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Brandon W. sent to me a photo of a white squirrel in Somerville. Why are squirrels – in this case eastern gray squirrels (Sciurus carolinensi) – sometimes white?
If you were to look closely at a gray squirrel’s hairs, you would notice that none of the hairs are actually gray, but a mixture of black, brown and white. It’s just when viewed from a distance that most squirrels look gray – and generally, eastern gray squirrels are grayish on top and white below, with dark brown or black eyes.
Squirrels produce a pigment called melanin that comes in two varieties: eumelanin, which produces black and brown pigments, and pheomelanin, which produces red and yellow pigments. Different combinations produce squirrels sporting a variety of shades of gray, black and red.
White occurs in places on a squirrel’s body where regulatory genes suppress melanin production. White undersides have a purpose: They make a squirrel less visible to predators looking up from below because the white belly blends in with the light-colored sky. Conversely, the gray on top helps squirrels blend in with trees when viewed from above.
Most white squirrels have genes that have halted the production of melanin in much of their coat, although some white squirrels have dark patches. Melanin is usually still produced in the eyes, and consequently, these squirrels have dark eyes.
If a squirrel cannot produce melanin anywhere in its body, it has pinkish, bluish or red eyes, making for a true albino. These are rare. (The gene producing albinism is recessive, meaning an albino squirrel needs to get two, one from each parent.)
Most albino mammals have vision problems. The lack of pigment in the eyes makes them sensitive to sunlight, which can damage their eyes and impair their vision. Because of their poor vision, predators often catch them.
Gray squirrels are unusual, though. Most mammals have rods and cones in their retinas. (Remember grade school science: Cones help you see color and rods help you see in the dark.) Gray squirrels have mostly cones – and albinism affects only rod cells. Therefore, albino squirrels have only minor vision deficits. For this reason, albino gray squirrels are the only known albino mammal to survive successfully in the wild. They can, for example, leap great distances from tree branch to tree branch, an indication that their vision is not impaired.
Most white squirrels are not albinos. They have dark eyes and are leucistic (that is, they have only a partial loss of pigmentation). Leucistic squirrels can be white, pale or piebald (with patchy white and gray).
White-colored squirrels act the same as gray-colored, even when it is not beneficial to them. For example, a threatened gray-colored squirrel will scamper to the nearest tree and spread-eagle against the trunk. This action camouflages the squirrel against the gray of the tree trunk. A threatened white squirrel will also scamper to the nearest tree and spread-eagle against the trunk. But this behavior does not help a white squirrel because the squirrel’s white coat does not blend in with the gray trunk.
There are white squirrels in our area. On Reddit, people have reported seeing white squirrels on the bike path in Arlington and Somerville, in Teele Square and on Sacramento Street in Cambridge. People have even named these white squirrels: One near Tufts has been named Blanca. Another has been named Whitey Bulger.
Some species of animals select mates based on color, but not gray squirrels. A female gray squirrel is in heat for only about eight hours in mid-January. During this short window, males come from hundreds of meters to chase her and try to mate. Scientists have observed as many as 22 males chasing a single female. The female does not have an opportunity to be selective, so coat color in squirrels is random. A female may mate with more than one male (of any color).
Male squirrels use one of two strategies to try to mate. The most dominant males (3 years of age or older) wait outside the nest of a female in heat early in the morning. When the female leaves her nest just after sunrise to find food, the males chase her. The female may race to the end of a branch high above the ground where she is cornered. Mating occurs, usually with the most dominant male. But the female may also manage to break away. When this occurs, low-ranking satellite males (younger than 3 years of age) join in the chase. The female finds a hidden spot close to the ground, where the chance of falling is lessened. There she mates with the first male to find her. If the female does such a good job of hiding that the males cannot find her at all, she clues them in to her location by chattering or yelping. Mating takes only about 20 seconds, but the male guards the female from interlopers for 20-ish minutes.
It is not unheard of to see a two-toned squirrel (often the tail will be white or blond and the rest of the body darker). I saw a blond-tailed squirrel a few years ago near Powder House Park in Somerville. You might also see black squirrels. In fact, we saw a black squirrel in our back yard for several years. (We were not that original, and my child called it Blacky the Squirrel.) Black squirrels are more common in cities, where their color reduces the risk of being hit by cars, and in northern regions, where their black coat keeps them warmer in winter.
Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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