Why Are There So Many Black Squirrels In Toronto?

Описание: In the second part of our series on black squirrels, we tackle the big question: if they’re a result of a rare, mutated gene, why do they seem to be by far the most common in places like Toronto?

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TRANSCRIPT:
Last time, we were introduced to the ubiquitous black squirrels of Ontario - or rather, melanized eastern grey squirrels, with a mutated gene that causes their colour.

And we saw how, once these squirrels appeared, they could start to overrun an ecosystem. So today the question is why?

It’s obvious strolling through Toronto - black squirrels are everywhere. In a study in Syracuse, New York, 65% of observed squirrels were the melanized variety. But if they’re a result of a rare mutated gene, how’d they end up in the clear majority? There’ve been a few theories, and only recently some solid and surprising answers.

The simplest explanation is that the gene in question is dominant. That would mean it gets inherited by offspring more often than not. Over time, melanized squirrels would outnumber grey ones.

But studies have shown it’s actually incompletely dominant. That means there are technically three possible colour outcomes - grey, black, or dark brown - and it means gene dominance doesn’t explain their numbers.

In fact, it gets even crazier. It’s estimated that across all eastern grey squirrels, only 1 in 10,000 are melanized. But…65% of squirrels in Syracuse are. And they seem to be a clear majority in Toronto. So they must have some huge advantage, but what is it?

For a long time people thought melanized squirrels were more aggressive. The idea was, melanin has a link to testosterone production. That could make melanized squirrels stronger and faster, outcompeting non-melanized squirrels through sheer aggression.

But that testosterone link is tenuous. And studies seem to show no behavioural difference between the two colourations.

So if melanized squirrels are behaviourally identical to non-melanized squirrels, what’s left?

Take a look at this map of squirrel sightings by colour. Something probably jumps right out at you. The further north you go, the more likely you are to see melanized squirrels.

And what do we know about melanin? As a black pigment, it absorbs almost all light and a lot of heat. Melanized squirrels can generate 11% more body heat and retain it 18% better than grey-coloured squirrels. Perfect for colder northern climates. Ontario is overrun with melanized squirrels because it’s at the northern limit of grey squirrel range, where that extra heat retention tips the scales.

So, mystery solved? Not quite. Because grey-coloured squirrels are still by far more common in rural or wild areas. It’s only really in urban settings that melanized squirrels have a big advantage. That can’t be epxlained by heat retention. So what is it about urban environments?

In a word: camouflage. But it’s more complex than that. In a forest shared with predators, you’re better off grey, blending in with the trees.

In urban environments, there are far fewer predators, and that camouflage advantage is nullified. Unless… you count these predators.

A study on red squirrels in 2021 showed that almost half of all their deaths were by car collisions. So, it’s a pretty significant factor. The difference is, with most predators you want to be invisible. With these predators, you want to be as visible as possible. A grey squirrel can blend right into the asphalt. But a melanized squirrel is much more likely to be seen and avoided.

And if you think that’s a little far-fetched, I’ve got numbers to back it up. A 2019 study showed that in an area where 33% of grey squirrels were melanized, they accounted for only 9% of vehicle deaths. That means they’re ten times less likely to be hit by a car than grey-coloured squirrels.

So of all the possible reasons why black squirrels might dominate places like Toronto, it may come down to the simple coincidence of sticking out better against asphalt.