A caribou (Rangifer tarandus caribou, and other trinomials under Rangifer t.) is any of several North American subspecies, ecotypes, populations, and herds of the species Rangifer tarandus, or reindeer. In North America caribou vary 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 porcupine caribou and the barren-ground caribou form large herds and undertake lengthy se