Up on the mountains the midday sun glared fiercely, but down in the valley near the swift-running stream it was cool and still. I stood listening until I heard a faint rustling of leaves—the only sound to betray the presence of the group of chimpanzees I was trying to approach.

Slowly and quietly, but making no attempt to hide, I moved toward the great apes until I was only 30 feet away. As I sat down, they watched me, staring rather hard, and a young female who had been lying on the ground climbed a little way up a tall fig tree.