Caribou (North America) refers to any of several North American subspecies, ecotypes, populations, and herds of the species Rangifer tarandus. In North America caribou range in size from the smallest, the Peary caribou, to the largest, the boreal woodland caribou. The North American range of caribou extends from Alaska, through the Yukon, the Northwest Territories, Nunavut, into the boreal forest and south through the Canadian Rockies and the Columbia and Selkirk Mountains. Barren-ground, Porcupine caribou and Peary caribou live in the tundra while the shy woodland caribou, prefers the boreal forest. Two major subspecies in North America, the R. t. granti and the R. t. groenlandicus form large herds and undertake lengthy seasonal migrations from birthing grounds, to summer and winter feeding grounds in the tundra and taiga. The migrations of R. t. granti Porcupine herds are among the longest of any terrestrial mammal. Barren-land caribou are also found in western Greenland, but the larger herds are in Alaska, the Northwest Territories, and Nunavut.