The fur, or hair, of all mammals is made up of the protein keratin and dead skin cells. It grows from follicles in the dermis, or inner layer of the skin. The follicles of humans have one hair each. The follicles of dogs, which depend on their fur to regulate their body temperatures, sometimes have several hairs growing out of one follicle.
Most hair follicles have an associated oil gland that works to keep the skin pliable and the hair smooth. Dog breeds that were developed to retrieve game from water have very active oil glands, a factor that helps to waterproof their fur and skin.
Most dogs have three types of fur: Undercoat, guard hair and whiskers. The undercoat lies closest to the dog's skin and grows in clusters from a single follicle. The undercoat has a soft, downy appearance and works to protect the skin from cold weather.
The guard hairs are harder in texture, thicker in diameter and longer than the downy undercoat. Considered the dog's primary coat, the guard hairs protect the skin from superficial injuries and form an additional layer of insulation to protect the dog from cold weather.
Dogs, such as the Collie, with both an outer primary coat and the inner undercoat, are said to have a double coat. Dogs with a coat that is made up mostly of the primary coat, with little or no undercoat are said to have a silky coat. Some breeds, such as the Pumi, have a coat made up of dense undercoat only. The guard hair of the wire-haired breeds is characterized by a crisp, hard texture. The corded coat of the Komondor and Puli looks as though each strand was twisted into individual mop-like strings. The Curly Coated Retriever has a coat characterized by tight, soft curls and Poodles have dense, harsh textured coats that will grow continuously if never trimmed.
The whiskers grow from deeply-rooted follicles on the muzzle and eyelids. These thick hairs function as sensory structures for the dog. When something brushes against these "feelers," the dog will automatically react by either closing his eyes or pulling away.
The color of a dog's fur comes from the cells in the skin that produce melanin. The shade of color depends on the dog's genetic make-up. Some breeds carry genes for a solid color (Samoyed, Maltese). Other breeds carry genes for multiple shades of a certain color (Weimaraner, Yellow Labrador). Some breeds carry the genetic code for a multi-color pattern (Doberman, Tricolor Collie). Other breeds can come in a wide variety of pattern and colors (Cocker Spaniel, Greyhound).
The color of a dog's fur can be black, liver (brown), red, yellow, white or combinations thereof. Dogs with dilution color factors in their genes will be a paler version of these basic colors. Even though breeders have come up with a wide range of descriptive terms for these primary colors and dilutions of these colors, all are actually just variations of these five primary colors.
Genes also determine the color patterns of a dog's coat. Coat patterns include spotted, masked, patched, striped (brindle), dappled, merle and agouti (where each individual hair shaft has several alternating bands of color).
Some coat colors and patterns are associated with genetically linked health problems. Collies, Great Danes, Dachshunds and Shetland Sheepdogs that have a merle pattern may also be deaf or have severe vision problems. The white coats of Boxers and Dalmatians are linked to deafness.
A dog's fur grows in seasonal cycles. When it reaches a genetically determined length, it stops growing and falls out -- a process known as shedding. Shedding is determined by the duration of daily sunlight as well as environmental temperature. (Some breeds, particularly terriers, shed very little and can be tolerated by some people with allergies.)
As days become longer and temperatures warmer, dogs shed their undercoat, allowing them to stay cooler in warm weather (vital for an animal whose only sweat glands are in his foot pads). As days grow shorter and temperatures cooler, the light summer undercoat is shed to make room for the thick winter undercoat. The dog's winter coat traps air warmed by the dog's body and keeps it next to the skin. Muscles in the dog's skin can fluff up the coat, creating even more insulation. Shedding can also occur after a dog has been given anesthesia or in females after they have given birth to a litter of puppies or come out of heat.
Regardless of the color, pattern or texture of a coat, it is an excellent barometer of your dog's overall general health. A dry, lifeless coat and itchy, irritated skin can indicate, internal parasites, a thyroid problem, and hormone imbalance or poor nutrition. If your dog's coat is in poor condition, consult with your veterinarian.