Article for village newspaper, April 2010

The New Black

Red, grey – and now black. Just exactly what is going on with our squirrels?

I’m sure that many people are aware that here in Cambridgeshire our native red squirrel was replaced long ago by the grey squirrel, an American species (grey squirrels were introduced from America in 1892 and quickly spread; by 1958 there were no longer any red squirrels in Cambridgeshire). However, in the woods and gardens around Histon and Impington there is another shade of squirrel to be seen: black. And they may be taking over.

You may be forgiven for thinking that the black squirrel is a new species – it certainly looks strikingly different from our ‘normal’ squirrels. However, that is not the case: the black squirrels are actually a colour variant of the grey. The normal grey colour is a result of a balance between two versions of the pigment melanin, a darker one and a lighter one. In the black squirrels, a genetic mutation causes overproduction of the darker pigment, resulting in the black coat colour. This mutation is dominant, which means that any squirrel that inherits a copy of the mutated gene from either parent will be black, with squirrels inheriting a copy from both parents being a particularly deep, glossy black.

There has been some speculation about the origin of the black squirrels here in the UK. The first black squirrels were spotted on the outskirts of Letchworth as long ago as 1912, and since then they have spread northwards and eastwards, reaching Cambridge in the 1990s. Initially scientists thought that the black mutation had occurred here in the UK; however, more recently, examination of the gene involved has shown that the mutation matches that found in black squirrels in the United States. This suggests that the black mutation was introduced into the UK population by American black squirrels that escaped from zoos.

Whatever the origin of the colouring, the black squirrel is here to stay, and it’s spreading. Again, it’s somewhat unclear why this is so. It’s hard to see how the black colour gives any survival advantage over being grey, unless perhaps their dark colour allows them to absorb more sunlight in winter and therefore stay warmer and thus need less food. There are also many reports of black squirrels being more aggressive than grey ones, perhaps because their genetic mutation also means that they produce more testosterone than greys, and are therefore able to out-compete them for resources such as food.

Perhaps one day all our squirrels will be black, but until then, keep an eye out for them (Girton is a hotspot, and I’ve seen them many times in Histon wood and the Coppice) – they are, after all, an interesting local speciality!

Details about the resource used in compiling this article are available on request from pennyjreeves@gmail.com.