An in-depth look at the genetic factors behind white squirrel coloration, differentiating between leucism and albinism, and exploring the prevalence of these variants in North America and beyond. The text explores the difference between mutation and variant, coat color variation in squirrels, and the origins of white squirrels.
Walker provided some fantastic photographs, capturing white squirrels enjoying peanuts on Pennsylvania Avenue. This prompts a common question: What exactly is a white squirrel, and what is its origin? Is it a distinct species, or a result of genetic mutation? The answer depends on the context. There exists one tree squirrel species where a white coat is a characteristic, particularly in parts of its distribution.
This is the Oriental Tree Squirrel (Callosciurus), found in Thailand and other Southeast Asian regions (Thorington and Ferrel, 2006). Additionally, a white-coated squirrel species, yet to be formally described, was recently discovered on Palawan Island in the Philippines and is considered endangered. Consequently, if one spots a white squirrel in North America outside of captivity, it is highly likely a color variant of a native species within the genus Sciurus. In most areas, including the neighborhood where the photographs were taken, this would be the Eastern Gray Squirrel (Sciurus carolinensis). The discussion primarily revolves around this species, although white-coated Fox Squirrels and Red Squirrels have also been observed. The information will gradually become more technical, examining the potential mechanisms that cause different variations. For those less interested in these details, the final three paragraphs offer a broader overview of white squirrels. \Squirrel coat color exhibits considerable local and regional variation. The typical pattern involves brown or gray on the upper body and white on the underside, known as countershading, considered the wild type from which other variations emerge. These wild types, like most mammals, possess dark eyes. The brown, reddish, gray, or even black coloration arises from the production of melanin. Melanin exists in at least two forms: eumelanin (black/brown) and phaeomelanin (red/yellow). Different combinations and arrangements of these pigments produce a range of hues. Alternating between the presence or absence of these pigments results in banded hairs, known as agouti. This variation can stem from changes in genes that control the production and packaging of melanin or genes that control melanin distribution. For instance, the white underside of most wild-type gray squirrels isn't due to mutated genes in those cells, but because regulatory genes suppress the activity of melanin genes in those areas, or prevent melanocytes (melanin-producing cells) from reaching such areas. The white abdomen provides camouflage against a light sky, reducing visibility from below. Some squirrels, however, have tan or ochre bellies, as seen in the case of Stan the rally squirrel for the 2011 St. Louis Cardinals. Black or melanistic squirrels produce excessive melanin compared to the wild type, and in extended areas of the body. These variants are often found at higher latitudes, and the darker color may assist in thermoregulation. The point being made is that while variations from the wild type result from genetic changes (mutations), the word mutation often carries a negative connotation, implying something unnatural. In this case, it leads to a diversity of appearances that might be well-suited to the environment. The term variant is preferred over mutant, although both will be used. \White squirrels are merely another color variant within this highly variable species. The most frequent observations involve isolated individuals with a fully white coat but dark eyes, a condition known as leucism. This variant seems to appear spontaneously across the species' range, only to disappear and reappear elsewhere (see lists of colonies and sightings). It is believed these are spontaneous mutants of a gene influencing the use of pigment (melanin) genes, not mutants of the melanin gene itself (a potential mechanism is detailed below). Rather than being produced in skin/hair cells and eyes, melanin is only produced in the eyes. In a separate mutation, involving genes directly responsible for melanin production, no cells produce the pigment. As a result, the squirrel is both white and has pink or blue eyes. These true albinos are even less common, likely because their vision is impaired due to the lack of eye pigment to reduce glare, increasing the risk of falls. Additionally, they might be more vulnerable to predation; a decrease in the closely monitored Olney, Illinois, albino population has been associated with cats. Albino squirrels cannot produce melanin in either their skin or their eyes. The structures within cells where melanin is typically stored are empty (Searle, 1968)
White Squirrels Genetics Color Variation Leucism Albinism
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