DOG, a carnivorous mammal, probably the first animal domesticated by man. How or when this domestication took place is not known with certainty, but the dog’s association with man began at least 10,000 years ago.
Dogs are found throughout the world, wherever man lives. Statements as to the size of the world dog population are little better than guesses and vary widely, but informed estimates place the number between 120 million and 150 million. In the United States, according to estimates based on projections of market research reports for certain areas of the country, the dog population is probably between 29 million and 35 million.
General Characteristics. The dog (Canis familiaris), possesses great genetic variation, and widely differing breeds can be developed from the same stock within a relatively short time. This genetic plasticity has been utilized in producing the more than 400 distinct breeds in the world today.
These breeds vary from those that weigh as little as 11/2 pounds ( 680 grams ), such as the chihuahua, to those that exceed 200 pounds ( 90 kg) such as the St. Bernard. Breed heights also vary markedly. A toy poodle may stand only 8 inches (20 cm) high at the shoulders, whereas some great danes and Irish wolfhounds ma reach 37 inches (94 cm).
The dog’s coat, which ranges from short to long and from wiry to straight, may be almost entirely lacking, as in the Mexican hairless, or as luxuriant as that of the old English sheepdog. whose guard hairs may be 10 inches ( 25 cm long.
Greyhounds and borzois are very long muzzled, while pugs, pekingese, and bulldogs have “pushed-in” faces. Bulldogs have small. folded ears; German shepherds have large erect ones. A basset hound, which may be only 14 inches (35 cm ) high at the shoulders may have ears that span 24 inches ( 60 cm ) from tip to tip.
Dogs are digitigrade animals; that is, they walk on what is anatomically their four fingertips. The fifth finger, or thumb, known as the dewclaw, does not reach the ground. Some breeds lack dewclaws on their hind feet.
The dog, like man, has two sets of teeth. The 32 milk, or baby, teeth appear at about 3 to 5 weeks of age and fall out when the puppy is from 4 to 6 months old. The adult dog’s teeth. which includes the molars, number 42.
The dog can hear pure tones of 35 kiloherz ( kHz; 1 kHz = 1,000 cycles per second), whereas in man, 20 kHz appears to be the top limit. The dog’s visual acuity is not especially sharp and is roughly comparable to man’s; however, the dog is unable to perceive colors and sees only shades of gray. The dog’s most important sense is that of smell. Exact measurements of this sense have been rather difficult to make, and there have been conflicting results; but it is unquestionable that the ability to track faint or old trails is an extremely sensitive one. For some substances the scenting ability of dogs is not very different from that of man, but in the detection of certain aliphatic ( “fatty”) acids—acids present in the skin secretions of mammals and left behind in their track dogs have been shown to be more than a million times more perceptive than man.
Usefulness of Dogs. Man has used dogs for many purposes, including hunting, guarding, racing, companionship, and leading the blind. Some applications have been quite odd. The French have used dogs to locate Perigord truffles edible fungi that grow underground. Modern narcotic officers have used dogs trained to detect drug odors in luggage and packages. St. Bernards have been used to search out snow-covered trails and warn of hidden crevasses. They have also been used to locate people buried snow avalanches.
In earlier centuries dogs worked as turnspits r keep meat revolving over a fire, and they served as footwarmers during services in unheated churches. The Spanish conquistadores are ported to have taught dogs to attack and kill Indians Modern armies have used dogs to lead scouting patrols, locate wounded, carry messages and supplies, and guard bases. Sled dogs helped the Eskimos adapt to the rigors of Arctic life. The American Plains Indians used dogs to haul the travois, a primitive type of sled made by hooking two poles into the dog’s harness.
The use of dogs in medical research and teaching has steadily increased. Testimony before committees of the U. S. Congress indicates that every year more than 3 million dogs are killed for this purpose. The demand for research dogs is so great that legislation has been enacted in both Britain and the United States to stem the traffic in stolen dogs and to foster more humane treatment of dogs in laboratories.