dog, carnivorous, domesticated wolf (Canis lupus familiaris) of the family Canidae, to which the jackal and fox also belong. The family Canidae is sometimes referred to as the dog family, and its characteristics, e.g., long muzzle, large canine teeth, and long tail, as canine traits. However, the unmodified term dog usually refers only to the domestic subspecies Canis lupus familiaris.
Two characteristics distinguish the dog from other canids and, indeed, from all other animal species. The first is its worldwide distribution in close association with humans, encompassing both hemispheres from the tropics to the Arctic. The second is the enormous amount of variability found within the subspecies. For example, the Irish wolfhound may stand as high as 39 in. (99.1 cm) at the shoulder, while the Chihuahua's shoulder is usually no more than 6 in. (15.2 cm) from the ground; the silky coat of the Yorkshire terrier may be 2 ft (61 cm) long, while a few breeds of dog (such as the Mexican hairless) are entirely without hair. The evolution of such widely differing breeds has been heavily influenced by conscious human selection, in addition to natural evolution.
Dogs have been selectively bred through the centuries for special purposes, notably to pursue and retrieve game, as draft animals, as guides (e.g., for the blind), and as companions. Although dogs possess hearing abilities far superior to humans', their acute sense of smell is probably the sense most utilized. In addition to traditional hunting and tracking, the dog's sense of smell has been put to such diverse uses as the location of exotic foods and the detection of drugs and explosives, e.g., in luggage and packages.
Dogs can be protected against serious diseases for which vaccines are available; these include distemper, canine hepatitis, leptospirosis, and rabies.
The dog is descended from the wolf. True wolves appeared in Europe about one million years ago and in the Americas some 700,000 years later. Dog remains estimated to be about 14,000 years old have been found in Germany, and younger remains have been found in Israel (about 13,500 years old) and Idaho (about 10,500 years old). It is probable that the dog was the first animal to become domesticated, certainly by 15,000 years ago, but possibly long before that. Genetic studies comparing dogs with surviving and extinct wolf species indicate that wolves and dogs separated 27,000 to 40,000 years ago, with the implication that domestication may have occurred as early as 30,000 years ago. Domestication may have occurred independently in a number of different areas of the world, but genetic tests show that all dogs are descended from an Eurasian stock, even the now extinct pre-Columbian dogs of the Americas.
It is thought that the earliest domesticated dogs resembled the present-day dingo, the wild dog of Australia. The dingo is believed to have come to Australia as a domestic dog with the aborigines from Southeast Asia. Although more historical information exists on the forerunners of European dogs (such as the British hounds, terriers, and shepherd dogs) than on those of other areas, there is evidence that dogs have existed in most areas of the world throughout the period of recorded history. One of the oldest known breeds is the basenji, which originated in central Africa and is still used as a hunter by certain tribes in that region. Several distinct breeds were known in ancient Egypt and a mastifflike breed (resembling the Kurdish dog in present-day Iraq) is found in Babylonian illustrations of c.2200 BC
The Purebred Dog
A breed of dog is produced by selecting and mating dogs with certain desired characteristics. The offspring of such matings are then inbred, i.e., mated with litter mates or close relatives. After about eight generations, the line usually breeds true, i.e., most offspring resemble each other. Then standard traits can be established for the new breed. A purebred dog is one that conforms to the standards of a certain breed and whose lineage, or pedigree, has been recorded for a certain period of time.
One of the principal functions of a kennel club is to maintain the records of lineage of individual purebred dogs in order to preserve breed standards. The stud books of the AKC contain entries for all purebred dogs whose owners have elected to register their dog's pedigree. Other stud books, such as those of the United Kennel Club, often record dogs of breeds not recognized by the AKC but which have a considerable following in the United States. Dogs of mixed origin or whose parentage is unknown are called mongrels.
Classification of Breeds
Attempts to classify dogs probably date from the time when humans discovered that certain canine traits were more useful than others. The earliest known system of classification, that of the Romans, included categories for house dogs, shepherd dogs, sporting dogs, war dogs, dogs that ran by scent, and dogs that ran by sight. Today there are systems of classification and breeding in most countries of Western Europe and in North America, many using a variation of the standard British system.
In the United States, the classification system most frequently encountered is that employed by the American Kennel Club (AKC), which recognizes more than 150 of the more than 200 known breeds. The breeds are grouped into six classes. In the sporting dog group are pointers, retrievers, setters, and spaniels. These dogs hunt by air scent as opposed to those of the hound group, e.g., beagles, foxhounds, and bloodhounds, which track their prey by ground scent. Also classified as hounds are those dogs of the greyhound type, e.g., whippets, borzois, and Salukis, which hunt mainly by sight. The many breeds of terrier go to earth after their burrowing prey. Among the working dog group, used as guards, guides, and herders, are the collie, the German shepherd, and the St. Bernard. Such diminutive pet dogs as the Pekingese, the Pomeranian, and the pug belong to the toy dog class. The nonsporting dog group is a class of dogs bred principally as pets and companions and includes the Boston terrier, the bulldog, the chow chow, the Dalmatian, and the poodle. In addition to the breeds in the above classes, the AKC currently places additional breeds in a miscellaneous group; breeds recently recognized by the club are placed in this class until they become established. Included are the Akita of Japan, the Australian cattle dog, the Australian kelpie, the Bichon Frise (a French descendant of the water spaniel), the border collie (an English shepherd dog), the cavalier King Charles spaniel, the Ibizan hound (of Spanish origin), the miniature bull terrier, the soft-coated wheaten terrier (from Ireland), the Spinone Italiano, and the Tibetan terrier.
Dogs registered by the AKC and other registry associations compete regularly in dog shows and field trials. In dog shows, the various breeds are judged solely on appearance, while in field trials they are rated according to their hunting skills.
See articles on individual dog breeds.
Female dogs, or bitches, will mate only when in heat, or estrus, which occurs about every six months and lasts from 18 to 22 days. Whelping (giving birth) occurs after a gestation period of about nine weeks. The size of the litter varies to some extent with the size of the dog: toy dogs rarely bear more than 2 puppies, while the largest breeds average closer to 10.
See E. Schneider-Leyer, Dogs of the World (1960); American Kennel Club, The Complete Dog Book (1968); E. H. Hart, Encyclopedia of Dog Breeds (1968); H. P. Davis, ed., The New Dog Encyclopedia (rev. ed. 1973).