The link between red squirrels and pine martens

As you'll know if you've been following my blog for a while, there are two local animals which I see an awful lot of, despite them being rare in most of the UK. These are red squirrels, Sciurus vulgaris,(which there are loads of around here), and pine martens (which are hard to spot, but I have lots of trail camera footage of them).

Despite them looking slightly alike in the photos above, and both living in trees, they are completely unrelated. Squirrels are part of the rodent family (which have two sharp front teeth, with an orange outer edge), which also includes mice, voles and beavers. Pine marten are bigger, about the size of a small cat. but are from the mustelid family which includes weasels, otters and badgers. But there's one unusual link between them which not a lot of people know about.

Before I tell you that, I'll explain more about why red squirrels are so rare. This might seem unusual since they are fast, can run and climb away from predators, and are big enough to not be preyed by many things - at least compared to mice or voles, although birds of prey will take they if they have a chance.

The problem that red squirrels have is not from other animals - but from other species of squirrel ! There are 140,000 squirrels estimated to be in the UK, and almost all of these are in Scotland. That seems like a lot until you realise that there are 2,500,00 grey squirrels (Sciurus carolinensis) in the UK ! The red squirrel is the native species, and the grey squirrel was introduced from America, and it's numbers have grown, forcing out the red squirrels.

Red and grey squirrels

This gets weirder when you remember that grey squirrels and red squirrels don't actually fight each other. So why do greys force out the reds ?

One reason is that grey squirrels are much fatter, meaning they have food reserves to survive cold winters. Winter is the worst time for the thinner red squirrels. About three-quarters of red squirrels die in their first winter, and about half die in winters in subsequent years. Grey squirrels have a better survival rate because they have four times as much fat stores.

But that in itself doesn't explain why Scotland, which has colder winters, has much more many red squirrels than England.

One other problem is that grey squirrels carry a virus, the squirrelpox virus, which the grey squirrels are immune to, but which causes horrible swellings and scabs on the red squirrels eyes, mouth and paws. The disease is relatively new, having first been recorded in the UK in 1980s (although maybe we weren't looking for it before then).

So grey squirrels are bad news for red squirrels. I can find both round my village, although there are far more red squirrels, and often traps like these are set to catch grey squirrels:

So what has this got to do with pine martens ?

Pine martens like all mustelids are pretty tough. They eat almost anything - and that includes squirrels which are smaller than them. But you often find pine martens and red squirrels living in the same area - but hardly ever pine martens and grey squirrels. Why is that ?

This article by George Monbiot earlier this year was the first one I read which made me think about pine martens actually keeping grey squirrels away. Pine martens are tough and territorial. They will chase squirrels - but red squirrels can hide at the end of thin branches, and red squirrels also stay int he trees much more than greys. Greys, being bigger and fatter and about the size of a pine marten have nowhere to hide.

I thought this was a bit of a bonkers theory, since red squirrels are mainly out in the daytime, and pine martens are nocturnal, but there is lots of evidence to show that pine martens are a great way to keep grey squirrels away. This is from the same George Monbiot article:

It’s not just that pine martens are eating the squirrels: they are terrifying the living daylights out of them.
If grey squirrels have no defences against martens, they must spend much of the time they would otherwise have spent feeding trying to avoid them. They are likely, metaphorically or perhaps literally, to spend so much time looking over their shoulders while they should be foraging during the summer that they don’t accumulate sufficient fat to get through the winter. The pine martens are starving them out.

So in a weird roundabout way, it seems that pine martens actually protect red squirrels. by keeping grey squirrels away, which also explains why I am lucky enough to have not one but two relatively rare species around my village !

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