New research has revealed the secret behind black squirrels – first spotted in Letchworth GC more than a century ago.

A study carried out at Anglia Ruskin University shows that a molecular switch in DNA is the reason some ‘greys’ have black fur instead.

Comet country still has black squirrels today, and Letchworth GC has been widely reported as a place where they were first spotted in 1912.

The research, led by the university’s Helen McRobie and published by the journal Federation of European Biochemical Societies (FEBS), reveals how a tiny molecular switch controls fur colour.

Grey squirrels inherit a normal pigment gene from both parents (genotype E+E+), while black squirrels inherit a gene with a missing piece from both parents (EB EB).

Ms McRobie, senior lecturer in biomedical science, said: “If you look closely at grey squirrel hairs, they are not actually grey at all but are a combination of white, black and orange stripes. However, the hairs of black squirrels have no stripes – they are just plain black.

“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 mutation is found in the black jaguar and Ms McRobie predicts that this is caused by the same thing – a broken molecular switch.

For more information about the research visit www.blacksquirrelproject.org