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Tamiasciurus hudsonicus - Red Squirrel


Head and body length: 165-230 mm, tail length: 90-160 mm.  Weight: 141-312 grams (no more than 5 ounces).  Color varies from a tawny red to tan on head, back and sides.  Faint black line is apparent on ventral portion of sides, but is more prominent in summer pelage.  Underside and rings around eyes are whitish in color.  Color of tail is similar to that of the body but the tip is often tipped with black.  Summer pelage is usually slightly grayer in color, especially on the head.  Ears have small tufts of fur at tips.  Variation between sexes is absent, making identification on site very difficult (Nowak, 1991).  Dental formula: I 1/1, C 0/0, P 2/1 or 1/1, M 3/3 = 22 or 20. Most cases have found T. hudsonicus with only 1 upper premolar, but 2 upper premolars has also been documented (Palmer 1957, Hartley 1961).


Distribution Map for Tamiasciurus hudsonicus:


Distribution and History:

T. hudsonicus diverged from Sciurini approximately 3 million years ago during the end of the Pliocene.  Nearctic: currently found from southern Alaska, throughout most of Canada, the Rocky Mountain and Appalachian Mountain ranges, and the northeastern U.S.  T. hudsonicus was introduced to Newfoundland by man in 1963, and are still flourishing (Gurnell 1987).

Distribution in Wisconsin:

Most common in conifer stands in the northern part of the state, but found throughout the rest of the state.  Currently uncommon in the southwestern 1/5th of the state: from Dane county, south to Rock county, and west to Grant county.

In the middle 19th century was once absent in from this region (Hartley 1961, Kurta 1995).

Ontogeny and Reproduction:

Breeding occurs during two seasons.  The first occurs between February to March and second season lasting from June through the end of July.  Only one breeding season a year has been reported in many parts of northern Canada and parts of Alaska.  Nest types of the T. hudsonicus vary.  According to Nowak (1986), locations for building nests are in abandoned holes in trees (dens); old woodpecker holes are common sites.  During warm periods, nests called “dreys” are often loosely constructed in the crotch of a tree and composed of grasses and foliage of trees (Gurnell 1987).  Winter dreys are located in very dense foliage of trees and are built weather-tight.  Males tend to exhibit a polygynous reproductive behavior and females will occasionally mate with more than one male.  Females will go into estrous for no more than one day.  Breeding is triggered mainly through olfactory communication.  When the female squirrels go into estrous, males from other territories will enter the female’s territory.  Pre-mating chases are common until copulation takes place. Gestation will last somewhere between 31-35 days.  Litter sizes range between 1-8 young, but 4-6 is most common.  Newborns are altricial; without hair, closed ears, and blind, but develop rapidly.  Average birth weight is approximately 7 grams per individual in the litter.  After 4 weeks the ears finally open and the fur is done growing.  Usually on the 27th day, the eyes will open.  By the 6th week from birth, their eyes are fully open.  After 7-8 weeks the young are weaned but still remain within the mother’s territory.  After approximately 18 weeks the juvenile squirrels disperse and search for their own territories.  One year after birth, T. hudsonicus  is reproductively mature and is ready to mate (Craven, S. and S. Keith. April 25, 1991).

Ecology and Behavior:

Coniferous forests are usually associated with T. hudsonicus, but they can also be found in mixed stands or deciduous forests, which must contain at least Quercus spp. (oaks), Carya spp. (hickories), or Juglans spp. (walnuts).  The primary source of food in their diets is composed of seeds from conifers, nuts, fruits, bark, twigs, buds, mushrooms, sap and seeds.  Seeds from Pinus spp. (pine), Picea spp. (spruce) and Tsuga spp. (hemlock) are the preferred diet selections of T. hudsonicus when available.  T. hudsicus is rarely found in urban settings like Sciurus carolinensis (Gray Squirrel).  The home range is in size of 0.2 to 0.7 ha. in area.  Home range is usually larger when in mixed or deciduous habitats, and studies have shown that males have a smaller home range.  When food availability is higher, territories decrease in size.  In the center of established territories one or more food caches are found called “middens.”  Middens are often found directly below a branch where the squirrel eats and discards cone scales from conifers.  T. hudsonicus often burries up to 20 entire cones inside the debris of the middens for food in the winter months.  Middens are also commonly found near stumps, and underground cavities or burrows, which can run up to 5 meters in length.  Foraging activities takes up 60-90% of the activity time outside of the nest and takes place the most in summer months.  The first strategy of foraging for T. hudsonicus is cutting cones or nuts from trees and letting them fall to the ground.  Then the squirrels will collect all of the cones or nuts later and cache them in middens or retrieve them to a common feeding perch in a tree.  Mould (1983), found in deciduous forest settings, T. hudsonicus prefers white oak acorns over red oak acorns because they are lower in tannin content (Gurnell, 1987).  Before burying the white oak acorns in middens, T. hudsonicus will cut the embryos from the acorns with their sharp incisors.  This is done to prevent the quick germination of white oak seedlings during autumn, as they do not like the taste of the taproot. T. hudsonicus is known to be very aggressive and territorial species, showing inter- and intra-specific aggression toward other squirrels especially shown in males between March and April.  It is very common for a T. hudsonicus to chase other squirrel species that are over twice their own size.  Territorial activity increases as the young disperse in the late summer and autumn.  T. hudsonicus create “marking points,” where they gnaw on the bark of a trees and deposit small amounts of urine.  This is a mechanism, which is used between males to communicate and gather information about other squirrels (Laycock, 1975).  Cheek rubbing behavior is also exhibited in T. hudsonicus, where squirrels rub saliva from the corner of their cheeks to a branch on a tree.  This behavior is understood to be a mechanism for dominant squirrels to note familiarity within their home-range (Gurnell 1987).  Activity is diurnal, but T. hudsonicus may also be active during warm moonlit nights.  Peak activity lasts the first couple of hours after daylight and the last couple of hours before sunset.  Red Squirrels are very agile and have good dexterity.  Although a tree squirrel, T. hudsonicus feels more comfortable on the ground than its Sciurus cousins in Wisconsin.  On the ground, bounding types of locomotion are most common.  In trees, it can run very quickly with ease up and down the trunk and from limb to limb.  Jumps of 10 feet between tree limbs have also been documented.  T. hudsonicus has also been documented with excellent swimming capabilities, as they are well adapted to swim across streams and rivers.  Two molting periods are found: one during the months of May and June for the summer pelage and August through September for the winter pelage.  The order of molted fur usually starts in the order from head to tail.  As activity during winter months slows down, T. hudsonicus will stay in their dens most of the time, but does not exhibit “true hibernation.”  Sometimes they will winter in an underground burrow under a stump or stone with their food cache if the weather is too harsh, if no other suitable habitat is found.  Food caches of nuts and mushrooms are buried in mass quantities at once, not one-at-a-time characteristic of the Gray squirrel (Barkalow 1973).  T. hudsonicuss has also been reported to be slightly predatory, unlike other squirrels.  Sometimes if the opportunity presents itself, they will raid the nests of birds and eat the eggs or fledglings.  Mice and baby rabbits also have been reported to be preyed upon by T. hudsonicus (Nowak 1991).  Many different vocalizations are found.  They range from, rattles, screeches, growls, and growls; which are all forms of aggression or alarm calls, buzzes; which is a call for contacting or seeking mates, and squeaks; which is another seeking call.  Most vocalizations are responses to either alarm others of predators, or to fend territory (Gurnell 1987).


The lifespan of the T. hudsonicus is up to 7 years, but 10 years has been reported in domestication (Laycock 1975).  Common names and nicknames associated with the T. hudsonicus are; American Red Squirrels, Pine squirrels, Chickarees, Boomers and Chatterboxes.  A 1991 census showed 25 subspecies under T. hudsonicus (Gurnell 1987).  The common subspecies in Wisconsin is: Tamiasciurus hudsonicus f. minnesota (Jackson 1961).  The sub-species grahmensis from the Grahm Mountains of Arizona is currently listed as an endangered species with a population of less than 200 individuals.  Their existence is threatened by development in their short range (Nowak 1991).  Other than size and color, there are a few major differences between the Tamiasciurus and Sciurus genera.  The main phylogenic difference is the complete suppression of a bacula in T. hudsonicus opposed to a bacula found in the Sciurus tree squirrels.  An enlarged bulla proportional to skull size is shown in T. hudsonicus compared to that of the Sciurus carolinensis (Barkalow 1973).  T. hudsonicus has been considered to be a nuisance to man; associated with damage to small trees in pine plantations, crop damage, killing poultry, raiding bird feeders, entering house fixtures and attics.  These issues are not uncommon but are not major economic concerns.  T. hudsonicus has been accused to inhibit forest generation since they eat many pine seeds.  The fact is that T. hudsonicus  often helps forest regeneration from old seeds hid in moist cache sites, which is beneficial to seed germination.  The most common predators of T. hudsonicus include; Martes Americana (Pine Martens), Accipiter gentilis (Goshawks), Buteo jamaicensis (Red-Tailed Hawks), Strix varia (Barred Owls), Bubo virginianus (Great Horned Owls), and Homo Sapiens (Humans).  Predation from humans is minimal in most areas compared to other squirrel species, since they are not considered to be game animals, and their pelts are currently not in high demand in the fur trade (Gurnell 1987).             

Literature Cited:

Barkalow, Frederick S.; and Shorten, Monica. 1973. The World of the Gray Squirrel, 1st ed. J.B. Lippincott Co., Philadelphia and New York. p. 39.

Craven, S. and S. Keith.  April 25, 1991. “Squirrels in Wisconsin: Benefits and Problems” University of Wisconsin-Extension, College of Agriculture and Life Sciences, University of Wisconsin-Madison.

Ellermann, J.R. 1966. The Families and Genera of Living Rodents, Vol I. British Museum (Natural History), London. pp. 345-346.    

Gurnell, J.  1987. The Natural History of: Squirrels. Facts on File Publications, New York.

Hayssen, V.  “Mammalian Species: Tamiasciurus hundsonicus” (On-line),  Available .  Accessed November 04, 2003.

Hoffman, R.S., C.G. Anderson, R. Thorington Jr. and L. Heaney.  1993. “MSW Scientific Names” Smithsonian Institute. (On-line), Available .  Accessed November 03, 2003.

Jackson, H.H.T.  1961. Mammals of Wisconsin. University of Wisconsin Press, Madison. pp. 169-174.

Kurta, A.  1995. Mammals of the Great Lakes Region. The University of Michigan Press, Ann Arbor. pp. 129-131.

Laycock, G.  1975. Squirrels. Four Winds Press, New York. pp. 43-45.

Nowak, R.M.  1991. Walkers Mammals of the World, 5th ed., Vol I.  The John’s Hopkins University Press, Baltimore and London. pp. 586-589.

Palmer, Dr. E.L.  1957. Palmer’s Field book of Mammals. E.P. Dutton & Company, Inc., New York. p. 89.

Sullivan, J.  1995. “Wildlife Species: T. hudsonicus” (On-line), Available .  Accessed November 03, 2003.

Reference written by Jason Fleener, Biol 378: Edited by Chris Yahnke. Page last updated 4-29-04.

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