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Pelt

The color of the fur varies considerably, both between individuals and depending on season and species. Northern populations, which usually are relatively small, are whiter, while southern populations, which typically are relatively large, are darker. This can be seen well in North America, where the northernmost subspecies, the Peary caribou, is the whitest and smallest subspecies of the continent, while the Selkirk Mountains caribou (Southern Mountain population DU9) is the darkest and nearly the largest, only exceeded in size by Osborn's caribou (Northern Mountain population DU7).

The coat has two layers of fur: a dense woolly undercoat and a longer-haired overcoat consisting of hollow, air-filled hairs. {{efn|According to Inuit elder Marie Kilunik of the Aivilingmiut, Canadian Inuit preferred the caribou skins from caribou taken in the late summer or autumn, when their coats had thickened. They used it for winter clothing "because each hair is hollow and fills with air trapping heat." Fur is the primary insulation factor that allows reindeer to regulate their core body temperature in relation to their environment, the thermogradient, even if the temperature rises to 38 °C (100 °F). In 1913, Dugmore noted how the woodland caribou swim so high out of the water, unlike any other mammal, because their hollow, "air-filled, quill-like hair" acts as a supporting "life jacket".

A darker belly color may be caused by two mutations of MC1R. They appear to be more common in domestic reindeer herds.