The domestic dog (Canis lupus familiaris), is a subspecies of the gray wolf (Canis lupus), a member of the Canidae family of the mammilian order Carnivora. The term “domestic dog” is generally used for both domesticated and feral varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in human history. The word “dog” may also mean the male of a canine species, as opposed to the word “bitch” for the female of the species.
The present lineage of dogs was domesticated from gray wolves about 15,000 years ago. Though remains of domesticated dogs have been found in Siberia and Belgium from about 33,000 years ago, none of those lineages seem to have survived the Last Glacial Maximum. Although mDNA testing suggests a split between dogs and wolves around 100,000 years ago, no specimens prior to 33,000 years ago are clearly morphologically domesticated dog.
Dogs’ value to early human hunter-gatherers led to them quickly becoming ubiquitous across world cultures. Dogs perform many roles for people, such as hunting, herding, pulling loads, protection, assisting police and military, companionship, and, more recently, aiding handicapped individuals. This impact on human society has given them the nickname “Man’s Best Friend” in the Western world. In some cultures, dogs are also source of meat. In 2001, there were estimated to be 400 million dogs in the world.
Most breeds of dogs are at most a few hundred years old, having been artificially selected for particular morphologies and behaviors by people for specific functional roles. Through this selective breeding, the dog has developed into hundreds of varied breeds, and shows more behavioral and morphological variation than any other land mammal. For example, height measured to the withers ranges from 6 inches (150 mm) in the Chihuahua to about 2.5 feet (0.76 m) in the Irish Wolfhound; color varies from white through grays (usually called “blue”) to black, and browns from light (tan) to dark (“red” or “chocolate”) in a wide variation of patterns; coats can be short or long, coarse-haired to wool-like, straight, curly, or smooth. It is common for most breeds to shed this coat.