The clouds are a blessing. it’s 97 degrees in the shade, and the air is wet as kelp. For six miles the trail has wound in and out of the Costa Rican rainforest like leather stitching—here a narrow footpath under towering beach almond trees, there a black-sand trudge along a mile of rocky coast. Twice we’ve forded waist-deep rivers barefoot, crossing between low surf and mist-shrouded forest, watching for crocodiles and bull sharks with our shoes in our hands. We’ve cooled our heads in streams pouring onto the beach from 30-foot cliffs, and clambered over rusting shipwrecks and the graves of forgotten gold miners. We’re now six hours into a 12-mile hike, and ahead the shore of the Osa Peninsula curves into the distance, green forest spilling to the sand, surf gleaming like the edge of a scimitar. The trail jogs back toward the forest, and there my eyes fall on prints in the sand. Kneeling down, I place a hand beside them. The tracks are deer-like, but more heart-shaped...