Dogs range in height from 5 to 34 inches (12.7 to 86.4 cm) at the top of the shoulder blades. The smallest breed, the Chihuahua, weighs two to six pounds (0.9 to 2.7 kg); the largest breed, the mastiff, weighs up to 220 pounds (100 kg). Most dogs are born with a fairly long tail; the tail may be docked (cut) for appearance or cleanliness. The ears may be short or long, erect or hanging. In some breeds, the ears are cropped (cut) for appearance.
The dog has five toes on its forefeet and four toes on its hind feet. The innermost claw on each foot is called the dewclaw. The dewclaw is located slightly above the other claws and serves no apparent function. It is sometimes removed for cosmetic reasons. A dog's claws, unlike those of a cat, cannot be retracted.
There are five basic types of coats: short and smooth; wirehaired (hard and wiry); wavy, curly, or corded; long and silky with feathering (fringes of hair on the legs, ears, tail, or chest); and double. Dogs adapted to cold climates typically have a thick double coat. The outer layer is composed of guard hairs, coarse hairs that protect against ice and snow; the inner layer is composed of soft, woolly hairs that help prevent the loss of body heat. Coats are composed of either long or short hair, depending on the breed.
Most dogs have a thin, light coat during the summer and a heavier one during the winter. They shed the winter coat in spring and grow a new one in late fall. Some dogs do not grow winter coats and shed very little. Dogs kept indoors tend to shed all year. Some dogs have no hair at all.
A dog's skin contains glands that secrete oils onto the hairs, helping make the coat water-repellent. Dogs have sweat glands only on the soles of their feet, and pant to cool off. (Through the evaporation of saliva from the surface of the tongue, panting increases the rate of the loss of body heat.)
Dogs have very acute senses. The most highly developed are hearing and smell. Dogs can hear sounds at high frequencies, higher than the upper limit of human hearing. The sense of smell is the most important sense to a dog. Dogs can locate particular scents, follow them for long distances, and recognize them when reencountered. Taste buds on the tongue help the dog distinguish sweet, sour, salty, and bitter tastes.
Research indicates that dogs see colors, but cannot distinguish between the colors of the spectrum from green to red. Dogs have a relatively wide field of vision but a limited capacity to judge distances. A third eyelid, called the nictating membrane, is hidden behind the lower eyelid. It helps sweep away foreign particles from the eye.
A dog can feel pain, pressure, cold, and heat. Certain hairs on the body are sensitive organs of touch. Such hairs include the eyelashes, the hairs on the external parts of the ears, and the whiskers on the muzzle.
Dogs communicate by a variety of means. The sounds they make, such as barking, growling, and whining, can indicate a number of things, including aggression, excitement, fear, and submission. Dogs also communicate through physical means. When two unfamiliar dogs encounter one another, they typically sniff each other, holding their heads and tails erect, indicating interest and excitement.
In some cases, one of the dogs may act submissive by lowering its ears, tucking its tail under, and rolling over on its back. The other dog displays its dominance by tensing its body, keeping its ears and tail erect. It may bare its teeth, and the hair on its neck and back may stand on end.
Dogs also communicate with their tails. A tail held erect usually indicates cautiousness or excitement. A tail that is tucked under indicates fear or submission. Dogs wag their tails to indicate pleasure or to invite play.