I’ve been riding the trail from Northampton to UMass frequently for six years or so and for the first time I’ve noticed a few black Eastern gray squirrels along the way. That sounds goofy, but the species is called the Eastern gray squirrel and, well, these ones are black.
Before the incursion of Europeans into North America, most gray squirrels were in fact black (Penn State University’s Virtual Nature Trail). A black was advantageous for avoiding hawks and other winged predators as the color allowed blending into shadows when looked at from above. As humans proliferated, we became the more dangerous predator. Because we looke up from the ground – gray became more advantageous. But both black, gray and a sort of brownish in between squirrel existed all along.
Like the squirrel, most familiar wild animals have predictable coat colors. There is some variation to be sure, but in both predator and prey species the pressure is to blend into the background. Brightly colored mice don’t last long. Birds are a bit of an exception as many of them display bright colors. They do have the advantage of flying away. They’re also descended from dinosaurs and we all how ostentatious they were.
Domesticated animals are dramatically different. Cats, dogs, cows, and horses experience little selective pressure based on their coat color. It is true that cats probably did better for thousands of years when they were good mousers, but these days we look for cuter.
In a famous experiment conducted over five decades in Siberia, Russian silver foxes were selected for friendliness. Friendly foxes in each litter were bred with other friendly pups. Not surprisingly the foxes became more dog like after several generations. They are more than tame, they’ll lick your face and let you cuddle them. Astonishingly other traits cropped up. Their tails curled, their ear flopped over and their coats started getting spots: they began to look like dogs.
White spotting is extremely common in domesticated animals and virtually non-existent in the wild. The coat pattern is called piebald, apparently after magpies, and you see it in pigs, dogs, cats and horses. The etiology at the cellular level is the same in all of the animals. Melanocytes, the cells that make melanin (the dark color) fail to migrate from the neural crest (a cell population along the spine of the embryo that gives rise to many other cells) to their proper place in the embryo (Trut American Scientist, 1999).
It has been argued that domestication actually is selection for a whole slew of traits that trace from a delay in migration of various cells from the neural crest; domesticated animals are slightly delayed in many ways. The traits that stem from these delays include floppy ears, curled tails, calmer disposition, a shorter snout and spotty coats (Wilkins, Wrangham and Fitch Genetics 2014).
I have several domesticated animals in the household. The chickens are more “tame” than domesticated really, but both cats are arguably domesticated. The boarders are neither tame nor domesticated, but they don’t have spotty coats. One of the cats would never survive in the wild as she’s far too fat. The other shows piebald spotting. She’s the least domesticated cat I’ve ever had.
What’s really crazy to me is that we also have a copy of the gene that leads to piebald spotting. In some humans it is shut off and causes white patches, this is relatively rare. In others it become overly active causing more cell migration when we don’t want it which leads, ultimately, to tumors.
