First red, then grey and now black - watch out for the spread of new look squirrels of Cambridgeshire

Published: 24 April 2008 at 15:45

The frequency of ‘domineering’ black squirrel sightings is set to increase as researcher predicts further population increases.

A new study by Life Sciences researchers at Anglia Ruskin University has confirmed that a mutant gene from the grey squirrel has produced the growing colonies of black variants which are predicted to continue to populate the East Anglia region.  The story will be a feature of the BBC 1 ‘The One Show’ which will be broadcast on Monday 12 May.

In 1908 the squirrel visiting our Cambridgeshire gardens was likely to have been red, our beloved native species immortalised as Squirrel Nutkins in Beatrice Potter’s timeless tale. Fifty years later the squirrel raiding the bird table and irritatingly digging up our newly planted tulip bulbs would have been the bigger and bolder grey. By 1958, all red squirrels had vanished from the county, chased out by its more aggressive grey cousin.

Now, in 2008, it appears that the woods and gardens of Cambridgeshire

are being invaded by a new look squirrel, one with a glossy jet coat; and it seems that the black squirrels are now giving the grey ones a taste of their own medicine. But this time the challenge is coming from within the grey squirrel’s own ranks. The black squirrel is a colour variant of the normally grey Sciurus carolinensis. It is only a small genetic change that prompts the switch from growing a grey coat to growing a black one, yet the visual effect of this change is startling.

Anglia Ruskin University’s Dr Alison Thomas, from the Department of Life Sciences, became aware of their existence during the summer of 2004 when she saw one cross the road while she was driving in Comberton, west of Cambridge. 

It was this chance discovery that prompted the biologist to find out more about the rapidly expanding population of black squirrels in Cambridgeshire.

Dr Thomas recalls:

“I was so surprised to see my first ever black squirrel, and when I relayed the story to my family they were keen to know all about them and so I made it my mission to find out more.”

Explaining the importance of the study, she said:

“Many vertebrate species will occasionally produce black ‘melanic’ forms; including the leopard, tiger, jaguar, fox, rabbit, and coral snake. The dense black colour results from an imbalance in the pigmentation process.”

“The abrupt switch from a delicate silver toned squirrel’s coat to a dense black pelt occurs as a result of melanin pigments deposited in them during their development. The normally grey pelt of the grey squirrel results from a delicate balance between the deposition of two melanin pigments: the dark eumelanin and the paler pheomelanin. A single genetic mutation can upset this balance, resulting in the overproduction of the darker pigment and the birth of a black squirrel.”

“The relevant gene is known as the ‘melanocortin receptor’ and when mutated locks the hair cells into only forming the black eumelanin pigment. This is the gene that my student Helen McRobie and myself have found to be changed in the black squirrel.”

“We have been able to locate and sequence the gene producing the critical melanocortin receptor in the grey and black squirrel. Compared to the DNA sequence that is obtained for the gene from the grey squirrel, the sequence from the black squirrel had a chunk missing.”

“The genetic change is dominant - one dose of the mutant sequence and a black pelt results. However, one dose does not produce the glossy jet black coat. Both copies of the critical melanocortin receptor gene need to be changed to achieve this. Otherwise the ‘black’ squirrel has a brown-black appearance.”

At the time when grey squirrels were new to the United Kingdom, black squirrels started to be noticed on a Hertfordshire common. The first sighting was as early as 1912. Experts agree that it seems probable that, over many generations, the black mutation has slowly migrated northwards and eastwards into Cambridgeshire from the flourishing colony on Norton Common, on the northern outskirts of Letchworth. In 1942 black squirrels were sighted at Odsey, just on the Hertfordshire-Cambridgeshire border. Since then they have journeyed northwards, spilling over into Huntingdonshire in 1983, and eastwards, penetrating the Cambridge city boundaries some time in the 1990s, leaving many bemused citizens in their wake.

There is a precedent for the appearance of black squirrels among normally grey populations in their native territory of the eastern sea boards of their native USA and Canada. The wildlife artist John James Audubon had even depicted the Lousiana black squirrel in one of his 150 plates of The Quadrupeds of North America, published in 1849 as a sequel to his highly successful Birds of America.

There seems no doubt that black squirrel numbers are booming in East Anglia, though whether this is part of a general increase in squirrel numbers or something specific to the black kind is difficult to decide. It is also difficult to see that there is any specific survival advantage in being black or whether it is a case of the reverse.  There are numerous reports of the black squirrels being more aggressive which could be due to the presence of higher levels of testosterone.

On August 28, 1972, the Governing Body of Marysville, Kansas, USA passed legislation protecting the black squirrel (there is a fine of $25 for harming one), and making it the official town mascot.

Could Cambridgeshire follow suit? With an estimated three-quarters of its squirrel population being of the black kind, Girton would seem a prime location for declaring itself a black squirrel friendly zone or will these determined rodents simply become pests.

To date the limit of the black squirrel’s north-eastern migration seems to be Cambridge.  Perhaps in another ten years they might make it to the Suffolk or Norfolk borders.  Meanwhile do not be surprised if you see a yellow or, even, a white ‘grey’ squirrel.  These variants have also been observed in Hertfordshire.