You're Not Seeing Things, Squirrels 'Morphing' From Gray to Black

A black squirrel.

It’s that time of year when squirrels seem compelled to cross the road. They fill their mouths with nuts and suddenly lose all sense of caution in their rush to stash those nuts in hidey-holes on the opposite side of the highway.

Having just had a close encounter with one such manic creature, barely avoiding him with my front wheels as he skittered to and fro, my thoughts turned to the changes in squirrels as I have known them during my decades in northwestern Connecticut—specifically the change in the color of the ubiquitous rodents.

Forty odd years ago, the only black squirrels I had ever seen were found in Wisconsin. Their presence there was apparently noteworthy enough to be included in a tourist guide of the day. So, I was surprised in recent years to find melanistic squirrels not a thousand miles away but only one block from my home in Canaan. Last year, they had traversed that block and were found raiding our bird feeder.

Curious, I consulted James Fisher, a mammalogist and research director at the White Memorial Conservation Center in Litchfield. Mr. Fisher revealed that the black squirrel is the same species as the more familiar gray squirrel that risks its life daily in front of cars. The black squirrel, he said, is a “morph” of the Eastern gray squirrel, one that simply has more pigmentation than its brethren. Its color adaptation is offset by a “third morph” that produces a light-colored squirrel.

“Most white squirrels are not true albinos,” Mr. Fisher said, “because they have pigment in their eyes and skin. Some squirrels even have white blotches.”

He said the white morph is “very rare,” and that the black morph “is infrequently observed, but can be found.”

Mr. Fisher said the dark color variations are common throughout the state—as common as an animal can be that occurs only once in 10,000 squirrel births. There may be a slightly greater concentration in northern parts of the state, with pockets of them reported in Canaan, Torrington, New Hartford and the like.

Their preference for colder sections of the state is consistent with one theory of why they proliferate. It is supposed that black squirrels thrive where it is colder because the dark fur absorbs more heat from the sunlight. Indeed, black squirrels are particularly common in northern Midwestern states and in Canada.

Beyond that, there appears to be disagreement about the creatures. One Web site, which cites 2009 research by Helen McRobie and others, claims that two gray squirrels cannot produce a black offspring, because gray squirrels have “two copies of a normal pigment gene and black squirrels have either one or two copies of a mutant pigment gene. If a black squirrel has two copies of that mutant gene, it will be jet black. If it has one copy of a mutant gene and one normal gene, it will be brown-black.”

But Mr. Fisher cautioned that it is “not as categorical as we would like it to be.”

“There is a whole spectrum across the population,” he said. “If you look at the gray squirrel, there all kinds of colors in their pelts. If you look at a litter, every so often there will be a very dark or light one in the litter—but they are all the same species.”

He disputes the findings of Dr. Bill Hamilton, a biologist with Penn State New Kensington, who is quoted by nature writer Gayle Pille as saying, “Early northern forests were very primeval. They were very shaded, very dense … so dense that a squirrel could go from one end of the state to the other without ever touching the ground. The undisturbed North American population of gray squirrels was, according to historical records from the 1600s and early 1700s, predominately made up of black-phase gray squirrels due to the effectiveness of the black coloration as an aid in hiding from avian predators such as hawks or owls.”

Dr. Hamilton asserts that as the forests were cleared for farmland and squirrels were hunted, “The black squirrel was very clearly outlined against the light-colored sky … . This human hunting pressure apparently favored the mixed, ‘gray’ coloration that even today predominates in most North American populations.”

“There is no information to support that,” said Mr. Fisher. “There are lots of different biological collections—some fairly old stuff—and I haven’t seen anything to support that theory.”

He said that squirrel populations have increased in the past century as the hills have become more forested. “The Eastern Gray Squirrel can live in [areas with fewer trees] but they do tend to be found where there are lots of mature trees because of the food available,” he said, adding that they eat anything at hand, from bird eggs and nestlings to seeds and fruit.

“The gray squirrel is opportunistic, tenacious and intelligent,” he said. “They can do all sorts of acrobatics to get food.”

That is a characteristic that humans intent on feeding only birds will find annoying, but Mr. Fisher said he is appreciative of gray squirrels in the backyard.

“But if they get into a house, they can do damage,” he cautioned, “so we need to be sure they don’t enter. We need to be careful about feeding wild animals but we should remember that they are just doing what they do. We are all part of the same ecosystem.”