Researchers from Anglia Ruskin University in Cambridge believe they have worked out why black squirrels are black.
The experts say they are grey squirrels that have a faulty gene so their their individual hairs do not have white or orange in them.
Grey squirrels actually have fur made up of white, black, and orange hairs which give them their grey appearance.
Helen McRobie, of Anglia Ruskin University, said: “Our research shows that one hormone turns the switch on to make black fur, and a different hormone turns the switch off to make orange and white fur. So in the grey squirrel, as the fur is growing, the switch turns on and off to make the stripes.
“However, in the black squirrel, because there is a piece of DNA missing, a piece of the switch is also missing.
"The first hormone that switches it on still works and black fur is made, but the second hormone that should switch off, actually switches on as well. The off switch fails and the black fur continues to grow.”
A similar genetic mutation is found in the black jaguar.
The first black squirrel in the UK was spotted in Woburn, Bedfordshire, in 1912.
Over the last two years, 6,100 recordings have been submitted by the public to the Black Squirrel Project which show they have now spread as far as South West England, Wales and Southern Scotland.