This page is the first
in a series of future squirrel natural history pages .
This paper was researched and written in the summer of 2002 © by Barbara
Bellens- Picon and was
presented in November 2002 at New York State Wildlife Conference.All
rights reserved.
Sciurus carolinensis
The Sciuridae
Eastern Gray Squirrel
Sciurus carolinenis Gmelin, 1788
Graceful, acrobatic, charismatic ,one of the more familiar yet fascinating tree squirrels in
North America is the eastern gray squirrel (Sciurus
carolinensis)Eastern gray squirrels belong
to the order Rodentia
of the class Mammalia. Rodentia is the largest
order of mammals in
the
world and accounts for more than 1600 species, greater than one third of the
species of mammals living today. The word
Rodent is derived from Latin, Rodere meaning;
to
gnaw.Some rodents are arboreal (red, gray, and fox squirrels),
some are semifossorial -
burrowers(prairie dogs, ground squirrels, woodchucks),
and some do both (chipmunks).
Some hibernate (woodchucks),some enter deep
torpor and feed occasionally on cached
food supplies (chipmunks), and many
are
active year-round. Most rodents feed on
vegetation,
but a few species, notably the grasshopper mouse, feed extensively
upon animal matter
Specialized anatomical
features emphasize the diversity of the group and include the beaver
tail,
porcupine quills,
and the patagium of the flying squirrel. The order Rodentia
is further
divided into three suborders, one of which, the
sciuronathi, contains
the squirrel-like rodents.
The 367 or so species in this sub-order are further
divided into 7 families.
One of these, the Sciuridae, includes the tree
squirrels, the ground squirrels and the flying
squirrels. Currently
26 species
of tree squirrels in the genus sciurus are recognized with
three species
found in
Eurasia and across North and South America (Wilson and Reeder 1993).
The Sciuridae
The Sciuridae is made up
of 267 species in 49 genera. A large family represented in both old and
new
worlds. The Sciuridae includes arboreal tree squirrels, terrestrial chipmunks
and ground squirrels, and
semi fossorial-marmots and prairie dogs. In habits,
members of this order are diverse. There are
three basic body forms in squirrels: tree squirrels, ground squirrels and flying squirrels. Tree
squirrels
have long, bushy tails, sharp claws and large ears. Some have well-developed ear
tufts.
Flying squirrels have a furred membrane (patagium) extending between the
wrist and ankle that
allows them to glide between trees.
Ground squirrels often
have short, sturdy forelimbs that are used for digging. Their tails, while fully
furred, generally are not as bushy as those of tree squirrels.
Like many other
rodents, all squirrels have five functional toes on the hind feet and four on
the
forefeet, with a well-developed claw on each digit.
Squirrels occur
throughout the world except for Australia, Madagascar, southern South America,
parts of North Africa ,polar and some desert regions.( Gurnell ) They inhabit a
wide variety of habitats
from the tops of trees to underground tunnels.
There are seven native
species of tree squirrels in North America which belong to the two genera,Tamiasciurus
and Sciurus; Pine squirrel,(Tamiasciurus hudsonicus),Douglas squirrel (
T.douglasii)
Fox squirrel ( Sciurus niger) Eastern Gray squirrel(
Sciurus
carolinensis)Western gray
squirrel( S griseus)Arizona gray squirrel ( S
arizonensis)and the Abert squirrel( S aberti )
Appearing
on the USFW endangered species list are the Delmarva Fox squirrel
(Sciurus niger
cinereus) and mount graham red squirrel, (Tamiasciurus hudsonicus grahamensis).
Two Appalachian Mountain subspecies of the Northern Flying Squirrel also appear
on the US Endangered species list; the Carolina Northern Flying Squirrel (glaucomys
sabrinas coloratus), classified as endangered in North Carolina and Tennessee,
and the Virginia Northern Flying Squirrel (glaucomys sabrinus fuscus),
classified as endangered in Virginia and West Virginia. Loss of habitat is the
primary reason for the species decline.
“Sciurus" the Latin word for squirrel comes from
the ancient Greek "skia," meaning shadow,
and "oura," meaning tail
referring to the characteristic posture of a thick bushy tail held over
its
head. The eastern gray squirrel (Sciurus carolinensis) was first observed in
the Carolinas.
The earliest published record of this squirrel was in 1788 by J.F
Gmelin. James J Audubon was
one of several naturalists who referred to the gray
squirrel as the “migratory squirrel”
(Sciurus migratorius) in reference
to its tendency for mass migrations.
Fossil Records
Some biologists refer to tree squirrels as "living
fossils" because their form and structure have
been relatively unchanged for the
last 5 million years. Discovered in fossil records from the
Oligocene periods in
North America, Protosciurus is the oldest known member of the squirrel
family and its bones are very similar to those of the tree squirrel today.
Fossils of some early
squirrels are on display in the Smithsonian Museum.
Diet
Classified as granivore/herbivores
they primarily rely on the products of the trees in the
habitats in which they
live. They are known to eat bark of dead trees for the fungal mycelia
within.
Squirrels have a large cecum, optimized to digest plant protein, not animal
protein .
However they will resort to occasional omnivory when necessary. Adults
obtain minerals from
bones, antlers and turtle shells. They feed on nuts flowers
and buds of more than 24 oak
species, 10 species of hickory, and also pecan,
walnut and beech. (Fagus grandifolia)
(Korschgen, 1981;Nixon et al, 1968) Other
important foods include the fruits, seeds, buds or
flowers of maples (acer)
mulberry (morus) hackberry (celtis) elms (ulnus) buckeye and horse
chestnuts (aesculus)
wild cherries (prunus) dogwoods (cornus) hawthorn (crategus), black
gum (Nyssa
sylvatica) hazelnut (Corylus americana) hop hornbeam (ostrya virginiana) and
ginko (Ginko biloba).
Seeds and catkins of gymnosperms are also eaten including cedar (juniperus)
hemlock
(tsuga) pines (pinus) and spruce (picea) (Barkalow and Shorten
1973);
(Thompson and Thompson 1980) A variety of fungi are also eaten
particularly in the summer
along with insects. Other lesser-consumed items are
bones (Flyger and Gates 1982) bird
eggs and nestlings (bailey 1923) and frogs (Goodrum
1961) tree bark and cambium-
inner lining of trees.
Distribution/Habitat
Eastern gray squirrels
are found throughout the eastern United States; their natural range
extends from
Florida, north to Canada, and west to where deciduous forests meet the
Great
Plains grasslands. Due to introductions, they have become established in several
areas of western Us and Canada (Flyger and Gates 1982) Mature deciduous forests
provide
an ideal habitat.
Extremely adaptable, they
are found anywhere, except spruce/fir forests,
where red squirrels dominate.
Gray squirrels tend to prefer more dense woods than fox
squirrels ( Sciurus
niger). The more large trees an area has, the more gray squirrels will
inhabit
the area (larger cavities and more food). Adapted to a variety of wooded
conditions,
included urban area parks and suburbia that include oak, maple and
hickory trees.
Habitat loss worldwide greatly affects tree squirrels and is a central factor in
their conservation.
For example since 1820, nearly 80 percent of forests in Illinois have been
converted to agricultural
cropland (Iverson 1988) Habitat loss is the primary reason cited for the legal
protection of several
subspecies of fox squirrels in the eastern United States.
Home Range
The area where most of
its foraging for food, nest building, and breeding/rearing occurs.
The home
ranges of males are larger than those of females. One to four acres, up to ten
acres (varies with food availability), Chapman and Feldhammer report 2 acres for
males
and 1.25 acres for females in a Maryland study (Flyger). Home ranges can
overlap and
there is no evidence that of true territoriality (Don 1983) although
core areas are
sometimes defended (Kenward 1985)
Subspecies
There are 5 subspecies of
eastern grey squirrels, S. carolinensis carolinensis is the
subspecies
found in most of the south from northern Florida, to North Carolina, west to
Missouri, and eastern Texas. Others include S.cextimus,S.c.fuliginosus,
S.c. hypophaeus
and S.c. pennslylvanicus.
Sciurus carolinensis also
were introduced into various parts
of the pacific states and also into parts of
Canada, England, Italy, Scotland, South Africa
and Australia. The overseas
introductions occurred at the end of the 19th and beginning
of the 20th
century. The 1880's introduction from England to Australia failed by 1973.
(Purdue, 1980)
General Characteristics
Sciurus carolinensis are medium sized arboreal
dwelling quadrupeds weighing @ 300-700
grams with no sexual dimorphism in size
or coloration. 
Body length measures@ 16-21 inches with the distinctive trademark
tail
measuring 8- 10 inches. They have strong back legs and their back
feet contain 5
elongated claws with the 4th being the longest. Their back
feet can
turn 180 degrees, an adaptation that allows them to flexibly
climb head first,
down a tall tree. Their front feet have 4 claws and the
appearance of a
vestigial thumb. Diurnal in winter to crepuscular in
summer, primarily arboreal mammals that are
most active in
September and early October, burying
acorns (this is also the peak time
for their arboreal activity). They do not
hibernate like the woodchuck
(Marmota monax), but may rest to conserve energy in
response to
extreme temperatures. Active year-round, they may remain in their
nests
during inclement weather until hunger drives them to forage. Activity
levels
generally peak @ 2 hours post dawn and 2-4 hours before sundown.
Appearing predominantly gray up close, the fur color
palette actually is mixed with dark and
light gray hues. Underside fur color is
whitish and tail guard hair tips are white tipped. White or
buff colored fur
ring the eyes .A buff tinted color appears on top of feet, shoulders and head
area. Pads of feet are pinkish. Sometimes in winter, whitish fur grows around
the ears giving a
"tufted" appearance and thicker fur grows on the inner bottom
legs giving a “pantaloon”
appearance. In many areas of the country/state color
variations exist. Gray squirrels can have
brown, dark gray, orange, erythristic
(reddish) and tan color tone variations. Mixed breeding,
genetics and
environmental factors could possibly attribute to the reason behind these
variations of color. Melanism (black) and albinism (white) occur throughout the
species range,
although melanism populations occur more commonly in the north.
Seasonal breeders, they are genetically timed to breed and
reproduce coinciding with the
buds and flowering of indigenous trees .Two peaks
in breeding occur resulting in spring and
fall litters. Litter size ranges from
1-8,with an average of 2-3. Weather and food supply are two
strong factors that
affect their breeding behavior. Unfavorable weather and poor food supplies
can
delay or even offset breeding seasons. It can begin early as December and end
around
July or August although litters of young are sometimes born well into
fall and lactation frequently
continues in September and October. Females can go
into estrus and become sexually
mature at 9-15 months of age. Most females do
not give birth during their first year. They
have
a duplex uterus ,os clitoris measuring 3.6 mm ( Burt 1960 ) and eight
mammae (4 pectoral,
two abdominal, 2 iningual) that become black after the first
pregnancy. (Flyger and Gates, 1982)
Gestation is @ 44 days with neonatal
squirrels being born weighing 12-18 grams, and about 2
inches long, with claws
and vibrissae but without fur, sight and hearing .The umbilicus remains
attached
for @ 4 days. The dorsum darkens at 7-10 days preceding the development of
velvet
like fuzz toward the end of the second week. It takes @3 to 4 months to
achieve independence.
Most male eastern gray
squirrels become sexually mature at @ 9 - 12 months of age. Inactive
testes
weigh 1g, whereas active testes weight 6-7g. (Webley and Johnson 1983) The brain
weighs @ 7-10 grams (Meir, P.T. 1983) The cycle of testicular recrudescence and
regression
occurs twice a year. The baculum or os penis is thought to aid
in copulation and is 9.2-12.3 mm
in length. ( Burt 1960 )Their heart beats
150-450 BPM. (E Smith and Johnson 1984) and their
body temperature ranges
between 98 and 102 degrees F. (Bolls and Perfect 1972; Hoff Lassing
et al 1976)
Eastern gray squirrels have both a summer and winter coat, and therefore molt
twice each year,
coinciding with the
changing of the seasons. Molting lasts for @ 4-6 weeks but individual
differences vary. Spring molt begins at the face and head, progressing toward
the rump. The tail
and tips of ears only molt once. Tail molt begins at
base and ends at tip…this process can take
several months to complete. Fall molt
occurs in the opposite direction of spring molt. Variations
of color coincide
with the molt and most likely function in camouflage: to help them survive.
Squirrel fur is multi-layered providing insulation with short hairs, which lay
close to the skin, and
long outer fur, called guard hairs, which grow longer
during the winter months.
Like many other rodents lacking canine teeth, their incisors are hypsodont (grow continuously throughout life). They need to gnaw in order to keep these teeth trimmed. Each incisor is a segment of a true circle, continuously being pushed out of the end of the jaw. Unlike lagomorphs that have enamel covering the entire incisors, rodents only have hardened enamel on the anterior (front) side of the incisors. This keeps a sharp edge on the teeth, since the relatively soft posterior (back) side of the teeth wears more quickly than the front. They have 22 teeth in total. The total dental formula is 1 incisor, 0 canine, 1 premolar, and 3 molars on each side of the bottom mandible. A complimentary set of the same number of teeth occurs on each side of the top with a second premolar. Dental formula is expressed I 1/1,C 0/0, P 2/1 M 3/3 . The front incisors are a yellowish, orange color. Adjacent to the incisors is a gap referred to as the diastema into which the lips can be drawn, sealing off the mouth from inedible food fragments and soil. Following the diastema are the grinding teeth, or "cheek" teeth, molars and pre-molars which are a combination of vertical layers of dentine and harder enamel, which produce peculiar ridged patterns on the crown of the teeth, useful in classification by zoologists. Gray squirrels have 2 upper pre-molars. Other species like the fox squirrel do not have the first upper premolar. Permanent molars will replace the first premolars when the squirrel reaches adulthood.The molars are rooted.
Distinguished feature of sciuromorhs are a relatively primitive unspecialized
arrangement of jaw muscles and parts of the skull. The lateral masseter muscle
extends in front of the eye onto the nose, moving the lower jaw forward during
gnawing. The deep masseter muscle is short and used only for closing the jaw.
Reprinted with permission William Higgs-Dept of Archaeology, University of York
http://www-users.york.ac.uk/~wjh101/hedbone/Squirrels/squirrels.htm
Squirrels have 4 sets of
whiskers or vibrissae on the head. The whiskers are always black in
color. They
are located above and below their eyes, in front of the throat and along side
the
nose. There are also whiskers on the wrist (ulna) on the underside of the mammae, at the
base of the tail, and around the feet. (Laidler 1980, Shorten
1954). Some of these whiskers
are even present at birth. The sensitive whiskers
function as touch receptors that help relay
information about the squirrel’s
immediate surroundings. Eye- closed neonates use them to
locate the mammae in order to
nurse.
Squirrels have very good
eyesight with a wide field of vision and are able to see in dim light.
Evidence suggests that they possess dichromatic color vision which would
akin them with a
red/green colorblind person. They possess a well-developed sense of smell, which
aid in their
instinct to survive. It allows them to help locate buried (cached)
food and plays an important
role in their social communication. Scent glands can
be found on the underside of their paws
and they also deposit urine on tree
branches as a way to define their territory and home range.
These marking sights
can sometimes be found on the underside of large branches. Females
emit odor and
deposit scent at the onset of estrus to announce their availability to the male
population. Males reciprocate by scooting across branches to deposit their own
individual scent
on the branches and trees.
A squirrel’s glorious
tail is its characteristic trademark providing multifunctional uses, which
include protection, shade, balance, warmth, and communication. Stripes located
on the
underside provide a rough estimation of a young squirrels age. Adults do
not possess this
characteristic. The squirrel’s tail can also break at mid-point
and allow the squirrel to escape
while being held by a predator. Squirrels with
partial tails can function and survive successfully.
Depending on position and
action tail movement reflects emotion; it can indicate fear, curiosity,
aggression, or anger. A limp non-responsive tail could mean injury.
Gray squirrels are scatter
hoarders, placing single food items at different locations. Food is
buried
(cached) one half to 1 inch beneath the soil. Scatter hoarding is an important
survival
strategy that allows gray squirrels to remain active through the winter
and spring. They primarily
use their keen sense of smell to locate the buried
food but often, the squirrels do not locate
caches and the stored seeds
germinate to produce trees and other forest plants. Because fungi
is part of their
diet, the Eastern grey squirrel may play as important role in the dispersal of
hypogeous mycorrhizal spores. All conifers and many hardwoods strongly depend on
mycorrhizae (fungus roots) for growth and survival.
Generally socially tolerant, a dominance hierarchy in both females and males is
maintained by
bluffing or chasing rather than by actual fighting. Usually
considered solitary animals, studies
suggest that they operate within a complex
social system. (Thompson 1978a, 1978b:Koprowski 1991a, 1993a)
Males dominate over females, adults dominate over immature sub adults. Pregnant
and lactating
females are aggressive toward all.
Individual squirrels are often seen feeding close to
each other
without any aggressive activity
Nests
Three general types: Summer, winter dreys and winter den.
The leaf nests are called dreys and the
dens are formed in tree cavities and
crevices. Winter dreys are usually more sturdy and elaborate
then summer ones. Dreys that are @12- 15 inches wide and one foot high are
designed to be
waterproof, constructed with an outer layer of coarse natural items such
as larger twigs and
branches. The inner layer consists of leaves, bark and other insulating
materials .In urban
metropolitan areas unnatural items have been used as construction material such
as; ceiling
insulation, cereal boxes, cigarette cartons and plastic bags. It may take
squirrel one or two days
to complete building a nest. They can be seen adding to it over time with
the advent of inclement
weather conditions. A nest will always have one entrance and sometimes a second,
which probably
functions as an escape route. Groups of individuals can share a nest.
Unlike flying squirrels, grays
generally do not use their nest as a latrine or to store food. The nest is
usually clean. When the
problem of fleas arise they vacate and set up housekeeping elsewhere. Gray
squirrels can have
more than one nest. Perhaps this helps to control external parasites, elude
predators or establish
territory. Gray squirrels also use tree dens primarily in winter. These
dens exist in the main trunk of
large mature or dead trees. They offer substantial protection against cold
and inclement weather.
Predators
Natural predators include Largemouth bass (Micropterus
salmoides) Timber snake
(crotalus horridus) Diamond back rattlesnake
(C.adamanteus), Black rat snakes
(Elaphe obsoleta): moore 1941)
Bullsnakes (pituophis melanoleucus) Red tail hawk,
(Buteo jamaicensis)
Red shouldered hawk (B Lineatus), Broad winged hawk (Buteo platypterus)
Marsh Harrier (Circus
aeruginosus) Cooper’s hawk, (A. Cooperi) Goshawk (Accipiter
gentilis),
Great horned owl (Bubo virginiacus), Long-eared Owl (Asio
otus), Barred owl, (Strix varia)
American crow, (Corvus brachyrhynchos)Common raven,(Corvus corax) Raccoon ( procyon
lotor),
Opossum,(
Didelphis virginiana)
American martin(Martes americana),
Weasel( Mastela frenata;hamilton
1934), Red fox,(vulpes,vulpes),
Gray fox( Urocon cinereoargenteus)
Bobcat,( lynxs rufus) Coyote,( C Latrans) mink(M.Vison),
Red
Squirrel (tamiascirus hudsonicus)
Unnatural/introduced predators include; House cat,(Felis
catus) Dog,(Canis familiaris )
and Human beings.(Homo
sapiens)
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