A dog's hair follicles are arranged in groups consisting of a central primary hair surrounded by thinner, shorter secondary hairs. (Puppies have no secondary hairs.)
The density of the hairs depends on the breed and age of the dog. The softer the hair, the more dense it is. For example, German Shepherds have between 100 and 300 follicle groups/cm2 (650-2000 inch2), while a softer-haired dog can have from 400 to 600 groups/cm2 (2600-4000 inch2). The number of follicle groups a dog will have is already established at birth. However, in young dogs, only one type of hair (either primary or undercoat hair) is present, which is what gives puppies their well-known softness. As the dog grows, the angle of the hair with respect to the skin decreases, reaching about 45° in the adult.
Hair color is genetically determined, with certain colors being dominant over others. This explains the wide range of coat colors in dogs, as well as the specific types of spots and patches characteristic of certain breeds. For example, a Beagle's spots are not related to those of a German Shepherd.
Practically everyone has had the experience of having to pick up a large ball of hair from the rug in early summer. In fact, dog hair does not last forever. There is a cyclical loss of hair known as shedding. Like wild animals, dogs shed twice a year, resulting in a summer coat and a winter coat. These seasonal coats can be explained by the three stages of activity of the hair follicles:
Anagen is a period of growth for the hair and its follicle, which grows down into the dermis. It lasts about 130 days in the average dog and can be as long as 18 months in the Afghan Hound.
Catagen is a resting phase. Growth stops and the follicle shrinks.
During the telogen phase, the follicle shrinks all the way to the orifice of the sebaceous gland. The base of the hair shrinks to a cone shape and the hair falls out. Another hair begins to grow, starting with the anagen phase and growing in the same follicle as its predecessor.
Of course, the hairs do not all fall out at the same time. Shedding begins at the rear of the dog and progresses toward the front. The winter coat is much thicker than the summer coat and protects the dog from severe cold.
These changes in the coat do not happen by chance. The most important factor governing shedding seems to be the photoperiod (the duration of daylight in relation to darkness). The lengthening day triggers shedding in the spring and the decreasing amount of daylight triggers shedding in the fall. Temperature changes affect only the density of the coat and the speed with which the hairs are replaced and are not a major factor causing shedding.
Although individual hairs are replaced, a dog's coat keeps its color, in spite of the appearance of gray hairs on the muzzles of elderly dogs. Remember, a dog's coat should be groomed regularly to avoid disease.
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