Caribou and reindeer are North American and Eurasian names, respectively, for the same species of northern deer that exists in various subspecies across the higher latitudes of those continents. Woodland caribou of North America inhabit mountain forests and taiga of Canada and the northwestern U.S., but more northerly populations of barren-ground caribou range the open tundra. These tundra caribou display a number of adaptations to life on the cold, treeless plains.
The caribou's coat includes two layers: a dense, woolly undercoat shielded by longer, hollow guard hairs. The guard hairs trap air to insulate the animal's body, a factor that also makes caribou quite buoyant in the water. As herds often must cross big rivers during their migrations, such buoyancy is a useful adaptation in and of itself. Caribou grow woollier coats in the winter for further insulation, which contributes to their subtle seasonal shifts in colour. The Inuit of the North American Arctic incorporated caribou wool as snug clothing.
Caribou sport broad-splayed hooves, another of their prime adaptations for tundra life. Upon the winter crust, they mimic the effect of snowshoes, allowing the caribou to conserve the energy that would otherwise be lost floundering in drifts. The caribou also employ the sharp edges of their hooves to brush aside snow and access lichen and other forage beneath. Crossing rivers and sounds, the hooves double as effective paddles.
Among deer, caribou are supreme runners -- a good adaptation in the face of predation pressure by wolves, found nearly everywhere that tundra caribou are. Newborn caribou calves can run shortly after being born; unlike many deer, they do not rely on camouflage to avoid predators, but actively flee with the rest of the herd. The endurance of caribou is legendary: North American herds conduct the longest migration of any terrestrial mammal, itself an adaptation to the shifting resources of the high latitudes. Their yearly movements between wintering and calving grounds may cover 3,000 miles.
Barren-ground caribou have evolved a reproductive strategy called synchronised breeding to contend with the open, vulnerable nature of their environment. This means that cows drop their calves all at once, a feat requiring a scheduled mass breeding. Tundra bulls don't defend harems like their woodland cousins, but mate with individual cows within large combined herds. When the caribou arrive at their calving grounds, they give birth to such a large amount of calves at the same time that local predators -- from golden eagles and wolverines to wolves and grizzly bears -- are often overwhelmed.