Island Science

Squirrels – Fluffy Invaders

by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station

Every time I drive down the Field Station road, I see the same gray squirrel run across the road in almost the exact same location. Today he was followed by a second squirrel and I immediately thought, “there goes the neighborhood!” Not that I have anything against gray squirrels, but they are a recently introduced species and, although so far they have stayed in their own little niche, in some locations on the mainland they have pushed out their brethren and taken over the environment. Nantucket has such a limited number of mammals that a newcomer like the gray squirrel really sticks out. Gray squirrels (Sciurus carolinensis) are thought to have arrived around 30 or so years ago most likely on the ferry hidden on a pallet of wood or in the recesses of a truck.

When you go to the Massachusetts Division of Fisheries & Wildlife’s web page mammal web page (, you’ll find a list of every  mammal species found in the state. This is a fascinating list showing a variety of hair-brained introductions (European Hare and European Rabbit) across the state and short lived heydays for various creatures (wolves and cougars). And it includes our aquatic mammals too. You’ll see a lot of “except in  Nantucket County” entries on the right hand column of the table. Introduced Rodentia nuisance species include the house mouse (Mus musculus), brown or “Norway” rat (Rattus norvegicus) common on Nantucket and famous denizen of seedy parts of the cities, the black rat (Rattus rattus) which has been eradicated statewide.

Back to our fluffy friends with better publicity agents. The living squirrels are divided into five subfamilies, with about 58 genera and some 285 species. The oldest squirrel fossil, Hesperopetes, dates back to the Chadronian (late Eocene, about 40–35 million years ago) and is similar to modern flying  squirrels. North America supports 8 genera and 66 species of squirrels. The three main categories of squirrels are tree (Sciurus and Tamiasciurus), ground (Spermophilus and mmospermphilus), and flying squirrels (Glaucomys). Gray squirrels belong to the order Rodentia and the Family Sciuridae which includes tree Squirrels and Marmots, chipmunks, and woodchucks (neither of the last two currently are found on Nantucket). Nantucket is unique in that it no longer has many species of small and medium sized mammals such as skunks, opossums, porcupines, raccoons, foxes, coyotes, and many of the other types of squirrels like red squirrels and southern flying squirrels that can be found throughout Massachusetts.

The genus, Sciurus, is derived from two Greek words, skia, meaning “shadow,” and oura, meaning tail.  This name alludes to the squirrel sitting in the shadow of its tail. The species name, carolinensis, refers, of course, to the Carolinas, where the species was first recorded and where the animal is still extremely common. The eastern gray squirrel inhabits the forests of eastern North America, extending westward from the Atlantic Ocean to the Great Plains and eastern Texas and south from southern Canada to the Gulf Coast. Its distribution is closely correlated with the distribution of the eastern hardwood trees, especially oak, hickory, and chestnut.

Sciurus carolinensis is a medium sized tree squirrel with no sexual dimorphism in size or coloration. The coat ranges from grizzled dark to pale gray and may have cinnamon or reddish tones. The ears are pale gray to white and its tail is white to pale gray. Their fur underneath is gray to buff colored. Eastern gray squirrels exhibit melanism, the opposite of albinism, which is very dark skin pigmentation common to squirrels found in the northern portions of the range (in Canada). Albinism is rare in all areas. Of course, everyone recognizes squirrels by their large bushy tails. The head and body length is from 23 to 30 centimeters (9.1 to 12 in), the tail from 19 to 25 centimeters (7.5 to 9.8 in) and the adult weight  varies between 400 and 600 grams (14 and 21 oz).

Breeding occurs in December-February and May-June and is slightly delayed in more northern latitudes. Gestation lasts 44 days. Most females begin their reproductive life at 1.25 years but can bear young as early as 5.5 months! Females may bear young twice a year for more than 8 years. Males usually are sexually mature by 11 months but maturity can be delayed by as much as two years if the young males are housed with a dominant adult male (that may be true for many species). The first litter is born in  February to March, the second in June to July. There are normally two to six young in each litter, but this number can be as high as 8. Eastern gray squirrels are born naked with their eyes closed.

From spring to fall, Eastern gray squirrels are crepuscular (I love that word, so creepy), or more active during the early and late hours of the day, and they tend to avoid the heat in the middle of a summer day. This habit changes in the winter when they become unimodally diurnal, or awake and raring to go only once a day versus crepuscular, with a peak in activity just 2-4 hours before sunset. Generally, females are more active in the summer months and males are more active in the winter months. They do not hibernate.

The maximum longevity is 12.5 years in the wild but a captive female lived to be more than 20 years of age. Squirrels occupy two types of homes, including a permanent tree den as well as a nest of leaves and twigs on a tree crotch 30-45 feet above the ground. Females nest alone when pregnant, and lactating females are especially aggressive and avoided by others. Eastern grey squirrels communicate among themselves with a variety of vocalizations and postures, such as tail flicking. They also have a keen sense of smell. They use their sense of smell to determine many things about their neighbors. Some of the things they can determine are levels of stress and reproductive condition.

Sciurus carolinensis feeds mostly on nuts, flowers and buds of more than 24 species of oaks, 10 species  of hickory, pecan, walnut and beech tree species. Maple, mulberry, hackberry, elm, bucky and horse chestnut fruits, seeds, bulbs or flowers are also eaten along with wild cherry, dogwood, hawthorn, black gum, hazelnut, hop hornbeam and gingko tree fruits, seeds, bulbs and/or flowers. The seeds and catkins of gymnosperms such as cedar, hemlock, pine, and spruce are another food source along with a variety of herbaceous plants and fungi. Crops, such as corn and wheat, are eaten, especially in the winter. Insects are eaten in the summer and are probably especially important for juveniles. Cannibalism has been reported, and squirrels may also eat bones, bird eggs and nestlings, and frogs. Eastern grey squirrels are preyed on by many predators, including American mink, other weasels, red foxes, bobcats, grey wolves, coyotes, lynx, and birds of prey, such as red-tailed hawks. They emit warning calls to warn neighboring squirrels of the presence of predators. Their extreme agility in the trees makes them difficult to capture.

On Nantucket they have few predators, with the red tailed hawk most likely the primary doom bringer. Cars may be a close second with feral cats also playing a role. So far we have not seen a huge outbreak of squirrels, so their ability to reproduce is keeping pace with, but not exceeding, their mortality rate. Because there are no native species of squirrel on island, the eastern gray squirrel has not supplanted another creature and is not consider a pest or invasive creature of concern.

Like many members of the family Sciuridae, the eastern gray squirrel is a scatter-hoarder; it hoards food in numerous small caches for later recovery. Some caches are quite temporary, especially those made near the site of a sudden abundance of food which can be retrieved within hours or days for re-burial in a more secure site. Others are more permanent and are not retrieved until months later. It has been estimated that each squirrel makes several thousand caches each season. The squirrels have very accurate spatial memory for the locations of these caches, and use distant and nearby landmarks to retrieve them. Smell is used once the squirrel is within a few centimeters of the cache.

Squirrels have been known to pretend to bury the object if they feel that they are being watched. They do this by preparing the spot as usual, for instance digging a hole or widening a crack, miming the placement of the food while actually concealing it in their mouth, and then covering up the “cache” as if they had deposited the object. 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 that the claws of its hindpaws are backward pointing and can grip the tree bark. Anyone who has a bird feeder is highly aware of their intelligence and the lengths you must go to eliminate their raiding of bird seed in various feeders When they aren’t busy eating what people put out for the birds, they are important predators of seeds and other animals  in the ecosystems in which they live. Their seed-caching activities may help disperse tree seeds. Eastern grey squirrels are also prey animals themselves and are hosts for parasites such as ticks, fleas, lice, and roundworms. They are important members of the forest ecosystems in which they live.

Eastern grey squirrels provided food for Native Americans and colonists and are still eaten by some people today (I have, in fact, had squirrel pie while living in Oklahoma). They have economic  importance in some states, such as Mississippi where 2.5 million are harvested each year with an economic impact of 12.5 million dollars. Amazingly, squirrels are ranked second to birds in value to nature watchers when they are surveyed about their favorite wildlife species. But they can become a pest when they decide to make your attic or barn their home. I was able to find an excellent resource
which compares all the natural tendencies and history of various squirrel species to help wildlife managers deal with them effectively and humanely called “Utilizing Squirrel Natural History in Rehabilitation Decisions” by Shirley J. Casey with Wild Again Wildlife Rehabilitation Inc

The Grey Squirrel has made it to the Invasive Species Specialist Group of the World Conservation Union’s list of “100 of the World’s Worst Invasive Alien Species.” In Great Britain, Sciurus carolinensis is considered very destructive to property and is ranked second in negative impact only to the Norway rat. Sciurus carolinensis first appeared in the English countryside between 1876 and 1929 having been accidentally released from the London Zoo. The gray squirrel is larger than the native red squirrel (Sciurus vulgaris) in Great Britain and Ireland and is believed to be causing a drastic reduction in the populations of native squirrels. In Washington State, isolated populations of Western gray squirrels are negatively affected by competition for resources with the eastern gray squirrel. Whether you love them or hate them, for now it appears that they are here to stay on Nantucket.

Articles by Date from 2012