Of freckle-faced redheads and black squirrels

Don’t think they have much in common? Read on.

Like the highly variable hair color of humans, gray squirrels come in a variety of colors from all white fur with dark eyes (leucistic gray squirrel)…

leucistic gray squirrel

More about this guy in a previous post: “A white shade of tail

to normal gray fur with various brown-red highlights…

gray squirrel with red highlights basking on a tree trunk

Some gray squirrels have more red highlights than others — including all red tails like a red squirrel (different species)

to an all black-furred gray squirrel, like the one I saw trying to get on my bird feeder yesterday.

black-colored gray squirrel

Not only was he black, he was very big for a gray squirrel.

Fur color depends on the genetic expression of two color pigments that bind to the same receptor in the melanocytes (pigment cells) in the skin.  Pulses of expression of the red-brown pigment (called pheomelanin) mixed with the expression of a black-brown pigment (called eumelanin) as the hair grows and elongates produce the “agouti” or grizzled coloration of a single hair we recognize in the gray fur of the squirrel and many other animals (e.g., coyote or wolf).

agouti fur

However, if production of just one pigment is stimulated while the other is inhibited, coat color may be dominantly black, black-brown, red, or in the absence of both pigments — white.  Fur color can vary over the animal’s body as well, depending on local expression of color pigments.  For example, a bay-colored horse typically has red-brown hair on its body, neck and head, with dark brown legs, mane, and tail.

bay horse-wikipedia

Bay horse: Wikipedia

The same rules of genetic expression apply to human hair color — with the red hair variant produced by pheomelanin expression alone in the absence of eumelanin expression.  The gene for this condition is somewhat rare, occurring in only about 1/4 of the human population, but it is a recessive trait so you need both parents to be carriers of the gene for the “pheomelanin only” expression to show in the child. That’s a 1 in 64 chance, or about 1.5% of the population with true red hair.

princess merida of "Brave"

Princess Merida — an exceptional huntswoman.  From  Disney Wikia.

Now what’s really interesting about red-haired humans is that the genetic difference that produces their unique hair color is associated with lower pain tolerance and sensitivity to analgesics (e.g., morphine).  So think about that the next time you see an oddly colored animal and wonder what special powers they might possess.

black-colored gray squirrel

Another Ninja Squirrel focusing on raiding the bird feeder…

3 thoughts on “Of freckle-faced redheads and black squirrels

  1. Fascinating stuff Sue. I’ve seen grey greys and black greys in my village but never a leucistic grey.

    I’ve heard, albeit anecdotally, that the black greys are more aggressive then the grey greys but I had assumed that was an old wives tale. But now you’ve informed me that expression of colour genes is linked with other, apparently unrelated, traits, maybe there is a grain of truth in the rumour.

    Do you know what the genetic link is between redheads and enhanced sensitivity to opioids?

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