Domestic dogs vary widely in appearance, particularly in size. The Shih Tzu, for example, is 20 to 28 cm (8 to 11 in) in length and weighs 4 to 7 kg (9 to 15 lb). The Irish wolfhound is at the other end of the scale, measuring about 71 to 94 cm (about 28 to 37 in) at the shoulder and weighing up to about 61 kg (about 135 lb). Coat color, length, texture, and pattern also vary greatly. The muzzle may appear shortened, as in the Pekingese, or elongated, as in the Doberman pinscher. Limbs are relatively short in the basset hound and dachshund, but long in the greyhound. Ear shape and carriage also vary, but these characteristics may be influenced by a dog owner’s decision to crop, or cut, the ears to make them stand up. Some dogs, notably the chow chow, even have a naturally blue-black tongue.
A dog’s skeleton enhances agility and endurance. The strong front legs bear more than 60 percent of the animal’s weight, yet still permit flexibility and nimbleness. The hind legs, attached to massive muscles, enable powerful acceleration and help to maintain running speed.
Despite these differences, all breeds of the domestic dog are essentially identical in anatomy. The skeleton of the domestic dog has an average of 321 bones, with variation reflecting differences in the number of bones in the tail and the presence of a dewclaw, an extra digit on the paw that not all breeds have. The rib cage consists of 13 pairs of ribs; the spine has 7 cervical vertebrae, 13 thoracic vertebrae, 7 lumbar vertebrae, and 3 sacral vertebrae. Rear paws have four complete digits and front paws have four or five digits. Most puppies have 28 temporary teeth, which are replaced with 42 permanent teeth at about six months of age.
Some breed differences evolved to help dogs survive in their native environment or occupation. For example, dogs that lived and worked outdoors, such as the Komondor of Hungary, needed a thick, weather-resistant coat to protect them from the elements and, perhaps, the biting teeth of predatory animals. Similarly, the Labrador retriever developed an oily coat, webbed feet, and a rudder-like tail to help it perform better in recovering downed waterfowl.
Just as distinct physical characteristics became trademarks in some breeds, unusual sensory abilities characterize others. Most dogs are able to detect scents and hear high-pitched sounds that are beyond human perception, but some breeds have especially acute sensory skills. The bloodhound, for instance, can follow a four-day-old track using its highly developed sense of smell. Other breeds with a keen sense of smell include the German shepherd, golden retriever, beagle, and Newfoundland. These dogs have been trained for such varied duties as detecting hidden drugs, explosives, termites, and even a decomposing body immersed in deep water.