Sciurus carolinensis – (Eastern) Grey Squirrel

Phylum: Chordata    Class: Mammalia    Order: Rodentia    Family: Sciuridae

Species Status in Ontario: Least Concern

Image of Eastern Grey Squirrel digital stock illustration, Sciurus carolinensis, Colour, 2020 © Suzanne M Matheson.
Sciurus carolinensis, Colour, 2020 © Suzanne M Matheson

Family, Sciuridae:
The term, rodent, comes from the Latin rodere, which means “to gnaw”. All animals in the order Rodentia are mammals that gnaw. They are diagnosed by a single pair of continuously growing incisors in both the upper and the lower jaw. Due to an extra pair of incisors, rabbits are not considered rodents.

Most rodents are herbivores. While the herbivore crania (housing smaller temporalis muscles than that of carnivores and omnivores) comes in two basic cranial shapes, smooth-rounded and smooth-flat, rodents are exceptions. Their skulls can be round and smooth or slightly peaked.

Family Sciuridae includes small to medium-sized rodents. The list includes prairie dogs, groundhogs and other marmots, chipmunks, flying squirrels, ground squirrels and tree squirrels. Eastern Grey (Gray) Squirrels are considered tree squirrels, not because of their physiology, but rather due to their arboreal habitat. They nest in trees and most of their life-sustaining activities rely on trees. Though flying squirrels make their home in trees, their physiology sets them apart.

Genus, Sciurus:
Squirrel fossil records date back 56 – 33.9 million years, to the Eocene epoch, the “dawn” of modern fauna. They are closely related to mountain beaver and dormice and are indigenous to the Americas, Eurasia, and Africa.

“Squirrel” is an etymological descendant of the Latin word “sciurus”. This was a word borrowed from Ancient Greek, “skiouros”, meaning shadow-tailed, and refers to the bushy tail of many Sciurus members.

When referring to a group of squirrels, you might say “a dray of squirrels” or “a scurry of squirrels”.

Description/Morphology:
This omnivorous mammal is a medium-sized rodent, 8-10 inches long, and a bushy tail of similar length. Fur on the back is grey and on the belly, white. In more northern areas, including Southern Ontario, there are black grey squirrels. These are melanistic, that is, individuals with a greater concentration of the dark-coloured pigment, melanin, in their fur.
Front foot: 4 sharp-nailed toes for climbing, 3 palm pads, 2 heel pads, and a vestigial thumb located on the inside near the heel pad. In soft substrate, the vestigial thumb will show up in the track.
Hind foot: 5 toes with nails, 4 palm pads, 2 heel pads with a surrounding area lacking hair. Sometimes, as is commonly the case with Eastern Cottontail Rabbits, S. carolinensis’ hind feet will make elongated tracks. Usually, the hind tracks end with the 4 palm pads. The track pattern is typically trapezoidal, with two hind tracks ahead and wider apart than the two closely spaced front tracks.

Range:
Generally, S. carolinensis can be found from the southern portion of Manitoba down into Texas and east to the Atlantic coast. I’ve heard that Southern Ontario has the greatest population of melanistic grey squirrels.

​Habitat:
S. carolinensis has adapted quite well to urban environments, though this species prefers living in deciduous, hardwood forests where acorns, hickory nuts and beechnuts are plentiful. In Southern Ontario, the frequent discovery of gnawing signs on discarded Black Walnut shells, particular to Eastern Grey Squirrel, reveals that they are keen for these large nuts as well. In warm seasons, when nuts are unavailable, they will also take advantage of the following habitat food offerings: expanding buds of many trees, ripened seeds of elm, maple and basswood, caterpillars and cocoons, beetles and ants, and on occasion, bird’s eggs and nestlings. They enjoy the sweet sap that flows in the spring from a maple or birch injury. ​​​

Sketchbook illustration of Quercus alba (White Oak) leaf in graphite and various perspectives of an acorn in graphite and coloured pencil © Suzanne M Matheson, 2019.
Quercus alba (White Oak) leaf and various perspectives of an acorn © Suzanne M Matheson, 2019

In winter, in the northern parts of their range, they shelter in tree holes and tree crevices, insulated with leaves and other soft materials. Otherwise, they will nest in “outdoor” spherical leaf nests, 12” to 19” in diameter. These loose, leafy nests are built high up in mature deciduous trees, with the main entrance near the tree’s trunk. To discover where a squirrel may be nesting, follow one who is carrying leaves, twigs and/or shredded bark.

Behaviour:
I don’t think of squirrels as migratory, so this story really piqued my curiosity: Legend has it that in the early 1800’s there was an intense grey squirrel migration, with up to a billion animals moving into new territory, swaths a mile wide. When encountering a river, to make their passage across, the squirrels would float on pieces of bark and hoist their tails up for sails! Today, surplus and scarcity of acorns in any given year may instigate mass migrations of Eastern Grey Squirrel.

S. carolinensis is diurnal, with their day starting at sunrise and ending just a little after sunset. The home range of each individual is fixed. Within this range he or she acquires food, gains protection and locates a place to den. Ideally, this range would be several wooded acres, more for males, less for females. Not territorial creatures, these home ranges overlap significantly. Defense is reserved for females during breeding season.

The social group, consisting of all squirrels with overlapping ranges, has a hierarchy of dominance based on age and sex, with older males at the top. With this hierarchy very little aggression occurs. If you witness aggression, it could be a young squirrel attempting to establish a home range near its birthplace and within that of an older squirrel. In this case, the local hierarchy is being redefined.

Aggression for dominant squirrels takes the form of tooth chattering, sometimes audible, and forward and back tail waving. General alarm is also displayed as tail waving. On the ground, dominant squirrels will also jump towards, run at or chase others, often resulting in the two, classically, spiraling up a tree.

Squirrels have highways in the trees, efficient routes that they use regularly. When they pause, sniff and rub the sides of their face on a limb, they may be leaving scent marks to keep track of one another.

Life Cycle:
Mating/breeding occurs from December to February and again from June to July. The mating ritual begins with a pre-chase, the male following the female (who is giving off a scent) at a comfortable pace. The male will stop and sniff as he follows. Sometimes he gets close enough to sniff her directly. When the female is in full estrus the early morning mating chase begins. As many as ten males will chase a single female, all repeatedly calling “chuck”, among the trees. Females only mate on their home range, but males will travel out of their range at the sound of her chucks! There are occasional outbursts of aggression from both the female and among the males. One can determine who is who by the social hierarchy. In this case, the estrous female is in lead, followed by the older, most dominant males, with the younger males bringing up the rear.

Young are born in a tree den that is aggressively defended by the mother from all other squirrels, including the father. Gestation is 45 days, with an average of 3 young per litter. The infants are born hairless, with eyes and ears closed. As mammals, they gain their nourishment from mother’s milk. Four to five weeks after birth they make short trips out of the den, fully furred, with eyes and ears open. Weaning occurs at 8-9 weeks old and they are on their own when they disperse from their mother’s home range around 4 months of age.

Ecology:
One particularly heavy, cold winter I discovered many black-stained holes in stark-white snow under a large walnut tree. The holes did not extend into underground tunnels, but were only up to a foot deep and the size of a grey squirrel’s circumference. This is consistent with what I now read, that these squirrels do not store their food in large underground caches. Instead, they dig a shallow hole in the forest floor where they place and cover only one food item. They will also use tree cavities to store somewhat larger caches.

Apparently, Grey Squirrels are very adept at relocating their buried nuts. This may be due to scent marking the nut at the time of burial – pushing the nut down into the dug hole with their nose before covering it over with earth. The low percentage of “lost” nuts will germinate to become — seedlings … then saplings … join the understory … and finally reach the canopy as mature trees — a significant contribution to forest regeneration.

To protect their own hardwood community from an over-abundance of hungry squirrels, nut trees have evolved a sporadic crop pattern. Most years are lean, then once every 2-7 years all the trees throughout their range suddenly produce an abundance, referred to as a bumper or “mast year” crop. The lean years establish a low population of squirrels that then cannot make use of the abundance offered during the following mast year. I’ve been wondering what signals mast crops? Could it be the specific chemical levels injected into the soil by means of squirrel urine?

Threats:
Squirrels are often roadkill victims, especially the young ones in search of a home range.

When settlers first arrived on Turtle Island Grey Squirrel was so numerous as to be considered a significant threat to their agricultural crops. Some states declared a bounty on them, and this, along with the continuous and universal deforestation, caused such a drop in this species’ population there arose concern that Sciurus carolinensis would become extinct! With grey squirrel adaptation to urban environments, and with a decrease in farming, resulting in old field succession towards forest recovery, Eastern Grey squirrel has made a successful comeback.

Digital Stock Illustrations

Image of Eastern Grey Squirrel digital stock illustration, Sciurus carolinensis, Colour, 2020 © Suzanne M Matheson.
Sciurus carolinensis, Colour, 2020 © Suzanne M Matheson
Eastern Grey Squirrel (Sciurus carolinensis) with tracking tag & collar, digital colour stock illustration, for research presentation to the general public © Suzanne M Matheson, 2020.
Sciurus carolinensis with Tracking Device, Colour, 2020 © Suzanne M Matheson

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​Bibliography:

Kimmerer, Robin Wall. Braiding Sweetgrass. Minneapolis, Minnesota: Milkweed Editions, 2013.

Martin, Alexander C., Herbert S. Zim, & Arnold L. Nelson. American Wildlife & Plants A Guide to Wildlife Food Habits. New York: Dover Publications (1961), McGraw-HillBook Company, 1951.

Rezendes, Paul. Tracking & the Art of Seeing How to Read Animal Tracks & Sign. Charlotte, Vermont:
Camden House Publishing, Inc., 1992.

Searfoss, Glenn. Skulls and Bones A guide to the skeletal structures and behavior of North American
mammals. Mechanicsburg, Pennsylvania: Stackpole Books, 1995.

Stokes, Donald & Lillian. Stokes Guide to Animal Tracking and Behaviour. New York: Little, Brown and  Company, 1986.

On-line Resources:

“Eastern gray squirrel.” Wikipedia, The Free Encyclopedia. Updated 11 June 2020. Web. Accessed on 28 July 2020. https://en.wikipedia.org/wiki/Eastern_gray_squirrel.

“Rodent.” Wikipedia, The Free Encyclopedia. Updated 20 June 2020. Web. Accessed on 28 July 2020. https://en.wikipedia.org/wiki/Rodent.

“Squirrel.” Wikipedia, The Free Encyclopedia. Updated 10 June 2020. Web. Accessed on 28 July 2020. https://en.wikipedia.org/wiki/Squirrel.

“Tree Squirrel.” Wikipedia, The Free Encyclopedia. Updated 26 June 2020. Web. Accessed on 28 July 2020. https://en.wikipedia.org/wiki/Tree_squirrel.

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