Sciurus carolinensis The Eastern Gray Squirrel Prepared by: Luke R. Breitenbach Description

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Sciurus carolinensis

The Eastern Gray Squirrel

Prepared by: Luke R. Breitenbach

Description: The Eastern Gray Squirrel (Sciurus carolinensis) actually falls into five subspecies ofmedium-sized tree squirrels that occupy the eastern part of the United States and Canada (Koprowski 1994).

Eastern Gray squirrels have no sexual dimorphism insize or coloration. They are the size of a large rat andrange in length from 380-525 (tail length: 150-250) mm andweigh from 300-710 g. The dorsum of the Eastern GraySquirrel is grizzled dark to pale gray and may be washed with cinnamon on hips, feet, and head. Ears are buff to gray (white in northern climates), and their long bushytail is silvery gray and often brownish with long margined white hairs (Long 1970). Under parts are white to gray tobuff to cinnamon.

Photo by: Richard Thorington, Department of Systematic Biology Vertebrate Zoology – Division of Mammals, Smithsonian National Museum of Natural History (2000).

Melanism is common in the north, and albinism is rare (some“mutant” white squirrels are found in Olney, Illinois andMissouri) (Jackson 1961).

Pictures and text copyright ©1995-2000 by Craig S. Thom unless otherwise noted.

The skull is relatively short with a broad and expandedzygomatic arch.

Copyright 1998 The Mammals of Virginia by Donald W. Linzey. All rights reserved

The braincase is broad and posteriorly depressed. Therostrum is laterally compressed; the frontal area isflattened.

Copyright 1998 The Mammals of Virginia by Donald W. Linzey. All rights reserved

Auditory bullae are moderately inflated.

Copyright 1998 The Mammals of Virginia by Donald W. Linzey. All rights reserved

The Sciurus carolinensis dental formula is (I)1/1, (C)0/0,(P)2/1, (M)3/3, total 22 teeth; premolar 3 is small andpeg-like. The average cranial measurements are: greatestlength of skull, 60.7mm; zygomatic breadth, 34.5mm; palatallength, 17.9mm; length of nasal, 20.6mm; cranial height invault of skull, 28.4; length of maxillary tooth row, 10.9mm(based on 32 specimens from Ark., Iowa, Kansas, and N.Carolina – McGrath, 1987) (Koprowski 1994).

Distribution: The Eastern Gray Squirrel is found in theeastern United States, with a range that extends west tothe edge of the deciduous forest and north to Canada(Koprowski 1994).

Copyright 1999 The Smithsonian Book of North American Mammals edited by Don E. Wilson and Sue Ruff. All rights reserved.

Introductions occurred in California, Montana, Oregon,and Washington in the United States; Quebec, New Brunswick,British Columbia, Manitoba, Nova Scotia, Ontario, andSaskatchewan in Canada. Eastern Gray Squirrels wereintroduced to Italy and England from the United States, andto Scotland, South Africa, and Ireland from England(Koprowski 1994). Introductions to Australia from England inthe late 1800’s proved unsuccessful. Sciurus carolinensis is found throughout the state of Wisconsin, and actuallyhas its own subspecies that is native to Minnesota andWisconsin (Sciurus carolinensis hypophaeus - subspecies)(Jackson 1961).

Fossil data: Sciurus carolinensis occurs in twenty NorthAmerican Pleistocene faunas as early as the lateIrvingtonian in Florida. Body size increased in the earlyto middle Holocene and then decreased to present size(Koprowski 1994).

Historical Distribution Info: The Eastern Gray Squirrel hasbeen subjected to an array of land use changes since thefirst settlers came to America. In some cases, clearing offorested land for agriculture led to the replacement ofgray squirrels by other squirrel species (i.e. red and foxsquirrels) (Anderson 1981, Schorger 1982).

Ontogeny & Reproduction: Sciurus carolinensis has an eight-stage ontogeny. Light and temperature may beresponsible for breeding behavior in Sciurus carolinensis. They may breed twice each year (by 27% of adult females,with a range from 0-36%) with third litters occurringduring years of mild weather and good mast yields. Litter size may vary from 4 (year with abundant food) to 1 (yearwith scarce food). Most breeding occurs from December –March, and May to August (delayed slightly in northernlatitudes). Sciurus carolinensis has a duplex uterus whichaverages approximately 81mm long and a mean clitoridislength of approximately 4mm. As many as 34 males areattracted to one estrous female. A dominance hierarchyforms among males with females demonstrating polyandry.Male gray squirrels follow females for 5 days before theygo into estrous (who have the highest levels ofandrostenedione) and ovulation (ova are ~95um in diameter –ovaries of pregnant females weigh nearly twice those ofnon-pregnant females, ovarian follicles are numerous andlargest in pregnancy) is induced by the presence of afertile male. An enlarged pink vulva indicates estrus(behavioral estrus last less than 8 hours). Copulationlasts less than 30 seconds, with the male mounting dorsallyand implanting a vaginal plug after fertilizing the female.Implantations are evenly distributed in the uterine horns.The paraplacental chorion allows albumin to diffuse butrestricts large proteins. Placental scars (from birthing)remain for over a week (Koprowski 1994, Andersen 1981).

Females can bear young at approximately 5 months ofage, but usually don’t until after 1 year of age and have areproductive longevity of approximately 8 years (even 12.5yrs. in warmer climates). The vagina is closed inanestrous and prepubescent females. Plasma progesteronerises from less than 3nmol/l before gestation to 318nmol/lin day 35 of gestation with levels dropping afterparturition. Corpora lutea regress at day 30 of pregnancy.The corpora lutea and placenta are the sources ofprogesterone, however pregnancy continues after ovariectomy.The number of corpora lutea is usually equal to the numberof embryos. Combined ovum and embryo mortality isapproximately 9% (Koprowski 1994, Andersen 1981).

Average litter sizes range from 1 to 4 with a maximumof eight. The average percentage of adults producinglitters for spring is approximately 61% (with a range of 095%) and for summer is 66% (range 9-100%). About 90% of adults, 50-60% of yearlings, and less than 5% of sub-adultsand juveniles, less than 1 year of age, reproduce each year.

Frequency of lactation in summer may fall due to mastfailure. Precocious breeding by females is observed onlyafter a good mast crop (Koprowski 1994, Andersen 1981).

Males gain sexually maturity at approximately ten oreleven months of age. They have a structure in the penisknown as a baculum (~9-12.5mm long). Males undergo asemiannual cycle of testicular recrudescence and regression,with functional testes being pendant in the scrotum.Spermatogenesis usually occurs between December to Februaryand May to July. Regressed testes without spermatogenesisusually occur in August to October and last two months perindividual. Active testes weigh approximately six to seven(versus 1g. for inactive testes). Occasionally, entirebreeding seasons may be skipped (especially during times ofpoor mast yields) (Koprowski 1994, Andersen 1981).

The interstitial cells of leydig atrophy in regressedtestes result in lowered plasma androgen levels. Mean plasma testosterone levels in spermatogenically activemales may be .8-7nmol/l (but less than or equalto .05nmol/l in regressed males). Testosterone levels in males vary from .4 to 20 nmol/l with declines toundetectable levels 3 weeks after a castration (with anassociated fall in accessory gland weights). Progesteroneis produced in the seminiferous tubules and accumulated in regressed testes. Regrowth of the prostate is preceded bymitotic activity. The prostate has either two types ofsecretory cells or a single type that changes function seasonally. Only 42% of germ cells become spermatozoa,resulting in relatively low numbers of spermatozoa forrodents. Epididymal spermatozoa aggregate into uniquecylindrical bodies and then separate, becoming single orrouleaux(sp.) (Koprowski 1994, Andersen 1981).

Newborn (neonates) gray squirrels are naked except forwhiskers (vibrissae) and weigh 13-18g. The umbilicus is attached for less than four days. The dorsum darkens at 710 days preceding the emergence of hair. Hair occurs on the tail dorsum by three weeks and the ventral surface bysix weeks. Eyes open at 24-42 days, and ears open at 3-4weeks. Lower incisors erupt at 19-21 days followed byupper incisors in week four and cheek teeth in week six.Weaning begins at seven weeks and is complete by ten weeks.The juvenile pelage is lost after weaning. Adult body massis reached after 8-9 months (Koprowski 1994, Andersen 1981).

Ecology and Behavior: Sciurus carolinensis molt twice a year (starting with the head) and molt their tail once insummer. Their body mass gets smaller as their distributiongoes south. They are mostly active in the early and latterparts of the day (bimodal). Male gray squirrels marktraditional points on trees with urine. Sciurus carolinensis can swim (doggy-paddle) at speeds up to 27km/hr. They gather and bury nuts for storage during wintermonths (scatter hoarders) when food is not available(Koprowski 1994, Andersen 1981).

Sciurus carolinensis feeds on a variety of nuts, seeds,buds, and cultivated (i.e. corn and wheat). They have evenbeen known to eat tree bark, fungi, bird eggs, frogs, andinsects. They are typically found in hardwood forestsgenerally greater than 40ha (oaks, hickory, and walnut), orsmall woodlots interspersed with farmland. They make nestsin hollow trees or build leaf nests high in branches oftrees. They are found throughout the state of Wisconsin(Koprowski 1994, Andersen 1981, WDNR 1998).

Because Eastern Gray Squirrels are woodland creatures,formation of den cavities requires 8-30 years with aminimum of 1 den/.8ha to maintain a density of 1squirrel/1.6ha. Selective cutting of the basal area oftrees (greater than 30cm at DBH) has little effect ondensities, reproduction, or survival. Densities of Sciurus carolinensis are usually less than 3/ha in continuouswoodlots; whereas, densities in small woodlots can be 16/hawith varying fluctuations based on the availability of treeseeds. Long term densities remain constant (Koprowski 1994, Andersen 1981).

Home ranges vary from .5-20ha with males usuallyhaving a 120% larger home range than females (based onseasonal sexual activity). Overlap of home ranges isextensive, but territorially not evidenced. Sex ratios are as follows: 1:1 in nestlings, .8:1.4 in juveniles,and .8:1.6 in adults. Survivorship of gray squirrels fromless than 8 months age classes to 1 year age classesusually averages 25%. Mean mortality for adults is 40-60%.Predators (in Wisconsin) include: largemouth bass, timberrattlesnake, several hawk and owl species, weasel, mink,bobcat, coyotes and foxes (wolves too), dogs, cats, andhumans. Gray squirrels rarely die from fatal falls.However, they have several types of parasites (Koprowski 1994, Andersen 1981).

Although Sciurus carolinensis feed on as many as 97 plant and 14 animal items, 18 plant species account for 87%of stomach volume (Koprowski 1994).

Remarks: The genus term Sciurus is from the ancient Greek, skia meaning shadow or shade, and oura for tail;carolinensis refers to the type locality of the colony ofCarolina (Koprowski 1994). Additional common names for theEastern Gray Squirrel are cat squirrel, migratory squirrel,bushy tail, black squirrel (gray squirrels are known to gothrough a black color phase), dark-bellied squirrel,Merriam’s gray squirrel, stump ear, timber squirrel, andah-ji-duh-mo (“tail in the air” – Chippewa Tribe) (Jackson 1961).

Jane Abbott - Kent State University (1986)

Popular ways of obtaining squirrels, whether for research,food, or fur, are by using live traps (baited with nuts orgrains) or by several hunting methods (rifle, shotgun, bow,with or without the aid of hunting dogs) (Koprowski 1994).

With the expansion of urban America and the greatdemand to observe non-game wildlife, the gray squirrel isincreasingly gaining the attention of natural resourcepersonnel. Increased interest is due primarily to theexpansion of America’s cities into rural areas, causing aloss of wildlife habitat in some instances (Koprowski 1994).

Eastern Gray Squirrels are commonly seen in city parksand lawns and on people’s birdfeeders. Some of the more rare types of gray squirrels are not gray at all, they arecalled melanistic gray squirrels and are a blonde, mutant,type (Long 1970). The UW-Stevens Point campus(particularly around the dormitories) is an example ofwhere this type of blonde gray squirrel can be found.


Andersen, Kenneth K. 1981. The Density And Distribution Of Gray Squirrels In A Suburban Environment. A thesis submitted in partial fulfillment of the requirements for the degree Master of Science. College of Natural Resources Universtiy of Wisconsin – Stevens Point.

Flyger, V. 1999. Eastern gray squirrel. In: D.E. Wilson, and S. Ruff (eds.). The Smithsonian Book of North American Mammals. Washington, D.C.: Smithsonian Institution Press. p451 - 453

Jackson, Hartley H.T. 1961. Sciurus carolinensis hypophaeus Merriam. Mammals of Wisconsin. p155-164

Koprowski, John L. 1994. Sciurus carolinensis. Mammalian Species (American Society of Mammologists). No. 480, p.1-9, 3 figs.

Linzey, D. W. 1998. The Mammals of Virginia. Blacksburg, Virginia: The McDonald & Woodward Publishing Company, Inc.

Long, Charles A. 1970. Sciurus carolinensis hypophaeus Merriam. UWSP Museum of Natural History – Reports on the Fauna and Flora of Wisconsin. No. 3, p25-26

Madson, Chris. 1978. Squirrels on the Howard Potter Research Area. Wisconsin Acadamey of Sciences, Arts and Letters. p284-318

Schorger, A.W. 1982. The Gray Squirrel. Wildlife In Early Wisconsin. p206-250

Wisconsin DNR. 1998. Gray Squirrel. Wisconsin Wildlife Primer. p12-13

Reference written by Luke Breitenbach, Biol 378: Edited by Chris Yahnke. Page last updated

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