What does a species of rat have to do with the color of your dog's coat? Not much, although they may share a common name. "Agouti" is a term used to identify a specific class of rodents (and guinea pigs), but it's also short for agouti signalling peptide. This peptide is created through a specific gene, known as the agouti gene, which is a key player in determining the fur patterns and shadings in many animals, including dogs.
With agouti coloring each individual hair is banded in at least three or more rings of alternating dark and light colors, similar to that of a rabbit or a wolf. The colors on each hair change abruptly from dark to light, with no shadings in between. Agouti coats are found on Keeshonds, Norwegian Elkhounds, Siberian Huskies and many types of Spitz. Bands on these dogs change color abruptly from dark to light and differ from the gradually shaded bands that are found in brindle- and sable-colored dogs.
The colors within a dog's coat are caused by many genetically determined variables which affect pigmentation of the skin and fur. The specialized cells that control this are known as melanocytes, and they manufacture two types of melanin - black/brown eumelanin pigment and red/yellow phaeomelanin pigment. With the agouti gene, the dark coat consists of bands of eumelanin pigment and the lighter coat of bands of phaemelanin pigment, both of which occur on the same hair shaft. These bands will appear on the dog's neck, shoulders, back and rump, and usually appear lighter on the chest, belly and the insides of the legs.
Dogs with agouti coats generally have no pigment-related health problems. Merle, or white-haired, breeds, on the other hand, often have genetically-based defects that are connected with these pigmentations and can cause the affected animal to develop hearing and vision problems. These defects usually are connected to eumelanin, the same pigment responsible for the dark bands of fur in agouti-coated dogs.
Within breeding and veterinary circles, many professionals argue over whether agouti coats should be considered a separate color pattern or a subcategory of the brindle and/or sable dogs. In most breeds, however, it's recognized that agouti coats are genetically different from both of these. It's worth noting that just because a dog has a banded coat, it is not necessarily an agouti. Many types of dog coats are banded, especially the above-mentioned sables and brindles. (Sable coats have darker coloration only at the tips of the individual hairs.) These normally have shaded bandings, with the colors changing gradually between shades of gold, caramel, cream, white and other light colors. Other dogs have shaded darker coats, with gradually changing tones of black, blue, charcoal, gray, mahogany and silver.