Most mammals grow one hair per follicle. A chinchilla grows a bundle, reported at up to sixty or even seventy-five hairs, which produces a pelage of extraordinary density and the insulation needed on cold nights in its arid Andean range.
That density has a cost. Water entering the coat is held by capillary action between the hairs and evaporates slowly, so the skin stays wet. Keepers therefore give chinchillas shallow trays of fine pumice or volcanic ash; the animal rolls in it and the grit absorbs skin oils and lifts dirt without wetting the coat. Wild chinchillas dust-bathe in the same way in fine volcanic soils.
The fur is also why the species is Endangered. Because a single garment required many pelts, hunting in the late nineteenth and early twentieth centuries reduced wild numbers drastically. The chinchillas alive today are overwhelmingly captive-bred, and their abundance says nothing about the wild population's security.