The highlight of a two-week backpacking trip in Alaska’s Arctic National Wildlife Refuge (or “ANWR” as it is known by its reductive acronym), which I guided this summer, were thousands of caribous from the Porcupine Herd that streamed past our camp on two consecutive days. They formed the vanguard of the annual fall migration, during which the herd returns south to congregate near the tree line for the rut. After dropping their calves in June, the animals had been fattening up on the coastal plain for their 400-mile return trip in late July. The leaders first alerted us to their presence with old-man grunts. Emerging from our tents, we watched in awe as the brown tide flooded the hillside, pulled by millennia-old urges...