Black squirrels and that missing gene

Black squirrels and that missing gene
Black squirrel in autumn at Tylee Marsh, Rosemere, Quebec, Canada.

A ‘scurry’ of American grey squirrels was presented to the bride on the occasion of her marriage at Castleforbes, County Longford, in 1910. The descendents of these aliens are thriving now east of the Shannon. The species had been introduced to Britain 40 years earlier; having these exotic creatures on your country estate soon became a fashion statement.

A black squirrel was seen in Woburn, Bedfordshire, in 1912. Three decades later, the dark variety had become relatively common in three midland shires. There are now thought to be about 25,000 black ones nationwide, outnumbering greys by three-to-one in some areas.

Fears that black squirrels will completely eclipse grey ones are unfounded, but the emergence of a black sub-group raises some intriguing questions. What, for instance, are their origins and why are they doing so well?

Many animal species have a dark form. The ‘black panther’ is a ‘morph’ of the leopard. Irish rabbits are black occasionally and an all-black magpie turned up in South County Dublin a few years ago. ‘Melanistic’ red squirrels have been reported from the Isle of Skye.

The peppered moth is a celebrated case. Lightly-coloured moths blend in with lichens on tree barks. During the Industrial Revolution, however, a dark form of the moth emerged. Replacing the light-coloured one in urban areas, it offered better camouflage against dark soot-covered lichen-exterminated backgrounds. When the ‘Satanic mills’ closed in more enlightened times, pollution was reduced and the light variety returned.

Prior the arrival of European settlers, black squirrels were common in North American forests. Squirrels were hunted for their meat and pelts; like the dark variety of the peppered moth, black fur offered better camouflage against predators and hunters.

When the Europeans cleared the forests, grey fur became advantageous; it rendered an animal less conspicuous in the newly-created, more open, habitat. The black variant is commonest, today, in the north of the continent where the woods are denser and darker. Keeping warm in winter may also be a factor; black objects absorb heat more readily than light-coloured ones.

Five years ago Helen McRobie, of Anglia Ruskin University, showed that a missing piece of DNA is responsible for the black pelage in grey squirrels. The faulty gene is found also among fox squirrels, another American species; there are isolated communities of the black fox squirrel in the southern United States.

Cross-breeding with fox individuals may have led to the emergence of the black form among greys. Did the ancestors of Britain’s black animals escape from zoos which had imported them as exhibits? Having dark fur or plumage comes at a price.

Additional nutrients are required to create the pigment. Shortages of particular foods, for example, can give rise to white patches in the feathers of Irish blackbirds. The snow goose has white and blue ‘phases’. Feathers are naturally white but ‘blue-phase’ goslings need to have pigment added; with extra food needed to develop the colour, they are slower to fledge than white ones.

But why are black squirrels prospering in Britain now? They are not hunted there and they have few natural enemies. With temperatures rising due to climate change, there seems no particular body-heating advantage to being black.

The black variety has not been recorded in Ireland. Presumably, none of the individuals released here 100 years ago lacked the missing gene.

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