A pair of American Pronghorn damsels (Antilocapra americana) warily survey the skyline in the Rainbow District of Petrified Forest National Park. More usually encountered on the short-grass prairie, these two were part of small herd traversing the Painted Desert badlands in the south of the park. Lucky visitors will have a chance to glimpse the graceful animals without the usual entrance fees from now through Sunday, April 24, in celebration of National Parks Week. Widely admired as the swiftest land animal in North America, the American Pronghorn is the sole survivor of a family of fleet-footed plains dwellers. Less well known is the evolutionary motivation of their “need for speed”; before the last Ice Age, the prairie in this part of the world was also home to a North American Cheetah, capable, it is thought, of the same astounding acceleration possessed by the African speed-cats of today.