Black squirrel
Black squirrels are a melanistic subgroup of squirrels with black coloration on their fur. The phenomenon occurs with several species of squirrels, although it is most frequent with the eastern gray squirrel and the fox squirrel. Black morphs of the eastern gray and fox squirrels are the result of a variant pigment gene. Several theories have surfaced as to why the black morph occurs, with some suggesting that the black morph is a selective advantage for squirrels inhabiting the northern ranges of the species, with the black fur providing a thermal advantage over its non-melanistic counterpart.
Black squirrels share the same natural range as their non-melanistic counterparts. Black morphs of eastern gray squirrels occur most frequently in the northern portion of its range around the Great Lakes Basin. Conversely, black morphs of fox squirrels typically occur most frequently in the southeastern portions of the species' natural range, the southeastern United States. Although they are found more frequently in those regions, the coloration remains uncommon in most areas that these species inhabit. However, black morphs of eastern gray squirrels form the majority of the species' population in the Canadian province of Ontario, and the U.S. state of Michigan. In addition to their natural range, black morphs of eastern gray squirrels were also introduced into other areas of Canada, the United Kingdom, and the United States during the 19th and 20th centuries.
Several municipalities and post-secondary schools in the United States have adopted a black squirrel for branding purposes, using it as a symbol and/or mascot. Some municipalities that have adopted the black squirrels as a symbol for their community have also passed ordinances that discourage attempts to threaten them.
Description
The black coloration in both eastern gray squirrels and fox squirrels is believed to stem from a variant pigment gene. A study published by FEBS Letters in 2014 demonstrated how a pigment gene missing a piece of DNA, can be a determinant of an eastern gray squirrel's coat. The emergence of black fur in the eastern gray squirrel is believed to be the result of the 24 bp deletion from their melanocortin 1 receptor gene; with the specific allele referred to as MC1R∆24. A study published by BMC Evolutionary Biology pointed to evidence that the variant pigment gene originated from the black fox squirrel, and was later passed on to eastern gray squirrels as a result of interspecies mating; given that the variant gene in both species were identical. Black coat color is caused by a 24 base pair deletion in MC1R in the western population of fox squirrels and by a point mutation in the agouti-signaling protein gene in the southeastern population.Black morphs may also occur with Columbian ground squirrels, Eurasian red squirrels, Richardson's ground squirrels, and western gray squirrels, although it is far more unusual for the latter to display color polymorphism. No association between melanism and variations in their MC1R was found in Eurasian red squirrels; with researchers suggesting that the different color variations in Eurasian red squirrels, and fox squirrels being a polygenic result. Melanism with Richardson's ground squirrels is due to recessive genes.
Benefits of black fur
With regard to black squirrels and melanism, two major theories dominate the literature, that its frequency is the result of crypsis, and/or the result of thermoregulation.Concealment
It has been theorized that non-melanistic gray squirrels have a concealment advantage in forests dominated by deciduous trees, while black squirrels hold a concealment advantage in forested areas in the northern portions of its range, where conifer trees are more prevalent. The theory is based on the idea that forests where coniferous trees are predominant block more sunlight from reaching the forest below, providing a dimly-lit habitat in which a darker-coated squirrel could better conceal itself compared to its lighter counterpart. It is also suggested that non-melanistic squirrels have a concealment advantage over their melanistic counterparts in deciduous forests because deciduous trees shed their leaves on a seasonal basis, illuminating the forested area below it during the winter season. A study conducted in 1989 on melanistic fox squirrels found the non-melanistic coloration better for concealment while the squirrel was still, but a melanistic coloration provided better concealment for when it was in motion.The frequency of black morph eastern gray squirrels is thought to have been once relatively common throughout the eastern gray squirrel range, although their frequency and population have dwindled since the 1700s. It has been suggested that their population declined due to extensive deforestation and the hunting of squirrels for their meat and pelts; with the newly changed environment providing non-melanistic gray-colored squirrels an advantage in concealment. However, the theory that the black morphs squirrels were more prevalent prior to the 1700s, and that deforestation led to their decline has been challenged by some researchers. One study found a high frequency of black eastern gray squirrels lived in rural southern Ontario, an area primarily made up of farmland.
Melanism in fox squirrels in the southeast portion of its natural range has also been associated with crypsis, as it inhabits forests that go through periodic burnings. It has been suggested that black squirrels would be harder to detect in forests already burned, due to the blackened substrate.
Thermoregulation
It has also been suggested that black morph squirrels have a considerably higher cold tolerance than gray squirrels given the color of their coat. Black-coated animals were found to have 18 percent lower heat loss in temperatures below, a 20 percent lower metabolic rate, and a non-shivering thermogenesis capacity that is higher than a gray morph. Additionally, researchers of the color morph have noted a strong negative correlation with the frequency of black squirrels and areas with high air temperature.The black coat has been suggested as a selective advantage for squirrels inhabiting the northern ranges of the species, as it helps them inhabit colder regions. The apparent thermal advantage has contributed to the expansion of the eastern gray squirrel's range northward following the end of the last glacial period. Black morph eastern gray squirrels have been reported as far north as Sudbury, Ontario, past the traditional range of the eastern gray squirrels.
A study published by the European Journal of Ecology in 2019 on eastern fox squirrels found that the melanistic morphs of the species saw a noticeable increase in their surface temperature in both sunny and cloudy weather; whereas the non-melanistic fox squirrels only saw their surface temperature increase when it was sunny with no cloud cover. Its ability to gain heat in sunny and cloudy conditions is believed to be the reason why melanistic squirrels are more active during winter mornings. However, the same study noted that there was no difference in metabolic heat production between the color morphs.
Reproduction
Among eastern squirrels, gray mating pairs cannot produce black offspring. Gray squirrels have two copies of a gray pigment gene and black squirrels have either one or two copies of a black pigment gene. If a black squirrel has two copies of the black gene it will be jet black. If it has one copy of a black gene and one gray gene it will be brown-black. Approximately nine percent of melanistic eastern gray squirrels are believed to be jet black. In areas with high concentrations of black squirrels, litters of mixed-color individuals are common.Differences with non-melanistic squirrels
A study conducted in 1990 of black and gray morphs of the eastern gray squirrel concluded that there was no major difference in behavior between the morphs. The same study also found no difference between the morphs when reacting to either a human or canid predator. However, another study in 2010 also found that gray morphs of the eastern gray squirrel were more prone to initiate flight than black morphs after hearing a red-tailed hawk; although the fact that black morphs were less likely to initiate flight after hearing a red-tailed hawk may not be an effect of pigmentation, rather the environment they inhabit. Given the higher frequency of black morphs in an urban setting, it has been suggested that black morphs have a higher tolerance for human/urban stimuli. It has also been suggested that behavioral differences with regard to mating may exist between the urban and rural populations of eastern gray squirrels.A 2019 study on fox squirrels found that there was no noticeable difference in metabolism between the different color morphs of that species. However, the same study on fox squirrels found that melanistic fox squirrels were more active than their non-melanistic counterparts during the winter and spring months, with melanistic fox squirrels found to be 30 percent more active during the mornings than their non-melanistic counterparts. Conversely the non-melanistic fox squirrels were more active during the autumn season. It has been suggested that the black squirrel's higher heat gain for its surface temperature is the reason why they are able to be active earlier in the day and remain active longer.
Distribution
Natural populations of black morph eastern gray and fox squirrels can be found in the natural ranges of both species in North America, although their frequency varies depending on the area. Black fur for both species of squirrels is rare and occurs at rates of less than one percent. It has been suggested that one in 10,000 eastern gray squirrels are a black morph.It has been suggested that the frequency of the black color morph in the eastern gray squirrel populations has declined since the late 1700s, especially south of the Great Lakes. There is a higher frequency of the black morph in the northern portions of the eastern gray squirrel's range; which includes the southern portions of central Canada and northern United States. In particular, large populations of black squirrels are found within the Great Lakes Basin, with a notable increase in their frequency between the 41st parallel north and the 45th parallel north.
File:Melanistic Sciurus carolinensis.JPG|thumb|Jet-black eastern gray squirrels in Toronto, Ontario. Urban populations of the species were found to have a higher frequency of black morphs.
Black squirrels occur with the highest frequency in Ontario and Michigan, and are the predominant color morph found in those areas; with the black morph accounting for 66 percent of squirrels documented on iNaturalist in Ontario, and 56 percent in Michigan. Significant populations of black morphs are also present in the other provinces/states that surround the Great Lakes; with approximately 15 percent of the eastern grey squirrels in those regions reported to be melanistic. Black squirrel populations south of the Great Lakes remain largely localized, with the frequency of black squirrels varying from one region to another. Black squirrels were found to be more common in urban areas as opposed to rural areas and forests. Among exurban populations of eastern gray squirrels, the black morph only occurs in high frequencies in Ontario and northern Michigan.
Conversely, black morphs of fox squirrels occur with the highest frequency in the southeastern portion of its natural range, the southeastern United States. Like the eastern gray squirrels, the frequency of black fox squirrels is dependent on the area, reaching a maximum frequency of 13 percent. Although they occur more frequently in the southeastern United States, large populations of black morph fox squirrels may be found in other areas of the species' natural range; including Council Bluffs, Iowa, around the Missouri River. Approximately half of the fox squirrels found in Council Bluffs are melanistic. Melanistic fox squirrels in Council Bluffs have since expanded across the Missouri River to other areas in the Omaha–Council Bluffs metropolitan area; with melanistic fox squirrels now accounting for 4.6 to 7.6 percent of fox squirrels in Omaha.
The Delta fox squirrel is a subspecies of fox squirrel found in the Mississippi Alluvial Valley. There are two common phases, a glossy solid black phase and a reddish phase that lacks the white markings of the fox squirrels found in the surrounding hill country.