Reindeer are probably a domesticated form of caribou, says the Smithsonian Museum of Natural History. Reindeer are slightly smaller than caribou. Reindeer are shorter and stockier than caribou, and have thicker, denser coats. There may be two different species of caribou, one that is that same species as reindeer and one that is not, says Discovery News.
The domestication process is thought to have brought about a number of changes such that reindeer are considered a subspecies of caribou, says the University of Alaska. Female reindeer grow larger antlers than female caribou do. Reindeer bulls are smaller than caribou bulls, but reindeer cows are about the same size as caribou cows. Reindeer are more sedentary than caribou and migrate only short distances. Caribou migrate long distances seasonally. Reindeer give birth earlier in the year than caribou do.
The two major types of caribou were separated during the last glaciation event. One was pushed down into North America, and one survived in Northern Asia before re-populating Europe, Alaska and Canada, says Discovery News. This group of caribou is the one from which reindeer were domesticated in Europe and Asia. Reindeer are one of the earliest domesticated animals, having been domesticated at least 3,000 and possibly 7,000 years ago, says the San Diego zoo.