This article explores the origins of white squirrels, distinguishing between species-specific traits and color variations resulting from genetic mutations. It delves into the science of coat color in squirrels, discussing melanin, pigmentation, and the adaptive significance of different fur colors. The text also clarifies the difference between 'variant' and 'mutant' in this context.
What exactly is a white squirrel , and where do they originate? Is it considered a distinct species? Or is it a result of mutation? These are common questions. The answer is nuanced. In fact, there is one type of tree squirrel where a white coat is a characteristic of the entire species, at least in certain parts of its habitat.
This is the Oriental Tree Squirrel of the genus Callosciurus, which translates to beautiful squirrel, found in Thailand and other regions of Southeast Asia (Thorington and Ferrel, 2006). Another belongs to an as-yet-undescribed species recently discovered on Palawan Island in the Philippines and is considered to be endangered (for more details, click here). If you spot a white squirrel in North America outside of captivity, it is most likely a color variation of one of our native species within the genus Sciurus (only distantly related to Callosciurus). In my area, that would be the Eastern Gray Squirrel (Sciurus carolinensis). The following discussion primarily focuses on this species, but white-coated Fox Squirrels and Red Squirrels have also been observed. Note: The following text becomes more technical, comparing the potential mechanisms behind the emergence of different variants. If these mechanisms aren't of interest, feel free to skip to the final three paragraphs, which revisit white squirrels and are, hopefully, of more general interest.\There's considerable variation in squirrel coat color, both locally and regionally. The typical pattern of brown or gray on top with a white underside (countershading) is regarded as the wild type from which other variations arise. These wild types, like most mammals, possess dark eyes. The brown, reddish, gray, or even black coloration comes from the production of melanin, a pigment existing in at least two forms: eumelanin (black/brown) and phaeomelanin (red/yellow). Combinations and arrangements of these pigments create diverse hues. Alternating between one or the other (or neither) leads to banded hairs, known as agouti. All this variation can be caused by changes in genes that control melanin production and packaging, or by changes in genes that control the distribution of melanin production. Most wild-type gray squirrels have white bellies not because of mutated genes in those cells, but because of regulatory genes that suppress the activity of melanin genes in those areas, but not in others (or prevent melanin-producing cells called melanocytes from reaching those regions). The white belly is an adaptation; it makes the squirrel less visible from below against a lighter sky. However, some squirrels have tan or ochre bellies (consider the infamous Stan, the rally squirrel for the 2011 St. Louis Cardinals). Black or melanistic squirrels produce excessive melanin compared to the wild type, and this is spread across larger areas of their bodies. These variants are often found at higher latitudes, and the dark color is believed to aid in thermoregulation. The point here is that while variation from the wild type arises through genetic change, known as mutation, the word mutation carries a negative connotation, implying a strange or unusual condition. In this case, it results in a rich diversity that might actually be beneficial for local conditions (including human preference). Hence, the term variant is preferred over mutant, although both will be used here.\There is much variation in squirrel coat color both locally and regionally. The general pattern of brown/gray on top and white below (counter shading) is considered the wild type from which other variations arose. These wild types, like most mammals, have dark eyes. The brown, reddish, gray, or even black color comes from the production of a pigment called melanin which, itself, comes in at least two varieties, eumelanin (black/brown) and phaeomelanin (red/yellow). Different combinations and arrangements of these two pigments produces a variety of hues. Alternating between one or the other (or neither) results in banded hairs referred to as agouti. All this variation can arise from change in either the genes that control the production and packaging of melanin itself, or of genes that control the distribution of melanin production. After all, most wild type gray squirrels have a white abdomen not because there are mutated genes in those cells but because there are regulatory genes which suppress the activity of melanin genes in these locations but not others (or prevent melanin producing cells called melanocytes from reaching such regions as discussed below). The white abdomen is adaptive. It makes the squirrel less visible from below against a light sky. Yet some squirrels have tan or ochre bellies (witness the now infamous Stan the rally squirrel for the 2011 St. Louis Cardinals
White Squirrel Squirrels Mutation Genetics Melanin
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