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)…
to normal gray fur with various brown-red highlights…
to an all black-furred gray squirrel, like the one I saw trying to get on my bird feeder yesterday.
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).
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.
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.
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.