I'm kneeling on a sandy beach in Costa Rica on a balmy January night helping biologists administer an ultrasound to a leatherback sea turtle. They are hoping to learn more about her reproductive cycle, to better protect populations of this endangered reptile at its nesting beaches around the world. The turtle has come ashore to lay her eggs, as her kind have done since dinosaurs roamed the earth. I watched as she hauled her enormous bulk up the beach, pivoted slowly around to face the ocean, and began scooping out an oval pit in the sand with her hind flippers. She soon entered her egg-laying trance, a quiescent state in which she remains for an hour until she covers over her nest and heads back to the surf.

In her reverie, she takes no notice of us as we get to work. We unpack the equipment, which looks like a desktop computer, and set it down in the sand just behind her. A switch is thrown, and flickering light from the screen lights up the turtle's hind end. Two of us, one on each side, hold down her huge front flippers to ensure that she doesn't start flinging sand in our faces when she begins concealing her nest. Those oarlike flippers can toss any one of us aside like so much flotsam, but they lie idle at her sides. I feel for her in her labor. Her weight makes it difficult for her to breath, and she sucks in air in great gasps. Tears designed to carry away excess salt dribble like saliva from her eyes, making her look as if she is crying from the strain.