Dear Readers, I have always been fascinated by the diversity of individuals within a single species. To take one example: why do pigeons come in such a variety of colours? But something that I’ve never quite gotten to the bottom of is why the majority of Grey Squirrels in London, UK, look like the little chap in the photo above and yet Grey Squirrels in Toronto look, well, mostly like the animal below, when they both belong to the same species.
Not all the Grey Squirrels in Toronto are black, but from my observations I’d say that they make up at least 80% of the population. In London I have yet to see a black squirrel, although I do know that a tiny population exists. So why this difference between the two countries? To begin with, I had a little look at genetics. And to make things easier going forward, I’ll use ‘grey squirrel’ and ‘black squirrel’ to refer to the colour of animals of the same species, the Eastern Grey Squirrel (Sciurus carolinensis).
Two grey-coloured squirrels cannot make a black squirrel – each animal has two copies of the ‘normal’ pigment gene. A squirrel with one copy of the ‘mutant’ black gene and one copy of the ‘normal’ grey gene will be a brown-black squirrel, like the rather handsome creature below.
A squirrel with two copies of the ‘mutant’ black gene will be jet-black.
These colourings should not be mistaken for physical features that result from something that happened to the animal. For example, the black squirrel below, seen when I was with my friend Michelle at the Royal Botanic Gardens in Burlington, is jet-black with a white spot on the tail, which may be the result of injury or of some kind of skin damage. Whatever the reason, it gives the squirrel a rather attractive Cruella de Ville look.
So, the question remains: why are squirrels in the UK largely grey, and the squirrels in Ontario (and many other parts of North America) largely black? There are several theories.
One is that the black ‘mutant’ squirrels were actually the prevailing colour variation prior to the arrival of Europeans in the 16th Century. The animals’ darker colour may have helped to camouflage them in the densely canopied old-growth forests, but when these started to be cut down, the advantage shifted towards the grey squirrels in many areas. The remnant black populations, such as those in Ontario, may have survived because more of the northern forests remain, but also because black colouration absorbs and retains more of the sun’s heat, meaning that the animals can be more active and find more food during the long, cold Canadian winters.
As you will remember from previous posts, the Eastern Grey Squirrel is an introduced species in the UK, and the vast majority of animals are of the ‘normal’ grey colouration. If we think back to our discussion about genetics earlier, it is likely that any black animals seen are descendants of imported captive animals, rather than the mutation occurring simultaneously in the population. However, having said this, there are now large populations of black squirrels in Stevenage, Letchworth and Hitchin, where they are said to make up fully 50% of the resident squirrels. One further theory concerns female sexual preference: given a choice between a grey male squirrel and one with black colouration, it may be that the females prefer the latter, which would result, over time, in an increase in black squirrels over grey-coloured ones. This is not as unlikely as it sounds – the peacock’s tail, the long neck of the Giraffe-necked Weevil and the extravagant dances of birds of paradise are an indication to the female that the male is healthy, and hearty, and free from parasites – only the most hale of individuals can sustain such extravagant accouterments. So, does the glossy coat of a black-coloured squirrel give a clearer indication to the female that her partner-to-be is in the first glow of health, and will she therefore prefer him, all other things being equal? It will be interesting to see how the squirrel population develops here in the UK. Maybe, one day, London’s squirrels will be as diverse as the humans who surround them.