When the caribou walks, a clicking noise is produced by a tendon slipping over a bone in the foot. Most caribou inhabit arctic tundra and surrounding boreal coniferous forest. Some populations are found in mountainous areas farther south. Rangifer is active primarily during the day and is almost constantly on the move. Its eyesight is apparently poor, its hearing is adequate, and it depends in large part on the sense of smell to detect food and danger.
Most populations have seasonal shifts in range.
Those to the south may simply move to lower elevations for the winter or to areas of favorable food supplies and shelter. Northern populations, however, make extensive spring and fall migrations, sometimes traveling over 1,000 km between the summer range on the tundra and the wintering grounds in timbered areas. During migration, however, when a herd concentrates, there may be up to 19,000 animals per sq km. Rangifer is highly gregarious. The animals in a given region of the world are divided into populations, or herds, which move about as a unit, keep to the same general ranges and migration routes over the years, and do not mix extensively with other herds.