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Black squirrels are the most ubiquitous of Toronto’s urban wildlife. So it might surprise you to learn that, technically, there’s no such thing as a ‘black squirrel’. Part one in a two-part series investigating where these squirrels came from and why they’ve taken over.

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TRANSCRIPT:
Today we’re going to continue exploring the strange and surprising world of melanin. Last time we saw how it could help seagulls soar. Now we’ll take a look at another ubiquitous animal to find out how melanin has changed the course of their evolution and what it means for their future.

Toronto has been called the raccoon capitol of the world. They’ve become an official mascot. But those shy, nocturnal critters aren’t exactly the most visible of the city’s urban wildlife.

That honour probably goes to the black squirrel. Visit any patch of green space year-round and you’re likely to see one - at least one. No surprise, Ontario has the largest population of them in the world.

But what might be a surprise is that ‘black squirrels’ don’t actually exist. What you’re seeing is an eastern grey squirrel, identical in every way but colour to this guy.

Which leads to two questions. Where exactly did these black grey squirrels come from? And, wherever they came from, why do they seem to be dominating the squirrel scene in places like Toronto? What have they got that grey grey squirrels… don’t?

So let’s tackle the first of those questions. And it was only in 2014 that we got a handle on why, exactly, this black fur appears on some squirrels.

Eastern grey squirrel fur isn’t actually grey, either. Close up, you can see it’s a combination of black, white and orange stripes. The growth of each of those colours is controlled by hormones. Think of them like switches turning on and off each colour as the hair grows.

But in some squirrels, there’s a mutation that breaks one of those switches. So black turns on, but doesn’t turn off. The production of melanin, the black pigment, goes into overdrive. That’s why the ‘proper’ name for a black squirrel is a ‘melanized grey squirrel’.

What’s especially weird is that mutant gene didn’t develop in eastern grey squirrels. In 2019 scientists were able to trace it back to fox squirrels, who can also produce melanized individuals. So the black grey squirrels we see are an accidental result of interbreeding with fox squirrels, allowing it to hop species and spread. And spread.

Maybe the best example is in the UK. Melanized grey squirrels were accidentally introduced there when they escaped a private zoo. The first report of one in the wild was in 1912 and they’ve been expanding their territory about a half mile a year ever since. When they appear in a new area they can quickly take over the local ecosystem. In Cambridgeshire, they’ve completely wiped out the endemic red squirrels - there are only melanized grey squirrels left.

Or just look at their success in a place like Toronto where they clearly seem to be dominating the scene. And that leads to our next big question. What is it about these squirrels, seemingly identical in every other way to their grey-coated counterparts, that lets them thrive? There have been a few possible answers to that question over the years, and recent research that points to a surprising new conclusion. And that, all of that, is what we’ll be getting into next time.