Dear Readers, a few years ago I did a blog about the colouration of squirrels in Toronto, where a surprising number of grey squirrels (Sciurus carolinensis) are black, whereas this is very unusual in UK grey squirrels. So, I was very interested to read that scientist Bradley Cosentino at the Hobart and William Smith Colleges in Geneva, New York state had been doing some studies into why this might be. Cosentino had already shown that, in rural areas, predators spot the black squirrels more easily than the grey ones, which explains why the grey morph is more common in woods in the countryside.

However, in the city of Syracuse in New York State, black squirrels make up half of the population, and Cosentino wanted to find out why they were so much more common in an urban area. He had a hunch that it might have been connected to roadkill – we rarely seen squirrels killed by traffic in the UK, at least in cities (maybe everything is moving too slowly), but it’s a very common cause of death in US cities. After 50 roadkill studies around Syracuse, involving more than 100,000 photos of dead squirrels (the joys of being a scientist), Cosentino found that within 10-12 kilometres of the city, the black squirrels were about 30 per cent underrepresented. It’s already been found that drivers can spot a black squirrel faster than a grey one, which may mean that they’re easy to avoid, and this is the current working hypothesis.

However, this correlation doesn’t yet prove that this is why black squirrels are so much more successful in urban areas. It could also be that black squirrels have some other survival advantage, such as being more careful about crossing the road, or even that they’re faster than their grey conspecifics. Still, it’s an interesting hypothesis. I would love to see a similar study for Toronto.

Black squirrel in a patch of scilla…

You can read the scientific paper here.