, using a series of images of vegetation growth and rainfall taken over days and weeks. This sheds unprecedented¹⁰ light on what drives animals to migrate, he said, what cues they use, and how animal migrations respond to environmental change.

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