The  sky over South America’s rain forest was turning that hard-to-describe color it gets right before the tropical night erases the color. Then, suddenly and silently, there the puma was! It had warily stepped into a forest clearing and stopped in its tracks.

For a moment the big cat stood motionless, except for the tip of its tail, which kept moving like a low-speed windshield wiper. Then, when it noticed that it was being watched, the puma leapt across the clearing and dashed into the f...