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Black wolves do not occur naturally. A 2008 study at Stanford University found that the mutation responsible for black fur occurs only in dogs, so black wolves are the result of gray wolves breeding back with domestic canines. The mutation is a dominant trait, like dark hair in humans, and is passed down to the majority of offspring. It is not entirely clear what benefit black fur has for the animals; they do not seem to be more successful hunters, but do show a marked improvement in immunity to certain infections.