Of squirrel tails and coat colors

In paying more attention to what’s going on in my own backyard these days, I’ve discovered there really is a lot of variation in Gray Squirrel looks and behavior, enough to kind of be able to tell them apart.  But first, let’s examine the obvious:  gray squirrels come in a variety of colors, and I’ve just seen my first blonde!

A novelty color variation in a “Gray” squirrel I’ve never seen before — a blonde!

But there are various mixtures of gray, brown, red, even black tones in the true “Gray” Squirrel coat.

Pretty red toes and red highlights in its fur and tail make this Gray Squirrel stand out.

Now we know that variation is the “spice of life”, and speaking biologically, it’s most often the thing that drives natural selection and evolution in species.  But inquiring minds want to know:  how does this variation in coat color in squirrels happen?

First, gray fur isn’t just a gray color — the grizzled look of the gray fur is achieved by a complex process of regulated pigment deposition in hair over time.  This is best seen in a comparison of the tails of two “gray” squirrels.

The individual hairs of reddish and grayish tails actually have a lot of color variation along their length. 

Gray hair in squirrels is actually a combination of brown pigment (eumelanin) deposition in the hair interspersed with a little yellow/red pigment (pheomelanin) deposition that is turned on and off as the hair grows so that there are spaces where no pigment is deposited and the hair is white.  Complex indeed — imagine the number of genes controlling that process: genes to stimulate specific pigment production, genes to regulate when that pigment is produced; genes to promote the other two sets to turn on and off; genes to control where in the body all the rest of those genes turn on and off. And so on…

The squirrel tail on the left (above) must have had more of the yellow/red pigment deposition than the brown pigment as the hair was growing, so its tail looks significantly redder.  Both tails have frosty tips (white = no pigment deposition), but the red tail has much longer white tips, meaning the pigment production was stopped for a longer period of time during tail hair growth.

But what about Blondie? How would this coat color be produced?

I bet you know enough about how color is produced and controlled in squirrel fur to answer this question now.  Please take a stab at it in your comments.

As an added challenge — how can we explain the fur color in this guy?  Note not just the dark color, but lack of pale whitish fur on its underside, a typical characteristic of Gray Squirrels.

A black Gray Squirrel acrobat

You might think at first glance that this was a different species, but it isn’t. Black-furred Gray Squirrels are common in some areas of the UK and the US, more common than the gray variety, yet the genes coding for black color are recessive to those that produce the “wild” type gray pattern and so the black color should be rarer.

Which brings us back to the role of variation in evolution, and it seems that black-furred Gray Squirrels have a significant thermal advantage in cold climates (as you might expect because of black color’s heat absorbing quality) and they also seem to be quite well camouflaged in the dark coniferous forests they inhabit.

As for Blondie, there are hawks and owls in my backyard, and it will be interesting to see how long she/he sticks around.

3 thoughts on “Of squirrel tails and coat colors

  1. Not many people take the time to recognize such variation. They are missing out. Individual animals also have individual personalities. Rarely do we take the time to notice unless it’s a pet.

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