A wolfdog (also called a wolf–dog hybrid or wolf hybrid) is a canid hybrid resulting from the mating of a gray wolf (various Canis lupus subspecies) and a dog (Canis lupus familiaris). The term "wolfdog" is preferred by most of the animals' proponents and breeders because the domestic dog was taxonomically recategorized in 1993 as a subspecies of Canis lupus, which includes gray wolves. (By extension, a wolfdog is also a member of Canis lupus.) The American Veterinary Medical Association and the United States Department of Agriculture refer to the animals as wolf–dog hybrids. Rescue organizations consider any dog with wolf heritage within the last five generations to be a wolfdog, including some established wolfdog breeds. In 1998, the USDA estimated an approximate population of 300,000 wolfdogs in the United States (the highest of any country world-wide), with some other sources giving a population possibly as high as 500,000.