It may sound a bit nuts but black squirrels are the result of the grey squirrel interbreeding, a study suggests.
The two squirrels are the same species, with the only difference being their fur colour.
Now research published in the journal BMC Evolutionary Biology has shed new light on why some grey squirrels are black.
In 2014, Dr Helen McRobie, of Anglia Ruskin University (ARU), found that the black fur is caused by the grey squirrel having a pigment gene with a missing piece of DNA.
The new study has revealed that the faulty gene is identical to one found in the closely-related fox squirrel – a species native to North America.
Dr McRobie, senior lecturer in biomedical science, said: “Squirrels take part in ‘mating chases’ where a female squirrel is pursued by lots of male squirrels and eventually one male mates with the female.
“People have spotted ‘mixed species’ mating chases, with a mix of grey and fox squirrels pursuing a female.
“The most likely explanation for the black version of the gene being found in the grey squirrel is that a male black fox squirrel mated with a female grey squirrel.
“The fact black grey squirrels have become so common right across North America is possibly because black fur offers a thermal advantage, helping them inhabit regions with extremely cold winters.
“This may have contributed to the expansion of the grey squirrel’s range during the past 11,000 years, following the end of the most recent Ice Age, helping them spread further north into Canada.”
Researchers from the University of Cambridge and the Virginia Museum of Natural History in the United States tested DNA from grey and fox squirrels found across the United States and British Columbia, Canada.
They discovered that other signatures on the mutated gene are more closely related to the fox squirrel.
According to the study, this suggests it is highly likely that the mutation first arose in the fox squirrel and passed to the grey squirrel through interbreeding.
The black squirrels living in the UK are believed to have escaped from a private zoo, having been imported from the United States.
The first wild black squirrel was recorded in Woburn, Bedfordshire, in 1912, and they are now found across many parts of south-east England.