Red squirrels, grey squirrels and a poxvirus

My fellow blogger nahrvalur tells us that in a forest park in County Down, the native red squirrel (Sciurus vulgaris) is recovering from a population crash associated with squirrel pox.

Since the North American grey squirrel (Sciurus carolinensis) was introduced here in the late 19th century, its populations have soared while the red squirrel has become rare. Is that because of the squirrel pox? Maybe so. Squirrel pox kills the red squirrel whereas the grey squirrel can carry it without harm. So the greys may have infected the reds, causing the reds’ population crash. It’s a classic story of an invasive species bringing in an infectious agent.

But I’ve also heard other hypotheses. Some people say that the greys outcompete the reds. I’ve heard it said that the two squirrel species prefer different kinds of woodland. Conifers for the red, broadleaved trees for the grey. Whatever the reason, it’s good to know that the reds are coming back in Co Down.


About argylesock

I wrote a PhD about veterinary parasitology so that's the starting point for this blog. But I'm now branching out into other areas of biology and into popular science writing. I'll write here about science that happens in landscapes, particularly farmland, and about science involving interspecific interactions. Datasets and statistics get my attention. Exactly where this blog will lead? That's a journey that I'm on and I hope you'll come with me.
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16 Responses to Red squirrels, grey squirrels and a poxvirus

  1. It is a combination of all those factors 🙂

    The squirrel pox is of course, a main factor, the reds get sick and die, whilst the greys just transmit it.
    Whilst this of course will cause the population to drop, the other factors come into play.

    The carrying capacity for grey squirrels is higher (8 per hectare) than for reds (1 per hectare, or 0.1 per hectare in coniferous forests). What this means is that each area of forest can support a larger population of greys than reds, or, looking the other way, if each area of forest can only support 1 red squirrel, and a pair of greys move in, its not looking good for the red.

    This is due to the grey squirrels more varied diet, it can better process certain nuts (those with high tannin content) than reds, which means they can choose from more food sources, bizarrely, it means they are better at digesting acorns than reds. This means they put on more weight, increasing their winter survival. They also produce more young per litter, again adding to the resource pressure on red squirrels, as the decreased mortality of greys, and the increased fecundity means their population naturally grows at a higher rate than the reds.

    They spend more time foraging as well on the forest floor if I remember correctly, which means the availability of food increases again, as red squirrels do not utilise the fallen nuts so much.

    Finally, the red squirrel does better in oak forests etc, and here its habitat is reduced, or fragmented. If a road, or other break in the forest is put in, then the squirrels range for foraging is cut, as whilst they can cross roads of course (although that increases their mortality), they will effectively view their territory as reduced if a break is introduced, and so will not be able to access as much food as possible.

    As far as I know, habitat destruction, whilst a major problem in the past has been reduced now, as there are more interests in preserving woodlands than in the past.

    From my understanding, the competition for resources alone would leave the grey squirrel the winner out of the two, and then the pox on top of that is just an exacerbating factor which wipes out reds fast.

    (Can you tell I have been doing population dynamics and life tables far too much!)

    • argylesock says:

      You’ve just answered all my squirrel questions! Thank you!

      This is what comes of writing a very rushed post to my blog, which I believed to be one of my weaker efforts. I must be a poor judge of blog quality because you’ve written a lot of very interesting words, so has Waldo (below) and I’ve e-met evilsquirrel13.

  2. I hope your reds come back in great numbers. They are an interesting squirrel. We ( here in upstate NY) have an abundance of greys and quite a few reds also. I wish to see more black squirrels (don’t ask me why, I just like them). We are seeing some every once in a while. I understand they are prevalent in Canada. I have visited the Bronx (NYC) since I was a kid (1947) and always enjoyed seeing black squirrels there. Does anyone know why they would not expand beyond the Bronx?

    • argylesock says:

      Are your red squirrel S. vulgaris like ours? As for your black squirrels I’d never heard of that species. Do tell, if you want to.

      • This seems to fit my concept of them. Picture is true also. From what I see on Nahavular’s (SIC?) blog and other places your reds look just like our reds. I have no credentials to comment beyond that.

        • argylesock says:

          She’s nahrvalur, aka Ann Novek, blogging at Confused? So am I, but she writes a good blog.

          • Yes, Ann Novek. I follow her but she has not posted for quite some time. I am not sure what the official term fro “varient” means but if it means that it is the same as a grey (close DNA) but with a different hue then I understand. I just like the Blacks for no particular reason other than they look sleek and pretty.

            • argylesock says:

              There must be some mistake. Ann Novek’s blog ‘With the sky as the ceiling and the heart outdoors’ is v busy with several posts every day.

              By ‘colour variant’ I mean that the black squirrel is the same species as the grey (S. carolinensis), just with a difference in one gene. It’s analagous to humans having different hair colours. The variants of S carolinensis can interbreed whereas the grey and the red (S. vulgaris) can’t. You probably know that the ability to produce viable offspring is one of the criteria used to define species.

              I agree that the black squirrel looks pretty. Maybe it’s well suited to particular environments – better camouflage or something.

        • argylesock says:

          It seems that the black squirrel is a colour variant of S. carolinenis, according to your link and also the one skepticalsquirrel linked to downthread. Or perhaps I should call it a ‘color’ variant 😉

      • They are these ones as far as I know:

        It seems to be a genetic mutation in greys (or vice versa, as greys have 2 copies of “normal” and blacks have either 1 or 2 copies of the “mutant”), like hair colouring in people.

  3. sharechair says:

    I live in PA and all we have are greys. But when I visit my son in Washington DC, there are many black squirrels there and in the Maryland suburbs of DC. I’m always a little startled to see the black squirrel since we don’t have any where I live.

  4. Hi there. I live in Toronto, Canada, and earlier this year, my wife, Jean, and I were in Ireland where we came upon the rarely seen Red Squirrel. They actually look somewhat like our Canadian Red Squirrels, but boy, do they have long ears! We were shocked to learn that the United Kingdom, and Ireland’s Red squirrels are contracting the pox virus from Grey Squirrels, and dying. As we know, Grey Squirrels originally came from Canada. We have posted some of our pictures and video for anyone interested at:

    • argylesock says:

      Hello and thank you for the photos! Please bear with me as I’m not v well today and nor is my internet connection, but I’ll reply properly when I can.

    • argylesock says:

      I’ve now looked at your photos. Here’s the comment I made, reposted here on my own blog.

      What fantastic photos. Thanks for sending me the link to them. Such beautiful views and such adorable animals. I’m well jel that you see chipmunks and bears!

      The red squirrels you were lucky enough to see in Ireland will have been Sciurus vulgaris, not Tamiasciurus hudsonicus as you have in Canada. It seems that T. hudsonicus survives the poxvirus as Sciurus carolinensis does (though perhaps not by the same mechanism: I don’t know) whereas S. vulgaris dies. Btw the S. vulgaris ears that look long aren’t really long. They just have long tufts of fur on top.

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