In late June our floatplane lifted off from Deadhorse, Alaska, at the top of the state, and arrowed west. As it gained altitude the industrial spraddle of the Prudhoe Bay oil field shrank beneath the plane’s pontoons. Soon there was nothing below but land the color of wet cardboard, an earth still waking from its long winter.

About 110 miles to the west, the plane skidded down on a half-frozen lake. We pitched our tents and ringed the camp with an electrified bear fence against curious grizzlies. Then, Martin Robards and Peter Detwiler—a scientist and a senior field technician for the Wildlife Conservation Society, respectively—headed out across the tundra. Robards wore a Remington 870 shotgun slung over his