Gray
squirrel
Gray squirrel is the common name for two species
of squirrel native to North America:
The Eastern Gray Squirrel (also introduced
elsewhere)
The Western Gray Squirrel.
Eastern
Gray Squirrel
(Sciurus carolinensis)
The Eastern Gray Squirrel (Sciurus
carolinensis) is a tree squirrel that is
native to the eastern to midwestern United States
and the eastern provinces of Canada. The species
name carolinensis refers to the Carolinas, where
they were first recorded by zoologists and are
still extremely common. The native range of the
Eastern Gray Squirrel overlaps with that of the
Fox squirrel, with which it is sometimes
confused.
A prolific and adaptable species, it has been
introduced to and thrives in several regions of
the western United States. The Eastern Gray
Squirrel has also been introduced to the United
Kingdom where it has successfully spread across
the country and displaced the native Red
Squirrel. There are concerns the same will happen
in Italy.
Description
As its name suggests, the Eastern Gray Squirrel's
fur is predominantly gray, but it can have a
reddish tinge. Its belly is white. They have a
large bushy tail. Particularly in urban
situations where predation risk is reduced, both
albino and melanistic forms of the Eastern Gray
Squirrel are quite often found. The melanistic
form, which is nearly black all over, is
predominant in certain local populations as well
as in large parts of southeastern Canada.
Behavior
Like many members of the family Sciuridae, the
Eastern Gray Squirrel is a scatter-hoarder; that
is, it hoards food in numerous small caches for
recovery later. Some of these caches (especially
those made near the site of a sudden abundance of
food) are retrieved within hours or days for
re-burial in a more secure site. Others 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. Olfaction is used
only once the squirrel is within close range (a
few centimeters at most) of the cache site.
The Eastern Gray Squirrel makes a variety of
noises, including a loud screeching, a "buck
buck buck" sound and a chattering, often
followed by "kyukyukyuuuu".
They make these noises to communicate with other
gray squirrels, and sometimes they make noises
during mating.
These squirrels build a type of nest, known as a
drey, in the forks of trees. These consist mainly
of dry leaves and twigs. Sometimes they will also
attempt to build a nest in the attic or exterior
walls of houses, often to the great annoyance and
frustration of the homeowner. They also invade
bird feeders for millet and sunflower seeds, but
safflower is often used instead, as they seem to
have no taste for it. Some seed is sold with hot
pepper coating, because only mammals such as
squirrels can taste its capsaicin, while the
birds cannot. Mixing hot pepper flakes into
regular birdseed works well as a squirrel
deterrent. They have also been known to dig up
bulbs from gardens. Their reputation for these
habits has led some to call them "tree
rats" or "rats with fuzzy tails".
Predators include hawks, mustelids, skunks,
raccoons, snakes and owls. On occasion, this
squirrel may lose part of its tail while escaping
a predator.
Distribution
The Eastern Gray Squirrel is common throughout
most of its natural range and wherever it has
been introduced. It readily becomes tolerant of
humans and learns to take food left or offered by
picnickers.
Introductions
The Eastern Gray has been introduced at a variety
of locations on the west coast of North America,
including the states of Washington and Oregon,
San Francisco and the Peninsula area of San Mateo
and Santa Clara counties south of the city. It is
now the most common squirrel in many urban and
suburban habitats in the western U.S. north of
central California. At the turn of the 20th
century the Eastern Gray Squirrel was introduced
into South Africa and England, spreading across
the latter and then invading both Wales and parts
of southern Scotland where it has almost
exclusively displaced the population of the
native Red Squirrel. It has also been introduced
to Italy, and the European Union is concerned
that the gray squirrel will displace the Red
Squirrel from parts of the European continent as
well.
Displacement of red squirrels
In the UK, the animal is known simply as the grey
squirrel and has no natural predators. This has
added to their rapid population growth and has
led to the species being classed as a pest.
Measures are being devised to reduce their
numbers, including one plan for famous television
chefs to promote the idea of eating gray
squirrels.[1] In areas where isolated populations
of red squirrels survive, such as the island of
Anglesey, eradication programmes for gray
squirrels are in progress to allow red squirrel
populations to recover [1].
Although the matter is controversial and complex,
the main factor in the displacement of red
squirrels by gray squirrels is thought to be
competition for resources, leading to a decrease
in fitness of the red squirrels on all measures
(e.g. Wauters, Gurnell, Martinoli & Tosi,
2002). Gray squirrels tend to be larger and
stronger than red squirrels and have been shown
to have a greater ability to put on fat before
the winter. These factors are thought to result
in grey squirrels competing effectively for a
larger share of the available food, resulting in
lower survival and breeding rates in red
squirrels. Parapox virus may also be a strongly
contributing factor. Red squirrels are fatally
affected by this disease, while gray squirrels
are unaffected but thought to be carriers. Red
squirrels are also more affected by habitat
destruction and fragmentation than the more
adaptable gray squirrel, which has also
contributed to a decrease in their numbers and a
linked increase in the numbers of gray squirrels.
This has also been the case in the Pacific region
of North America, where the native red squirrels
have been largely displaced by gray squirrels in
parks and forests throughout the region.
Western
Gray Squirrel
(Sciurus griseus)
The Western Gray Squirrel (Sciurus griseus)
is a tree squirrel found along the western coast
of the United States and Canada.
At various times and places, this species has
also been known as the Silver-gray Squirrel, the
California Gray Squirrel, the Oregon Gray
Squirrel, the Columbian Gray Squirrel, the
Banner-tail, and also simply as the Gray
Squirrel. There are three geographical
subspecies: Sciurus griseus griseus
(central Washington to the western Sierra Nevada
in central California); S. g. nigripes
(from south of San Francisco Bay to San Luis
Obispo County, California; and S. g. anthonyi,
which ranges from San Luis Obispo to
south-central California).
The Western Gray Squirrel was first described by
George Ord in 1818 based on notes taken by Lewis
and Clark at The Dalles in Wasco County, Oregon.
Compared with the Eastern Gray Squirrel S.
carolinensis or the Eastern Fox Squirrel
(which have been introduced into its range),
these squirrels are shy, and will generally run
up a tree and give a hoarse barking call when
disturbed. Weights vary from about 400 g to
nearly 1 kg, and length (including tail) from 45
to 60 cm. It is the largest native tree squirrel
in the western coastal United States. Western
Gray Squirrels exhibit a form of coloration known
as counter shading. The dorsal fur is a silver
gunmetal gray, with pure white on the underside;
there may be black flecks in the tail. Ears are
large but without tufts. The ears turn
reddish-brown at the back in the winter. The tail
is long and typically very bushy. Tree squirrels
undergo a complete head-to-tail molt in the
spring and a rump-to-head molt in the fall. Tail
hair is replaced only in the spring. Also,
nesting mothers will use their tail hair to line
birthing nests.
Western Gray Squirrels mate over an extended
period ranging from December through June. Young
are born after approximate 44 day gestation
period. Juveniles emerge from nests between March
and mid-August. Litter sizes range from 1 to 5.
Kits remain in the nest for a longer period that
other squirrels, and are relatively slow in
development, another detriment to the species
when it is in conflict with other, more-rapidly
independent squirrels. Young gray squirrels will
not leave the nest until 6 months or more. They
have "furled" tails which will not
reach fullness until adulthood. This is a good
indicator of age and maturity. Mother squirrels
often have a harassed, overworked look, complete
with bruised and battered nipples. Mating
squirrels can be very sadistic and will bite and
injure each other. Females can be quite
territorial, and will chase others away and have
fairly violent altercations between themselves.
Western Gray Squirrels are forest dwellers, and
can be found at elevations up to at least 2000 m.
Time on the ground is spent foraging, but they
prefer to travel distances from tree to tree.
They are strictly diurnal, and feed mainly on
seeds and nuts, particularly pine seeds and
acorns, though they will also take berries,
fungus and other soft food. Pine nuts and acorns
are considered critical foods because they are
very high in oil and moderately high in
carbohydrates, which help increase the
development of body fat. They feed mostly in
trees and on the ground. They generally forage in
the morning and late afternoon for acorns, pine
nuts, new tree buds, and fruits. When on alert,
they will spread their tails lavishly, creating
an umbrella effect that shields them and possibly
provides cover from overhead predators. They are
scatter-hoarders making numerous caches of food
when it is abundant, and thus contribute to the
seed dispersion of their food trees. Although
they show relatively good scent relocation
abilities, some food caches will never be
reclaimed, becoming seedlings in the spring. They
do not hibernate, but become less active during
the winter. Like many prey animals, they depend
on auditory alerts from other squirrels or birds
to determine safety. Once an alarm call is
transmitted, those present will join in, and the
trees become a cacophony of barking squirrels.
Tree squirrels are prey for bobcats, hawks,
eagles, and mountain lions.
Nests can be seen in tall trees, built from
sticks and leaves wrapped with long strands of
grass. There are two stick nest types made by the
Western Gray Squirrel, although one is not
properly termed a "nest." The first is
a large, round, covered shelter nest for winter
use, birthing, and rearing young. The second is
more properly termed a "sleeping
platform," a base for seasonal or temporary
use. Both types are built with sticks and twigs
and are lined with leaves, moss, lichens and
shredded bark. The birthing nest may be lined
with tail hair. The nest may measure 43 - 91 cm
(17 to 36 inches) in length and up to 46 cm (18
inches) in height. It is usually built within the
top third of the tree. Young or traveling
squirrels will also "sleep rough" when
weather permits, balanced spread-eagled on a tree
limb high above the forest floor. This attitude
is also adopted for cooling purposes in hot
weather (a behaviour also observed in Raccoons).
The Western Gray Squirrel was listed as a
threatened species in Washington state in 1993.
Populations of the Western Gray Squirrel have not
recovered from past reductions. They are being
threatened with habitat loss, road-kill mortality
and disease. Habitat has been lost due to
urbanization and catastrophic wild fires.
Previous areas of forest have been degraded by
fire suppression and overgrazing, allowing the
invasion of Scot's Broom. Notoedric mange, a
disease caused by mites, becomes epidemic in
Western Gray Squirrel populations and is a major
source of mortality. Other species of Eastern
Gray Squirrels, fox squirrels, California ground
squirrels and Wild Turkeys are expanding and
compete with the Western Gray.
Listed as extirpated in some California areas,
the Western Gray Squirrel in southern California
is generally found only in the mountains and
surrounding foothill communities. Local
rehabilitation experts recount the Eastern Fox
Squirrels were released in urban regions Los
Angeles in the 1970s, and these aggressive
cousins drove the more reclusive Western Grays
back into the mountains, where competition was
not so strong. This non-native species
introduction appears to be the largest threat in
the southern California area.
References
Baillie (1996). Sciurus griseus. 2006 IUCN Red
List of Threatened Species. IUCN 2006. Retrieved
on 12 May 2006.
Sciurus griseus (TSN 180176). Integrated
Taxonomic Information System. Accessed on 23
March 2006.
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American
Red Squirrel
(Tamiasciurus hudsonicus)
The North American red squirrel (Tamiasciurus
hudsonicus) is one of two species of tree
squirrel currently classified in the genus Tamiasciurus
and known as pine squirrels (the other is the
Douglas squirrel, T. douglasii). It is a
medium sized (200–250g) diurnal mammal that
defends a year-round exclusive territory. The
diet of these tree squirrels is specialized on
the seeds of conifer cones. As such, they are
widely distributed across North America wherever
conifers are common, except on the Pacific coasts
where they are replaced by Douglas squirrels.
Recently red squirrels have been expanding their
range to include primarily hardwood areas (Goheen
et al. 2003)
Nomenclature and Identification
North American red squirrels are also referred to
as pine squirrels, American red squirrels and
chickarees. They should not be confused with
Eurasian red squirrels (Sciurus vulgaris);
since the ranges of these species do not overlap,
they are both commonly referred to as "red
squirrels" in the areas where they are
native. The species name hudsonicus refers to
Hudson Bay, Canada, where the species was first
catalogued by Erxleben in 1771 (Woods 1988). Red
squirrels can be easily identified from other
North American tree squirrels by their smaller
size, territorial behavior and reddish fur with a
white venter (under-belly; Steele 1998). Red
squirrels are also much bigger than chipmunks.
The Douglas squirrel is morphologically similar
to the American red squirrels but has a rust
venter and is restricted to the southwestern
coast of British Columbia and in the Pacific
Northwest of the United States. These two species
do not co-occur. 123
Evolution/systematics
North American red squirrels are widely
distributed across North America. Their range
includes most of Canada, and extends into the
United States in the Rocky Mountains, the North
Central and North East. There are 25 recognized
sub-species of red squirrels (Steele 1998). The
sub-species of red squirrel found in the
southwest Yukon is T. hudsonicus petulans
(Osgood, 1900) (see Steele 1998). A recent
phylogeny (Mercer and Roth, 2003) suggests that
squirrels as a family can be divided into five
major lineages. Two of these clades are
monotypic. Red squirrels (Tamiasciurus)
fall within one of three remaining clades that
includes flying squirrels and other tree
squirrels (e.g. Sciurus). This clade is
thought to have diverged from ground squirrels,
chipmunks and marmots approximately 23 million
years ago. See Mercer and Roth (2003) for details
and interesting discussion of the climatic and
tectonic events associated with these
diversifications.
Status
Red squirrels are abundant and not of
conservation concern throughout much of their
range. However, an isolated population of red
squirrels in Arizona has experienced considerable
declines in population size. In 1987 this
population was listed as Endangered.
Food habits
Red squirrels are primarily granivores, but
incorporate other food items into their diet
opportunistically (Steele 1998). In the Yukon,
extensive behavioral observations suggest that
white spruce seeds (Picea glauca)
comprise over 50% of a red squirrel’s diet
(McAdam & Boutin unpub. data), but squirrels
have also been observed eating the following:
spruce buds and needles, mushrooms,willow (Salix
sp.) leaves, poplar(Populus sp.)
buds and catkins, bearberry (Arctostaphylos
sp.) flowers and berries, and animal
material such as bird eggs (Willson, et al.
2003). White spruce cones mature in late July and
are harvested by red squirrels in August and
September. These harvested cones are stored in a
central cache and provide energy and nutrients
for survival over the winter and reproduction the
following spring. The fallen scales from consumed
seed cones can collect in piles, called middens,
more than a meter across. White spruce exhibits
2-6 year masting cycles, where a year of
superabundant cone production (mast year; Kelly
1994) is followed by several years in which few
cones are produced (Nienstaedt & Zasada
1990). Red squirrel territories may contain only
one or several middens.
Nesting Behavior
Nests are most commonly constructed of grass in
the branches of spruce trees. Nests are also
excavated from witches’ broom –
abnormally dense vegetative growth resulting from
a rust disease – or cavities in the trunks
of spruce, poplar trees, and walnut. Kluane red
squirrels rarely nest below ground. Each
individual red squirrel has several nests within
its territory and females with young move
offspring between nests. Some behaviour has been
reported within human dwellings using insulation
as nesting fodder.
Survival
Red squirrels experience severe early mortality
(on average only 22% survive to one year of age).
The survival probability, however, increases to
age three at which point it begins to decrease
again. Females that survive to one year of age
have a life expectancy of 2.3 years and a maximum
lifespan of eight years.
Reproduction
Red squirrels are spontaneous ovulators (Layne
1954, Millar 1970). Females enter estrus for only
one day, but venture from their territory prior
to ovulation and these exploratory forays may
serve to advertise their upcoming estrus. On the
day of estrus, females are chased by several
males in an extended mating chase. Males compete
with one another for the opportunity to mate with
the estrous female. Estrous females will mate
with 4 – 16 males. Gestation time has been
reported to range from 31 to 35 days (Lair 1985).
Females can breed for the first time at one year
of age but some females delay breeding until two
years of age or older. Most females produce only
one litter, but in some years reproduction is
skipped while in other years some females
attempted to breed twice. Litter sizes typically
range from 1 to 5, but most litters contain 3 or
4 offspring. Offspring are pink and hairless at
birth and weigh approximately 10g. Offspring grow
at approximately 1.8 g/day while nursing but do
not reach adult body size until 125 days of age.
Offspring first emerge from their natal nest at
around 42 days of age but continue to nurse until
approximately 70 days of age.
Dispersal
Juvenile red squirrels must acquire a territory
and midden prior to their first winter. Juveniles
without a midden do not survive their first
winter. Offspring can acquire a territory by
competing for a vacant territory, creating a new
territory or by receiving all or part of a
territory from their mother. This somewhat rare
(15% of litters) female behavior is referred to
as breeding dispersal or bequeathal and is a form
of maternal investment in offspring (Berteaux and
Boutin 2000). The prevalence of this behavior is
related to the abundance of food resources and
the age of the mother. In some cases females will
acquire additional middens prior to reproduction,
which they later bequeath to their offspring
(Boutin et al 2000). Offspring that do not
receive a midden from their mother typically
settle within 150m (3 territory diameters) from
their natal territory (Berteaux and Boutin 2000).
Predators
Red squirrels are preyed upon by Canada Lynx (Lynx
canadensis), coyote (Canis latrans),
great horned owl (Bubo virginianus),
northern goshawk (Accipiter gentiles),
red-tailed hawk (Buteo jamaicensis),
American Marten (Martes americana), fox
(Vulpes vulpes), wolf (Canis lupus)
and weasel (Mustela sp.).
Other
The red squirrel has been found in a black phase
and recently, a white phase (not albino) in
Alaska.
References
Belitsky (1996). Tamiasciurus hudsonicus. 2006
IUCN Red List of Threatened Species. IUCN 2006.
Retrieved on 09 May 2006.
Tamiasciurus hudsonicus (TSN 180166). Integrated
Taxonomic Information System. Accessed on 18
March 2006.
Kluane Red Squirrel Project web page, Accessed on
September 6, 2006.
Berteaux, D, and S. Boutin. 2000. Breeding
dispersal in female North American red squirrels.
Ecology, 81: 1311-1326.
Boutin, S., K.W. Larsen, and D. Berteaux. 200.
Anticipatory parental care: Acquiring resources
for offspring prior to conception. Proceedings of
the Royal Society of London, Series B, 267:
2081-2085.
Goheen, J.R., R.K. Swihart, and J.H. Robins.
2003. The anatomy of a range expansion: changes
in cranial morphology and rates of energy
extraction for North American red squirrels from
different latitudes. Oikos, 102: 33-44.
Kelly, D. 1994. The evolutionary ecology of mast
seeding. Trends in Ecology & Evolution
9:465-470.
Lair, H. 1985. Length of gestation in the red
squirrel, Tamiasciurus hudsonicus. Journal of
Mammalogy 66:809-810.
Layne, J. N. 1954. The biology of the red
squirrel Tamiasciurus hudsonicus loquax in
central New York. Ecological Monographs
24:227-267.
Mercer, J. M., and V. L. Roth. 2003. The effects
of Cenozoic global change on squirrel phylogeny.
Science 299:1568-1572.
Millar, J. S. 1970. The breeding season and
reproductive cycle of the western red squirrel.
Canadian Journal of Zoology 48:471-473.
Nienstaedt, H., and J. C. Zasada. 1990. Picea
galuca (Moench) Voss white spruce. Pp. 165-185 in
R. M. Burns and B. H. Honkala, eds. Silvics of
North America. Vol. 1. Conifers. U.S. Department
of Agriculture Agricultural Handbook.
Osgood, W. H. 1900. Results of a biological
reconnaissance of the Yukon River region. General
account of the region. Annotated list of mammals.
Annotated list of birds. North American Fauna
19:1-100.
Steele, M. A. 1998. Tamiasciurus hudsonicus.
Mammalian Species 586:1-9.
Willson, Mary F; De Santo, Toni L; Sieving,
Kathryn E (Jul 2003). "Red squirrels and
predation risk to bird nests in northern
forests". Canadian Journal of Zoology 81
(7): 1202-1208. DOI:10.1139/z03-096. Retrieved on
2007-03-02.
Woods, S. E. J. 1980. The Squirrels of Canada.
National Museums of Canada
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