According to some estimates, nearly 40 percent of U.S. households own at least one dog—including pups like Duke here.

Dogs were probably the first tame animals. They have accompanied humans for some 10,000 years. Some scientists assert that all dogs, domestic and wild, share a common ancestor in the small South Asian wolf.

Today humans have bred hundreds of different domestic dog breeds—some of which could never survive in the wild. Despite their many shapes and sizes all domestic dogs, from Newfoundlands to pugs, are members of the same species—Canis familiaris. Although they have domestic temperaments, these dogs are related to wolves, foxes, and jackals.

Domestic dogs still share many behaviors with their wild relatives. Both defend their territories and mark them by urinating on trees, rocks, fence posts,and other suitable sites. These scent posts serve notice to other dogs that an animal is occupying its territory.

Many pet dogs also bury bones or favorite toys for future use, just as their wild relatives sometimes bury a kill to secure the meat for later feasts.

Dogs communicate in several ways. Scent is one method, another is physical appearance. Body position, movement, and facial expression often convey a strong message. Many of these signals are recognizable even to humans, such as the excited tail-wagging of a happy dog or the bared teeth of an angry or threatened animal. Vocally, dogs communicate with a cacophony of sounds including barks, growls, and whines.

Domestic dogs serve as more than companions; many earn their keep by working hard. Dogs herd livestock, aid hunters, guard homes, and perform police and rescue work. Some special animals even guide the blind—a poignant symbol of the dog's longstanding role as man's best friend.