Eastern grey squirrels come in a variety of color morphs, including grey and black. All individuals contain a gene, MC1R, that controls how much dark pigment is added as a squirrel’s hairs grow. When a tiny piece of DNA is missing from this gene, it boosts the production of dark pigment and makes the fur darker. It’s a small genetic difference but can have major implications for squirrel fitness and survival.
Black morph
150 years ago, this is what most squirrels in Eastern forests looked like. But after a century of hunting pressure and deforestation, this morph is now rare outside of cities. The black morph emerges due to missing bits of DNA on the MC1R gene.
This black coloration is considered adaptive in cold climates because it helps the squirrel stay warm. Black squirrels, however, are easier to spot in modern forests, which lack the complex structure (and deep shadow-y hiding spots) of the past.
Image: DaPuglet / flickr (CC BY-SA 2.0 DEED)
grAy morph
The gray morph is what most gray squirrels look like today. Unlike the black morph, the gray morph has two complete copies of the MC1R gene. The gray coloration blends in well with the vast younger forests that have regenerated across much of the eastern United States since the 1800’s. It is by far the most common color form of the gray squirrel.
Image: watts_photos / flickr (CC BY-SA 2.0 DEED)
white morph
This is the rarest color morph of all, and you never see this morph outside of cities. The white coloration is a mutation where the MC1R gene is completely non-functional, so no color is added to the fur as it grows.
White morphs only exist in urban areas, where predation pressure is low. In addition, humans often preferentially feed white squirrels, providing a competitive advantage when living alongside humans.
Image: White Squirrel Research Institute (CC BY-SA 2.0 DEED)
coats of many colors
As you can see, gray squirrels come in many different colors …usually gray but also black, white, auburn, blond...but why?
Black and gray squirrels are only different based on one gene. Alleles are variations on DNA sequences at a specific location on a chromosome (usually different forms of the same gene). The allele for squirrel fur color has two different possible sequences – one that codes for gray fur and one that codes for black fur. Each squirrel has two copies of this allele. If a squirrel has two copies of the gray allele, then it will be gray, but if it has either one copy of the gray and one of the black or if it has two copies of the black allele, then it will be black. The gray type is actually genetically recessive, which is somewhat counterintuitive since it is the more common genotype these days!
The species that we commonly call the eastern gray squirrel was, according to historical documents, originally mostly black. One historical account says: “Grey-squirrels were rarely met with before the commencement of the [20th] century, though they are now quite numerous; while black-squirrels, though now nearly if not quite extinct, were then found in large numbers” (John M. Weeks, History of Salisbury, Vermont. Middleburg, 1860, p. 112). “Squirrels in Early Wisconsin” published by A.W. Schorger in the Wisconsin Academy of Sciences, Arts and Letters is an excellent reference on this topic.
There is some evidence that the black morph has greater capacity to stay warm in cold conditions, and the black morph does appear to be more common in colder areas. But this probably doesn’t explain why the black morph appears to be more common in urban areas because cities are typically 2-3 C warmer than surrounding rural areas (known as the “urban heat island” effect).
Absolutely. Black squirrels were often introduced to cities as novelty. But this might not explain why black squirrels have remained common primarily in cities. Squirrels are very abundant in urban and rural areas and have great capacity to move long distances. Black squirrels were introduced to cities many decades ago, so there’s been plenty of time for them to move into rural areas. In addition to mapping squirrel color morphs, we’re conducting studies to more directly measure natural selection on squirrel color morphs to make sure their geographical distributions are not simply the outcomes of chance introductions.
Because squirrels are so visible and recognizable they are a great way to get non-scientists interested in evolution and involved in scientific studies. The simple genetic difference between black and gray squirrels also provides an easy way (relatively speaking!) to watch how our rapidly changing environment is affecting evolutionary processes within our own lifetimes. You don’t need a fancy machine to sequence DNA to watch this bit of evolution in action, just your own two eyes.
