The black squirrel coat is not simply a fashion statement, it is a genetic mutation. This is not a life threatening change, and is caused by a deletion of a specific gene which codes for the white and orange striping which gives the squirrels its grey coloring. The gene is found in the white blood cells and has been linked to higher aggression levels in other species. The hormone allows the black color to be switched off and the striping to come through, but it is never turned off in these squirrels. Mating can occur naturally with no harmful effects to the offspring. The "black gene" is a mutation acting as a recessive gene therefore no mutation will display unless both parents have the mutated genes. The occurrence is on the rise, in the United Kingdom. The first black squirrel was spotted in 1912, and today there has been over 6,100 spotting and a total grey squirrel population of 2 million.
This is a very interesting fact and a unique mutation. It leads me to believe that this may actually be a beneficial mutation for the squirrel, since the numbers continue to grow. Who knows some day we may be viewing the grey squirrel as the one with the mutation!