The eastern gray squirrel (Sciurus carolinensis), spelled as grey squirrel outside of the United States, is a species of tree squirrel in the genus Sciurus. It is native to eastern North America, where it is the most prodigious and ecologically essential natural forest regenerator. Widely introduced to certain places around the world, the eastern gray squirrel in Europe, in particular, is regarded as an invasive species.
In Europe, Sciurus carolinensis is included (since 2016) in the list of Invasive Alien Species of Union concern (the Union list). This implies that this species cannot be imported, bred, transported, commercialized, or intentionally released into the environment in the whole of the European Union.
Distribution Sciurus carolinensis is native to the eastern and midwestern United States, and to the southerly portions of the central provinces of Canada. In the mid-1800s, the population in the midwestern United States was described as being "truly astonishing," but human predation and habitat destruction through deforestation resulted in drastic population reductions, to the point that the animal was almost absent from Illinois by 1900.
The native range of the eastern gray squirrel overlaps with that of the fox squirrel (Sciurus niger), with which it is sometimes confused, although the core of the fox squirrel's range is slightly more to the west. The eastern gray squirrel is found from New Brunswick, through southwestern Quebec and throughout southern Ontario plus in southern Manitoba, south to East Texas and Florida. Breeding eastern gray squirrels are found in Nova Scotia, but whether this population was introduced or came from natural range expansion is not known.
A prolific and adaptable species, the eastern gray squirrel has also been introduced to, and thrives in, several regions of the western United States. In 1966, this squirrel was introduced onto Vancouver Island in Western Canada in the area of Metchosin, where it is considered highly invasive and a threat to both the local ecosystem and the native squirrel, the American red squirrel.
Overseas, eastern gray squirrels in Europe are a concern because they have displaced some of the native squirrels there. For example, in Britain, they have largely displaced the red squirrel; in Ireland, they compete with the Eurasian red squirrel. They have also been introduced into Italy, South Africa, and Australia (where they were extirpated by 1973).
Fossil record S. carolinensis is known to have occurred at least 18 times throughout the North American Pleistocene fossil record, across eight different US states,These are (not counting the single fossil specimen from South Carolina): Florida, Kentucky, Maryland, Missouri, Pennsylvania, Tennessee, Texas, and Virginia. with some fossils allegedly dating back as early as the late Irvingtonian period. Body size seems to have increased during the early to middle Holocene and then decreased to the present size seen today.
A single fossil specimen (held and cataloged by South Carolina State Museum's natural history collection as SC93.105.172–.174) is known from the Ardis local fauna site in Harleyville, South Carolina. This specimen consists of a partial humerus (.172) and two partial tibiae (.173, .174), which are anatomically indistinguishable from that of contemporary S. carolinensis. Surrounding fossil material from the site has been dated back to 18,940–18,530 years ago, during the Late Pleistocene (late Rancholabrean) epoch, and indicates that the site was likely a hardwood-conifer swamp during this time.
Etymology The generic name, Sciurus, is derived from two Greek words, skia 'shadow' and oura 'tail'. This name alludes to the squirrel sitting in the shadow of its tail. The specific epithet, carolinensis, refers to the Carolinas, where the species was first recorded and where the animal is still extremely common. In the United Kingdom and Canada, it is simply referred to as the "grey squirrel". In the US, "eastern" is used to differentiate the species from the western gray squirrel (Sciurus griseus).
Characteristics
The eastern gray squirrel has predominantly gray fur with brown spots and streaks. It has a white underside as compared to the brownish-orange underside of the fox squirrel; both exhibit countershading. It has a large bushy tail. Particularly in urban situations where the risk of predation is reduced, both white-colored and black-colored individuals are quite often found. The melanistic form, which is almost entirely black, is predominant in certain populations and in certain geographic areas, such as in large parts of southeastern Canada. Melanistic squirrels appear to exhibit a higher cold tolerance than the common gray morph; when exposed to −10 °C, black squirrels showed an 18% reduction in heat loss, a 20% reduction in basal metabolic rate, and an 11% increase to non-shivering thermogenesis capacity when compared to the common gray morph. The black coloration is caused by an incomplete dominant mutation of MC1R, where E+/E+ is a wild type squirrel, E+/EB is brown-black, and EB/EB is black.
The head and body length is from , the tail from , and the adult weight varies between .BBC: Science and Nature, "Grey squirrel: Sciurus carolinensis" They do not display sexual dimorphism, meaning there is no gender difference in size or coloration.
The tracks of an eastern gray squirrel are difficult to distinguish from the related fox squirrel and Abert's squirrel, though the latter's range is almost entirely different from the gray's. Like all squirrels, the eastern gray shows four toes on the front feet and five on the hind feet. The hind foot-pad is often not visible in the track. When bounding or moving at speed, the front foot tracks will be behind the hind foot tracks. The bounding stride can be long.Murie, Olaus Johan and Elbroch, Mark (2005). Peterson Field Guide to Animal Tracks, Houghton Mifflin Harcourt, p. 79, .
The dental formula of the eastern gray squirrel is , for a total of 22 teeth.
Incisors exhibit indeterminate growth, meaning they grow consistently throughout life (as is true for all rodents), and their cheek teeth exhibit brachydont (low-crowned teeth) and bunodont (having tubercles on crowns) structures.
Growth and ontogeny
Newborn squirrels are called kits, kittens, or pups. They weigh at birth. They are born blind, entirely hairless, and pink with whiskers present at birth. 7–10 days postpartum, the skin begins to darken, just before the juvenile pelage grows in. Lower incisors erupt 19–21 days postpartum, while upper incisors erupt after 4 weeks. Cheek teeth erupt during week 6. Eyes open after 21–42 days, and ears open 3–4 weeks postpartum. Weaning is initiated around 7 weeks postpartum, and is usually finished by week 10, followed by the loss of the juvenile pelage. Full adult body mass is achieved by 8–9 months after birth.
Diseases Diseases such as typhus, plague, and tularemia are spread by eastern gray squirrels. If not properly treated, these diseases have the potential to kill squirrels. When bitten or exposed to bodily fluids, humans can contract these diseases. Eastern gray squirrels may also carry parasites such as ringworm, fleas, lice, mites, and ticks which can kill their squirrel host. Their skin may become rough, blotchy, and prone to hair loss due to the mite parasite during the chilly winter months. The parasites are not transferred to people when these squirrels reside in attics or homes. A frequent illness spread by ticks is Lyme disease. Ticks can also spread Rocky Mountain spotted fever. It can result in damage to internal organs including the heart and kidney if not properly treated. An eastern gray squirrel is susceptible to diseases including fibromatosis and squirrelpox. A squirrel with fibromatosis, a virus-induced illness, may grow massive skin tumors all over the body. Blindness could result from a tumor that is located close to a squirrel's eye.
Behavior and ecology
The eastern gray squirrel is one of very few mammalian species that can descend a tree head-first. It does this by turning its feet so the claws of its hind paws are backward-pointing and can grip the tree bark.
Eastern gray squirrels build a type of nest, known as a drey, in the forks of trees, consisting mainly of dry leaves and twigs. The dreys are roughly spherical, about 30 to 60 cm in diameter and are usually insulated with moss, thistledown, dried grass, and feathers to reduce heat loss. Males and females may share the same nest for short times during the breeding season, and during cold winter spells. Squirrels may share a drey to stay warm. They may also nest in the attic or exterior walls of a house, where they may be regarded as pests, as well as fire hazards due to their habit of gnawing on electrical cables. In addition, squirrels may inhabit a permanent tree den hollowed out in the trunk or a large branch of a tree.
Eastern gray squirrels exhibit diurnal or crepuscular behavior depending on the season, and tend to avoid the heat in the middle of a summer day. They do not hibernate.
Reproduction Eastern gray squirrels can breed twice a year, but younger and less experienced mothers normally have a single litter per year in the spring. Depending on forage availability, older and more experienced females may breed again in summer.Curtis, Paul D. and Sullivan, Kristi L. (2001) Tree Squirrels, Wildlife Damage Management Fact Sheet Series, Cornell Cooperative Extension, Ithaca, N.Y. In a year of abundant food, 36% of females bear two litters, but none will do so in a year of poor food. Reproduction is also strongly tied to mast years of seed-producing trees, as both litter size and amount of lactation decrease during food-poor years.
Their breeding seasons are December to February and May to June, though this is slightly delayed in more northern latitudes. The first litter is born in February or March, the second in June or July, though, again, bearing may be advanced or delayed by a few weeks depending on climate, temperature, and forage availability. In any given breeding season, an average of 61 – 66% of females bear young. If a female fails to conceive or loses her young to unusually cold weather or predation, she re-enters estrus and has a later litter. Five days before a female enters estrus, she may attract up to 34 males from up to 500 meters away. Eastern gray squirrels exhibit a form of polyandry, in which the competing males will form a hierarchy of dominance, and the female will mate with multiple males depending on the hierarchy established.
Normally, one to four young are born in each litter, but the largest possible litter size is eight. The gestation period is about 44 days. The young are weaned around 10 weeks, though some may wean up to six weeks later in the wild. They begin to leave the nest after 12 weeks, with autumn born young often wintering with their mother. Only one in four squirrel kits survives to one year of age, with mortality around 55% for the following year. Mortality rates then decrease to around 30% for following years until they increase sharply at eight years of age.
Rarely, eastern gray females can enter estrus as early as five and a half months old, but females are not normally fertile until at least one year of age. Their mean age of first estrus is 1.25 years. The presence of a fertile male will induce ovulation in a female going through estrus. Male eastern grays are sexually mature between one and two years of age. Reproductive longevity for females appears to be over 8 years, with 12.5 years documented in North Carolina. These squirrels can live to be 20 years old in captivity, but in the wild live much shorter lives due to predation and the challenges of their habitat. At birth, their life expectancy is 1–2 years; an adult typically can live to be six, with exceptional individuals making it to 12 years.
Communication