The silver fox is a melanistic form of red fox. Melanism is when the animal's coloring is much darker than normal. Interestingly, fox squirrels, who have brownish body fur and reddish tummy fur also have a melanistic form that resembles this. I had a "black" fox squirrel that I hand raised. He lived to be 12 years old, a full ten years longer than he would have in the wild.