The coat of a reindeer insulates against polar cold with the help of dense underfur that traps air.

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“Naturally, animals that live in polar regions have the warmest coats of all. The reindeer’s coat combines long, water-repelling guard hairs with an extremely dense underfur, deep-piled like a shag carpet.” (Foy and Oxford Scientific Films 1982:84)

The grand design: Form and colour in animalsJanuary 1, 1970
Foy S

“The winter fur of adult reindeer consisted of thick guard hairs with air-filled cavities and an underfur of thin and woolen hairs…The density of guard hairs varied considerably and averaged 2000/cm2 and 12 mm on the legs, 1000/cm2 and 30 mm on the abdomen, and 1700/cm2 and 30 mm on the back. The corresponding count on the back of calves was 3200/cm2 and 10 mm…All hairs were wool-like and hollow…The thick underfur is very important, since it effectively prevents air movement within it and thus reduces heat dissipation…[T]he results suggest that the prime mechanism by which adult reindeer thermoregulate in a cold environment is insulation.” (Soppela et al. 1986:275, 277)

Journal article
Thermoregulation in reindeerRangifer, 1: 273-278January 1, 1986
Soppela P; Nieminen M; Timisjärvi J

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