The dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The domestic dog 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 dog quickly became ubiquitous across world cultures, and was extremely valuable to early human settlements. For instance, it is believed that the successful emigration across the Bering Strait might not have been possible without sled dogs. Dogs perform many roles for people, such as hunting, herding, 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. Currently, there are estimated to be 400 million dogs in the world.