Dog hair grows in cycles. Each follicle has a period of rapid growth (the anagen phase), followed by slower growth and then a resting phase (the catagen phase). During the resting phase, mature hair remains in the follicles and eventually detaches at the base. When the dog sheds her coat (the telogen phase), a young hair pushes out the old hair and the cycle begins anew.
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The average dog takes about four months to grow a coat, but there are individual and breed variations. The Afghan Hound, for example, grows her coat in about 18 months. There are dog breeds, such as the Chinese Crested (which has hair on the head, tail, and legs) and the Xoloitzcuintal (which is normally hairless except for a single tuft on the head and some long hairs on the tail). Both of these breeds have coated versions as well.
The hairless condition in these dogs is due to a genetic mutation, not a health problem. Most dogs shed their coat at least once a year. Dams often blow their coat six to eight weeks after delivering puppies. Major shedding may follow a bitch’s heat cycle as well, due to the hormonal swings. Many people assume that temperature changes govern when a dog sheds her coat. In fact, the seasonal length of daylight exerts the major influence.
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Longer periods of daylight in spring activate a shedding process that lasts four to six weeks. In fall, as the daylight hours grow shorter, many dogs may again shed their coat. Sensitivity to ambient light is most pronounced in dogs who live outdoors. Dogs who live primarily indoors are exposed to artificial light and a rather fixed photoperiod. These dogs may shed and grow new coats all year long.