Porcupine Caribou Herd ~ Rangifer tarandus granti ~ are a barren ground caribou herd numbering over 179,000 members ~ they winter in the Yukon, Alaska and Northwest Territories ~ During their annual spring migration to their calving grounds in Canada's Ivvavik National Park and Alaska's Arctic National Wildlife Refuge ~ they are thought to have the longest land mammal migration in the world ~ aerial images of migration during spring