Natural History of Sciurus carolinesis (Eastern grey squirrel)

Order:  Rodentia
Suborder:  Sciurognathi
Family:  Sciuridae
Tribe:  Sciurini
Subtribe:  Sciurina
Genus:  Sciurus
Subgenus:  Sciurus

Number of known species of Sciurus:  28
(Hoffman, et al; 1993)

Number of recognized subspecies
of Sciurus carolinensis:  5
(Koprowski, 1994)

Geographic distribution:  Eastern United States, extending west to the edge of deciduous forest and north into Canada.  Introduced to California, Montana, Oregon, and Washington (United States) and in Canada into Quebec, New Brunswick, British Columbia, Manitoba, Nova Scotia, and Ontario (Barkalow and Shorten, 973; Flyger and Gates, 1982) and Saskatchewan (Nero, 958).  They were introduced to Italy and England from the United States, and introduced into Scotland from Canada (Currado et al., 1984; Lloyd, 1983, Staines, 1986).  They were then introduced into South Africa (Millar, 1980) and Ireland (Lloyd, 1983) from England.  In the 1880s, an attempt was made to introduce them to Australia from England, but this failed by 1973 (Seebeck, 1984).

Fossil record:  Sciurus carolinensis findings date back to the late Irvingtonian age (1.5 – 1/2 million years ago) in Florida are indistinguishable from today’s genus.  (Emry and Thorington, 1984)

 Physical characteristics:  Sciurus carolinensis is a medium-sized tree squirrel with no distinction in size/coloring between males and females.  Adult weight ranges from 300-710 grams (just over 1/2 – 1 1/2 pounds); total body length ranges from 150-250 millimeters (15 – 20 inches) with the tail 200-330 millimeters (5.9 – 9.8 inches) in length (Barkalow and Shorten, 1973; Hall, 1981). 

Coat color in Michigan ranges from a grizzled dark to pale grey with some rusty-tan on feet and face to a full coat of ruddy tan; undersides range from pure white to cream.  Melanism (“black-phase”) is common, as well as a grizzled combination of predominant black with ticking of rusty brown, tan, or white.  It is not unusual to see a coal black Eastern grey squirrel with a blonde or rusty red tail.

 Albinism (“white”) is rare, but there are small, “pocket” populations located in Olney, Illinois and Marionville, Missouri. 

They have a total of 22 teeth (Hench et al., 1984), differing from Sciurus niger (fox squirrel) by having an upper P3 (premolar).  Mean rectal temperature is 97.52° F – 101.66° F (Bolls and Perfect, 1972; Hoff et al., 1976a) and venous shunts at the base of the tail suggest it is used to assist thermoregulation (Thorrington, 1966).  Black-phase Eastern greys are better able to thermoregulate at lower temperatures than their grey counterparts; metabolic rate when exposed to cold is 3.5 times the predicted rate, making Eastern grey squirrels one of the most effective hypotherms (Ducharme et al., 1989). 

Front foot:

Hind foot:

Development:  Females may bear young at 5 1/2 months in North Carolina (Smith and Barkalow, 1967) but most do not begin to reproduce until they are over a year old (Brauer and Dusing, 1961, Brown and Yeager, 1945).  Reproductive longevity potentially > 12 years (Koprowski et al, 1988).  Gestation is 44 days (Webley and Johnson, 1983).

Sciurus carolinensis is born naked, except for vibrissae (whiskers), with birth weight ranging from 13-18 grams.  Claws are already well developed. 

Hair begins to appear between 7-10 days, at 3-4 weeks the ears open and lower incisors erupt at 9021 days.

Eyes open and upper incisors erupt at 24-42 days with upper incisors erupting in week 4 and cheek teeth in week 6. 

Weaning begins at 7 weeks and completes at approximately 10 weeks.  Adult weight is reached at approximately 8-9 months of age (Horwich, 1972; Shorten, 1951).

 Ecology:  Similar to the fox squirrel, the Eastern grey squirrel is considered to be diurnal (awake during the day and sleeping through the night), with bimodal activity (two distinct activity patterns) occurring spring through fall, peaking 2 hours after sunrise and 2-5 hours before sunset.  During the winter they exhibit a unimodal pattern, with activity peaking between 2-4 hours before sunset   (Thompson, 1977b)

They are classified as granivores (“seed eater”) and feed heavily on tree seeds during much of the year; tree buds and flowers are used primarily in winter and spring (Korschgen, 1981; Nixon et al., 1968).  They also feed on the seeds and catkins of cedar, hemlock, pine, and spruce trees  (Barkalow and Shorten, 1973; Thompson and Thompson, 1980).  They eat a wide variety of vegetation, and will eat cultivated crops especially during the winter; females ingest gravel and soil in winter and early spring (Korschgen, 1981; Nixon et al., 1968; Thompson and Thompson, 1980).  They have been known to eat insects during the summer and these may be an important food for juveniles (Korschgen, 1981: Nixon, 1970).  Other foodstuffs include bones (Flyger and Gates, 1982), bird eggs and nestlings (Baily, 1923), and frogs (Goodrum, 1961) and cannibalism has been reported (Holm, 1976; Thompson, 1976).

Eastern grey squirrels swim by a dog-paddle, holding head out of the water (Barkalow and Shorten, 1973), and they can run at speeds of 27 km/h or 16.77 mph (Layne and Benton, 1954).

They are considered classic scatterhoarders who disperse food caches.  Nuts are buried <2 cm below the surface of the soil (VanderWall, 1990).  They learn to excise the taproot of white oak acorns to prevent overwinter germination (Fox, 1982) and prefer those acorns with a high lipid content (Smith and Follmer, 972), however will refuse even high-lipid acorns if the amount of tannins is too large (Smallwood and Peters, 1986).

Mass migrations of Eastern grey squirrels were recorded in the 1700s and 1800s, extending for greater than 160 km (99.4 miles) and lasting for 3 weeks.  These usually occurred during autumn and crossed large lakes and rivers, possibly in response to food shortages (Schorger, 1949).  More recent such incidents were recorded in 1968 (Flyger, 1969) and again in 1985 when emigration from a Lake Michigan island population last over a month (Long and Long, 1986).

The range of Eastern grey squirrels overlap with those of both the fox squirrel and the pine squirrel, and while the pine squirrel is generally the aggressor in encounters between the two species, they have been known to share a nest box when housed together (Ackerman and Weigl, 1970).  Eastern grey squirrels are often successfully raised and released with fox squirrels when no other grey squirrels are available as “foster siblings”.

After their introduction in England, the Eastern grey squirrel rapidly expanded its range, while the range of the native red squirrel (Sciurus vulgaris) diminished and in local cases disappeared entirely.  Competition as cause of the displacement is considered only a partial explanation because local disappearance of the red squirrel occur when there are no grey squirrels present.  Hypotheses about habitat change and the grey squirrel as a disease vector also have little support (Reynolds, 1985b).

The most common social behavior of Eastern grey squirrels is group nesting in single- and mixed-sex groups, and amicable behaviors are exhibited towards close relatives (Koprowski, 1991b, 1993b; Taylor, 1969).  Pregnant females nest alone (Cordes and Barkalow, 1972; Hampshire, 1985; Nixon and McClain, 1975).