Anticipation ripples through the crowd. Fingers tighten around binoculars. Camera lenses snap into focus. No fewer than 11 canopied safari buses, bright with tourists and bristling with long lenses, huddle near a solitary acacia tree in Tanzania’s Serengeti National Park. For the past half hour a mother cheetah named Etta has been sitting in the shade with her four young cubs, eyeing a herd of Thomson’s gazelles that drifted into view on a nearby rise. Now she’s up and moving, sidling toward the herd with a studied nonchalance that fools no one, least of all the gazelles, which are staring nervously in her direction.