Gray and black squirrels are two color morphs of the same species, Sciurus carolinensis. Historical documents tell us that until about 150 years ago, black squirrels were much more abundant. (See Squirrel FAQ). Yet now black squirrels are rare. Why are these two morphs - gray and black - more common in some areas than others? Why have black squirrels decreased? Despite how common these squirrels are few have delved into these questions.
We need to spend some time on genetics because only a tiny genetic difference separates black and gray squirrels. Here's how...
All squirrels contain a gene 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 production of dark pigment and makes the fur darker. So gray squirrels have two copies of the complete gene. When a squirrel has a single copy of the gene with the small piece of DNA deleted, or two copies of that altered gene, its fur is mostly black. Technically, black squirrels are melanic.
That's pretty much it. A tiny genetic difference, yes, but being black or gray has many consequences if you are a squirrel! (See Evolution FAQ for more details)
Variation in squirrel color could be an adaptation to a changing visual environment. Squirrels that don't blend in well with their backgrounds get killed more often by predators. What kills squirrels? Mainly hawks, foxes, and dogs. Humans also kill many squirrels. Squirrel hunting has long been popular in rural areas and remains so today. But you can't hunt in cities, towns, or near houses, so maybe that's partly why we find black squirrels often in cities? At the same time, if black squirrels are more conspicuous than grays where they are hunted then might hunters have removed the darker individuals from squirrel populations of rural areas?
There could be other forces at work! Originally forests of eastern North America were old growth with lots of pockets of darkness associated with enormous tree trunks, many vertical layers within the forest (strata), and presence of coniferous trees with dark foliage. In these darker conditions black squirrels could remain well hidden. Now those forests have been cut repeatedly throughout the gray squirrel's range. How many patches of original, old growth forest remain near you? Likely very few, if any.
The forests that have since re-grown throughout the gray squirrel's range are secondary forest with spindly trunks of mostly deciduous trees whose trunks and branches are much lighter-hued. In such secondary forests might gray squirrels be less visible and less vulnerable than black ones? Might this be particularly true in the late fall, winter, and early spring, when leaves are off, and squirrels are most vulnerable to predators (natural or human)?
Are gray squirrels harder to detect in second growth forests? What do you think?
Interestingly, urban ecosystems also have many darker aspects to their visual environment. Deep shading is caused by buildings that block the sun's penetration as well as many planted conifer trees. In a strange twist, these changes may make urban areas more like an old growth forest from a visual perspective. So independent of hunting pressures might black squirrels also be better able to hide from natural predators like hawks and dogs in urban areas? (see below)
Are black squirrels harder to detect in old growth or urban forests? What do you think?
Maybe you have another hypothesis? (See Evolution FAQ)
By mapping out where black and gray squirrels occur in our cities, towns, farms and forests throughout the species' range you can contribute to an important scientific discovery about evolution in action in our own backyards and neighborhoods. All you have to do is contribute records of the number of gray and black squirrels in your neighborhood. You can also contribute by measuring whether the squirrels' camouflage actually provides them with a selective advantage in different visual environments in our "squirrel hunt" exercise.