Red squirrels occupy northern boreal coniferous forests abundant with conifer seeds, fungi, and interlocking canopies. This limits them to mountain ranges on the southern and eastern boundaries of their range. In the Rocky Mountains they have been found at elevations up to 2,500 ft (762 m). Populations of red squirrels occur in different habit conditions due to the vastness of their range. They occur in both temperate and polar environments and are considered to be primarily arboreal. They can be found in a mixed variety of forests including coniferous, deciduous, and mixed forests and they are also able to thrive in suburban and urban settings, as long as cool, coniferous forests with dense, interlocking canopies and abundant fungal resources are present. (Ruff and Wilson, 1999; Steele, 1998)
Red squirrels differ from other tree squirrels by their deep reddish color, territorial behavior, and their smaller body size. They are less than 30% the size of grey squirrels (Sciurus carolinensis). While size and pelage color can vary geographically, they generally have a reddish back and white underside that is demarcated by dark lateral lines, which are especially visible in summer. A white eye ring is present year-round and tufted ears are during the winter. Variation in the dorsal surface color can range from reddish to ferruginous brown to olivaceous gray, usually with a distinctive reddish or brownish lateral band running down the back. The tail is smaller and flatter than that of other tree squirrels and varies in color from yellowish-gray to rusty red, with a band of black often extending the entire length of tail. Where its range borders that of Douglas squirrels (Tamiasciurus douglasii), red squirrels are distinguished by color of their pelage. The underside of red squirrels is all white or cream, whereas Douglas squirrels are rust colored or with a blackish wash. Tail hairs have yellowish to rusty tips with a black band in red squirrels, whereas those of Douglas squirrels are white-tipped with a black band, making them moderately easy to distinguish from a distance. Male and female red squirrels are very similar in appearance. (Flyger and Gates, 1982; Hall, 1981; Lane, et al., 2010; Steele, 1998)
Some ecogeographic variation is believed to occur, so length and weight measurements may only hold true for the population(s) in which they were measured. Twenty-five subspecies of red squirrels are recognized. They differ mainly by range distribution. Mass tends to range from 197.3 to 282.2 g, with an average of 212.97. Their total length ranges from 270 to 385 mm, with an average of 327.5. Much of their length is their tail, which is 92 to 158 mm long. Red squirrels have an average hind foot length of 35 to 57 mm and the ear length is 19 to 31 m. Condylobasal length is 42 to 50 mm. Red squirrels have a basal metabolic rate of 166 cm^3 oxygen/hour. (Hall, 1981; Kramm, et al., 1975; Lindsay, 1982)
Red squirrels generally experience two annual molts, although the tail molts only once per year. The spring molt occurs from late March through August and starts on the nose and front feet, and ends on the rump. The fall molt is from late August to early December and begins on the tail and then progresses to the rump and the head before ending on the legs and flank. Molt stage can be an important age indicator. Skulls can be aged by looking at dental characteristics such as visible wear and presence of permanent teeth. Their dental formula is I 1/1, C 0/0, P 1/1 or 2/1, M 3/3. The total number of teeth is 20 or 22 depending on the presence of upper P3. (Layne, 1952; Lindsay, 1982; Nellis, 1969; Steele, 1998)
Red squirrels have a defined breeding season lasting 105 days that can occur either once or twice a year. They will mate in early spring from March to May and then again in August to early September. A second breeding period tends to occur in warmer areas of their range. Although mate pairings may occur, red squirrels are generally characterized as promiscuous. Animals in the best condition tend to breed more regularly and successfully than animals in poor condition. (Lane, et al., 2010; Layne, 1952; Steele, 1998; Wirsing, et al., 2002)
Red squirrels exhibit a scramble competition mating system, in which the main costs to males are locating receptive females. Males typically invade the territory of females in estrus and pursue them in obvious mate chases. During mate chasing, a single dominant male actively pursues a female and drives off other subordinate males using calls or direct chase. Mounting and copulation usually lasts several minutes and occurs several times in the afternoon of the only day the female is receptive. Copulation is frequently initiated by the female and terminated by the male. Mating most often occurs on the ground or in the lower branches of trees. During mating, the male holds the female around the posterior abdomen while resting his head on her back. It is common for both males and females to engage in grooming of the genitals before and after copulation. The adults have no further contact after mating and the male returns to his territory. (Gurnell, 1984; Smith, 1968; Steele, 1998)
Red squirrels breed once or twice a year, depending on their geographic location. Populations in the south and east generally have two litters a year, one in spring and one in late summer. This pattern occurs all the way up to Quebec and has also occurred in British Colombia. In the northern extent of their range, breeding only occurs once a year, generally in spring. In the eastern United States, two breeding seasons per year is common, one in March and a second in late July. Harsher conditions at the northern latitudes and higher elevations of the squirrels' range most likely limit reproduction to a single breeding season. Interannual variation in female reproduction is also common; there was a halving in the number of breeding females over 2 years in British Columbia. Their reproductive cycle has adapted to the cyclical production of conifer cone output. Female estrus is highest in February to March and June to July. Pregnancy peaks in March to April and August to September in New York. In Colorado, conception generally occurs April to June. (Descamps, et al., 2009; Humphries and Boutin, 2000; Lane, et al., 2010; Layne, 1952; Linzey and Linzey, 1971; Millar, 1970; Smith, 1968; Steele, 1998; Wrigley, 1969)
Females are in estrus only 1 day per breeding season. Conception usually occurs within a few hours of mating. Gestation averages 35 days. Newborn young weigh 7.08 g on average. Litter sizes range from 1 to 8 offspring but averages 3.97. Altricial young are born without any hair except vibrissae and fine chin hairs, but develop quickly. The external auditory meatus is obvious at 18 days, eyes open at 26 to 35 days, and pelage is fully developed in only 40 days. Lactation occurs for the first 70 days. After that time, young are cast out to find their own territory. In some cases when the mother is in poor condition she will give part of her territory to her offspring. This increases the probability of survival for the offspring, increasing the overall fitness the mother. Young are active outside the nest in 7 weeks and fully independent shortly after weaning. Dentition is complete and external skeletal measurements reach adult size by 125 days. (Descamps, et al., 2009; Hayssen, 2008; Lane, et al., 2010; Layne, 1952; Nice, et al., 1954; Ruff and Wilson, 1999; Steele, 1998)
Most red squirrel nests are constructed within 30 m of cone caches. Red squirrels prefer natural cavities, but due to lack of such resources in coniferous forests, they construct leaf nests or occasionally underground nests. The most important factors influencing nest-tree selection are tree diameter and branching structure, and the availability of canopy escape routes. Nests are found at heights of 2 to 20 m, and while nest material varies with habitat, they typically include grasses, mosses, inner cambium, shredded bark leaves, feathers, and fur. (Steele, 1998)
Reproduction is dependent on resource abundance. Red squirrels live in a resource-pulse system, where the main food (in most cases the seeds of conifer species like white spruce) fluctuates annually, in some cases spanning three orders of magnitude between failure and mast years. Food available for reproduction in the spring is determined by the abundance of cones produced the previous year. The reliance on and the defense of individual caches allows females to have the potential to assess the level of stored food that is available for current reproduction. However, female squirrels do not have a smaller litter in years of crop failure, so reproduction is expected to be more costly during these years. (Boutin, et al., 2006; Descamps, et al., 2009; Lane, et al., 2010; McAdam and Boutin, 2003)
Age also affects reproductive cycles in both male and female red squirrels. Females and males are sexually mature at 1 year of age but are still developing. Despite the detriment to their own health by trying to breed and grow simultaneously, 1-year-old females tend to reproduce because they can achieve higher lifetime reproductive success than females delaying their first reproduction. This is also true for young males. Mature females tend to engage in a conservative reproductive strategy in order to allocate reproductive resources only when their own survival costs are maintained. This is not the case for females over 6 years old, who tend to sacrifice their own survival for reproduction because they are unlikely to be able to breed again the following year. Reproduction senescence occurs from 4 years of age onwards. (Descamps, et al., 2009; Humphries and Boutin, 2000; McAdam, et al., 2007)
Females raise the young without any help from males. Gestation averages 35 days and lactation is 70 days. After this the young are no longer dependent on the mother and the offspring relocate to find territories of their own. Mothers sometimes bequeath territories, giving part or all of their territory to one or more of their offspring. This increases the offspring’s’ probability of overwinter survival. (Boutin and Larsen, 1993; Humphries and Boutin, 2000; Steele, 1998)
Red squirrels exhibit a type 3 survivorship curve in wild populations. This means that most of the mortality is associated with deaths of the young and only 25% survive longer than 1 year. Despite their small size, red squirrels are considered relatively long lived, with the oldest known squirrel in the wild reaching 10 years of age. The longest recorded lifespan in captivity is 9 years. Mature squirrels are 2 to 4 years old and older squirrels are considered to be over 5 to 6 years old. Their average lifespan in the wild is 5 years. (Lane, et al., 2010; Ruff and Wilson, 1999; Steele, 1998)
Red squirrels are primarily diurnal, but on occasion exhibit nocturnal activity. During the spring and summer they are most active in the morning and afternoon, but as fall approaches they become highly active all day in preparation for the food shortages associated with winter. In the winter, red squirrels peak their activity around midday to take advantage of warmer temperatures. Adverse weather may result in reduced activity, but it is unlikely that squirrels will remain in the nest for more than 1 day without foraging. As the temperatures drop, squirrels become less active, and it has been reported that at temperatures of -31.6 degrees Celsius, red squirrels become completely inactive. This likely varies geographically because squirrels living in Fairbanks, Alaska would have to be actively foraging at these temperatures or they would be confined to their nest for weeks. (Clarkson and Ferguson, 1969; Pauls, 1978; Pruitt and Lucier, 1958; Steele, 1998)
Throughout most of their range, and especially in coniferous forests, both male and female squirrels vigorously defend exclusive territories from competitors. Defense of these territories occurs year-round but is most obvious in autumn when squirrels are stockpiling cones and when competition from subordinate squirrels is at its peak. In New York forests, which are dominated by deciduous trees, squirrels often exhibit overlapping home ranges and occasionally defend only nests and caches from intruders. Their territories range from 2400 to 48000 square meters. (Kemp and Keith, 1970; Layne, 1952; Steele, 1998)
Home ranges are usually 1 to 2.4 hectares in area. (Ruff and Wilson, 1999)
Red squirrels have well-developed and extremely acute senses of smell, sight, and hearing. They are well known for their ability to communicate by calls. These calls consist of rattles, screeches, growls, buzzes and chirps and are often used in defense of territory or in response to threats from predators. During mating, red squirrels use low aggressive calls or territorial calls to drive off subordinate males. Red squirrels may even be able to recognize each other by individual call. Communication is important because of their territorality. They intensively use vocal communication to advertise these territories and to threaten other squirrels. Both vocal and scent marking aid in the recognition of individuals. Olfactory communication is important because it can leave long-lasting impressions which advertise if the territory is taken and males can discriminate between different olfactory signatures of other males. Another reason chemical communication is advantageous is because it may also reduce predation risk. Scent marking may also enable the squirrels to avoid unnecessary chases and fights by becoming known to their neighbors. Red squirrels may also be able to make predator-specific calls, but evidence of this is still very inconclusive. Red squirrels tend to produce a high-frequency call for aerial predators and a harsher, bark-type call for terrestrial predators. However, it is more commonly observed that these two call types are mixed. (Digweed and Rendall, 2010; Greene and Meagher, 1998; Price, et al., 1986; Ruff and Wilson, 1999; Smith, 1968; Vache, et al., 2001)
Red squirrels are primarily granivorous, but they are also opportunistic omnivores in the absence of mast foods. Primary diet items vary with habitat and include the seeds of conifers and other tree types detailed below. They live in a resource pulse system, where foods (conifers like white spruce, Picea glauca) exhibit extreme annual variation. They consume a wide variety of mushrooms, including at least 45 species in the Cascade Mountains alone. Secondary food items include tree buds and flowers, fleshy fruits, tree sap, bark, insects, and other animal materials such as bird eggs or young snowshoe hares (Lepus americanus). During winter, spring, and early summer, bark stripping and tree girdling occurs commonly to access phloem and cambial tissues. Red squirrels are highly selective in their foraging behavior, harvesting cones from the tree species with the highest seed energy per cone first and systematically working their way through species of conifers by energy density per cone. (Boutin, et al., 2006; McAdam and Boutin, 2003; Steele, 1998; Wilson, et al., 2003)
Red squirrels are primarily larder hoarders. In late summer through autumn, they harvest cones and store them in one or a few central middens. Middens are a central hoard that is easy to defend from competitors and provides a moist, cool environment that prevents cones from opening. Middens vary in size and number depending on habitat, food availability, and individual squirrel. However, they contain enough food to last one to two seasons and are often used by several generations of squirrels In the eastern United States and Canada, red squirrels frequently engage in scatter hoarding, which is a system involving many small hoards instead of a large midden. This accounts for 85% of all hoards and more than 50% of all cones stored in this area The downside to this type of storage is that middens only provides enough food for about 37 days. Red squirrels have a great sense of smell which they use when looking for middens during the winter. Some individuals store their food caches underground and are able to locate these seeds, even under 4 meters of snow. Whenever they do not recover a stockpile of food, the seeds are left to germinate. (Dempsey and Keppie, 1993; Gurnell, 1984; Hurly and Robertson, 1986; Ruff and Wilson, 1999; Smith and Reichman, 1984; Smith, 1968)
Red squirrels pilfer food from other squirrels, but the extent varies greatly by population. Some squirrels the Yukon Territory of Canada almost never pilfer. Mt. Graham Red Squirrels, on the other hand, pilfer 97% of the time and in Vermont pilfering occurred 25% of the time. Age, boldness, and population density may play a role in pilfering. (Donald and Boutin, 2011; Ruff and Wilson, 1999)
Red squirrels are likely prey for a variety of animals including snakes, birds of prey, and carnivorous mammals. They are preyed upon by Cooper's hawks (Accipiter cooperii), northern goshawks (Accipiter gentilis), bald eagles (Haliaeetus leucocephalus), great gray owls (Strix nebulosa), great horned owls (Bubo virginianus), American kestrels (Falco sparverius), red-shouldered hawks (Buteo lineatus), northern harriers (Circus cyaneus), red-tailed hawks (Buteo jamaicensis), and sharp-shinned hawks (Accipiter striatus). Mammals that prey upon them them are American martens (Martes americana) and fishers (Martes pennanti), weasels (Mustela), mink (Neovison vison), as well as various canids like red foxes (Vulpes vulpes) and felids like lynx (Lynx canadensis). They are also preyed upon by timber rattlesnakes (Crotalus horridus). Humans hunt red squirrels for both their fur and meat. (Gurnell, 1984; Steele, 1998)
Alarm calls are the most highly used anti-predator adaptations of red squirrels. Red squirrels tend to produce a high-frequency call for avian predators and a harsher bark-type call for terrestrial predators. However, it is more common that these two call types are mixed when any predator approaches. Red squirrels have a high survival rate even when subjected to heavy predation. It is very hard to catch and kill these animals because they are quite agile and take to the trees or thick vegetation to escape predators. Red squirrels are also fairly aggressive and when cornered, will not hesitate to defend themselves. (Digweed and Rendall, 2010; Greene and Meagher, 1998; Ruff and Wilson, 1999; Stuart-Smith and Boutin, 1995; Wirsing, et al., 2002)
Red squirrels impact the forest ecosystem by dispersing seeds and fungi through caching and forgetting about or otherwise failing to return to food caches. The diversity and abundance of beneficial ectomycorrhizal fungi in these caches helps young trees acquire nutrients and grow. They limit the regrowth of trees by eating the seeds and inner tissues of the trees, which can cause significant damage to tree survival and value. Red squirrels provide a feeding opportunity for porcupines (Erethizontidae) during the winter by peeling away the bark of lodgepole pines. Their feeding habits also cause conifers to grow multiple tops, which reduces their timber value but increases suitable nest sites for many arboreal rodents (Rodentia) and passerine birds (Passeriformes). The increased availability of nest sites sustains species richness. (Aubry, et al., 2003; Ruff and Wilson, 1999; Sullivan, et al., 1993)
Red squirrels are host to a variety of endoparasites and ectoparasites. Endoparasitic species include 9 species of nematodes, 9 species of tapeworms including the genus (Hymenolepis). Other endoparasites include tularemia bacteria (Francisella tularensis) and Emmonsia crescens), and some kinds of protists like sarocysts (Sarocystis), and (Haplosporanigium). They are also vulnerable to fungal lung disease infection via adiaspiromycosis (Emmonsia parva). Ectoparasites of red squirrels include 31 species of mites, ticks, and chiggers (Glycyphagidae and Acarina), 25 species of fleas including Siphonaptera, Opisodasys robustus, Orchopeas caedens, Orchopeas neotomae, Orchopeas leucopus, Oropsylla idahoensis, Ceratophyllus vison. They may also carry botfly larvae (Cuterebra emasculator). Viruses that infect red squirrels are silverwater virus, California encephalitis virus, and Powassan virus. (Edwards, et al., 2003)
Red squirrels are the third-most commonly harvested furbearer in Canada, bringing in about $1 million annually. Each year in Minnesota, thousands of red squirrels are harvested for consumption by humans. Red squirrels are an important prey item for other economically important species like lynx (Lynx) and martens (Martes). (Kemp and Keith, 1970; Ruff and Wilson, 1999)
Red squirrels cause considerable economic losses. They interfere with conifer reforestation by eating 60 to 100% of cone crops and directly damaging trees through bud consumption and bark stripping. Squirrels also cause damage to human property by nesting in homes and gnawing on household items. They may bite humans if provoked. (Ruff and Wilson, 1999; Steele, 1998)
As of 2008, red squirrels are classified as Least Concern on the ICUN Red List and by the United States government. They are widespread and common, have suitable habitat throughout their range, and face no major threats. One subspecies, Mt. Graham red squirrels (Tamiasciurus hudsonicus grahamensis), is endangered according to the ICUN Red List. This subspecies is only found in southeast Arizona and its population is about 150 individuals. (Ruff and Wilson, 1999)
Tamiasciurus and Sciurus likely diverged in the late Pliocene. Red squirrels are first reported in the Irvingtonian and are now known from more than 30 late Rancholabrean faunas from the central and eastern United States. (Hafner, 1984; Steele, 1998)
Catherine Rubin (author), University of Alaska Fairbanks, Link Olson (editor), University of Alaska Fairbanks, Catherine Kent (editor), Special Projects.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
uses sound to communicate
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
Referring to an animal that lives in trees; tree-climbing.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
parental care is carried out by females
A substance that provides both nutrients and energy to a living thing.
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
an animal that mainly eats seeds
An animal that eats mainly plants or parts of plants.
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
the area in which the animal is naturally found, the region in which it is endemic.
chemicals released into air or water that are detected by and responded to by other animals of the same species
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.
places a food item in a special place to be eaten later. Also called "hoarding"
living in residential areas on the outskirts of large cities or towns.
uses touch to communicate
Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
living in cities and large towns, landscapes dominated by human structures and activity.
uses sight to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
Aubry, K., J. Hayes, B. Biswell, B. Marcot. 2003. Community and ecosystem relations. Ch. 12 The ecological role of tree-dwelling mammals in coniferous forests. University Publishing Online: Cambridge University Press. Accessed October 07, 2012 at http://ebooks.cambridge.org/chapter.jsf?bid=CBO9780511615757&cid=CBO9780511615757A019.
Boutin, S., K. Larsen. 1993. Does food availability affect growth and survival of males and females differently in a promiscuous small mammal, Tamiasciurus hudsonicus?. Journal of Animal Ecology, 62: 364-370.
Boutin, S., L. Wauters, A. McAdam, M. Humphries. 2006. Anticipatory reproduction and population growth in seed predators. Science, 314: 1928-1930.
Clarkson, D., H. Ferguson. 1969. Effect of temperature upon activity in the red squirrel. American Zoologist, 9: 1110.
Dempsey, J., D. Keppie. 1993. Foraging patterns of eastern red squirrels. Journal of Mammalogy, 74: 1007-1013.
Descamps, S., S. Boutin, A. McAdam, D. Berteaux, J. Gaillard. 2009. Survival costs of reproduction vary with age in North American red squirrels. Proceedings of the Royal Society of London. Series B: Biological Sciences, 276: 1129-1135.
Digweed, S., D. Rendall. 2010. Are the alarm calls of North American red squirrels (Tamiasciurus hudsonicus) functionally referential?. Behaviour, 147: 1201-1218. Accessed January 03, 2012 at http://classes.uleth.ca/201003/biol4850a/Ch14C%20Low%20Priority%20Digweed.pdf.
Donald, J., S. Boutin. 2011. Intraspecific cache pilferage by larder-hoarding red squirrels (Tamiasciurus hudsonicus). Journal of Mammalogy, 92/5: 1013-1020. Accessed October 22, 2011 at http://www.bioone.org/doi/full/10.1644/10-MAMM-A_340.1.
Edwards, J., M. Ford, D. Guynn. 2003. Wild Mammals of North America: Biology, Management, and Conservation. Baltimore, MD: John Hopkins University Press.
Flyger, V., J. Gates. 1982. Pine Squirrels: Tamiasciurus hudsonicus, T. douglasii. Pp. 230-237 in J Chapman, G Feldhamer, eds. Wild mammals of North America. Baltimore: John Hopkins University Press.
Greene, E., T. Meagher. 1998. Red squirrels, Tamiasciurus hudsonicus, produce predator-class specific alarm calls. Animal Behavior, 55/3: 511-518. Accessed January 03, 2012 at http://www.ncbi.nlm.nih.gov/pubmed/9514668.
Gurnell, J. 1984. Home range, territoriality, caching behavior and food supply of the red squirrel (Tamiasciurus hudsonicus) in a subalpine forest. Animal Behavior, 32: 1119-1131.
Hafner, D. 1984. Evolutionary relationship of the near arctic Sciuridae. Pp. 459 in J Murie, G Michener, eds. The biology of ground dwelling squirrels. Lincoln: University of Nebraska Press.
Hall, E. 1981. Red Squirrel, Tamiasciurus hudsonicus. The Mammals of North America. Ronald Press.
Halvorson, C., R. Engeman. 1983. Survival analysis for a red squirrel population. Journal of Mammalogy, 64: 332-336.
Hayssen, V. 2008. Patterns of body and tail length and body mass in Sciuridae. Journal of Mammalogy, 89/4: 852-873.
Humphries, M., S. Boutin. 2000. The determinants of optimal litter size in free-ranging red squirrels. Ecology, 81: 2867-2877.
Hurly, T., R. Robertson. 1986. Scatter-hoarding by territorial red squirrels: a test of the optimal density model. Canadian Journal of Zoology, 65: 1247-1252.
Kemp, G., L. Keith. 1970. Dynamic and Regulation of Red Squirrel (Tamiasciurus hudsonicus) Populations. Ecology, 51/5: 763-779. Accessed October 13, 2011 at http://www.jstor.org/stable/1933969.
Klugh, A. 1927. Ecology of the red squirrel. Journal of Mammalogy, 8: 1-32.
Kramm, K., D. Maki, J. Glime. 1975. Variation within and among populations of red squirrel in the Lake Superior Region. Journal of Mammalogy, 56/1: 258-262. Accessed October 14, 2011 at http://www.jstor.org/stable/1379633.
Lane, J., S. Boutin, J. Speakman, M. Humphries. 2010. Energetic costs of male reproduction in a scramble competition mating system. Journal of Animal Ecology, 79: 27-34.
Layne, J. 1952. The Os Genitale of the red squirrel, Tamiasciurus. Journal of Mammalogy, 33/4: 457-459.
Lindsay, S. 1982. Systematic relationship of parapatric tree squirrel species (Tamiasciurus) in the Pacific Northwest. Canadian Journal of Zoology, 60/9: 2149-2156.
Linzey, A., D. Linzey. 1971. Mammals of Great Smoky Mountains National Park. Knoxville: The University of Tennessee Press, 114 p.
McAdam, A., S. Boutin, A. Sykes, M. Humphries. 2007. Life histories of red squirrels and their contributions to population growth and lifetime fitness. Ecoscience, 14: 362-369.
McAdam, A., S. Boutin. 2003. Variation in viability selection among cohorts of juvenile red squirrels (Tamiasciurus hudsonicus). Evolution, 57: 1689-1697.
Mercer, J., V. Roth. 2003. The Effects of Cenozoic Global Change on Squirrel Phylogeny. Science, 299/5612: 1568-1572. Accessed January 03, 2012 at http://www.sciencemag.org/content/299/5612/1568.abstract.
Millar, J. 1970. The breeding season and reproductive cycle of the western red squirrel. Canadian Journal of Zoology, 48: 471-473.
Nellis, C. 1969. Sex and age variation in red squirrel skulls from Missoula County, Montana. The Canadian Field Naturalist, 83: 324-330.
Nice, M., C. Nice, D. Ewers. 1954. Comparison of behavioral development in snowshoe hares and red squirrels. Journal of Mammalogy, 37: 64-74.
Pauls, R. 1978. Behavioral strategies relevant to the energy economy of the red squirrel (Tamiasciurus hudsonicus). Canadian Journal of Zoology, 56: 1519-1525.
Price, K., K. Broughton, S. Boutin, A. Sinclair. 1986. Territory size and ownership in red squirrels: response to removals. Canadian Journal of Zoology, 64: 1144-1147.
Pruitt, W., C. Lucier. 1958. Winter activity of red squirrels in interior Alaska. Journal of Mammalogy, 39: 443-444.
Ruff, S., D. Wilson. 1999. The Smithsonian Book of North American Mammals. Washington (D.C.): Smithsonian Institution Press in association with the American Society of Mammalogists.
Smith, C. 1968. The adaptive nature of social organization in the genus of tree squirrels Tamiasciurus. Ecology Monographs, 38: 31-63.
Smith, C., O. Reichman. 1984. The evolution of food caching by birds and mammals. Annual Review of Ecology and Systematics, 15: 329-351.
Steele, M. 1998. Mammalian Species. Pp. 1-9 in Tamiasciurus hudsonicus, Vol. 586. American Society of Mammalogists.
Stuart-Smith, A., S. Boutin. 1995. Behavioral differences between surviving and depredated juvenile red squirrels. Ecoscience, 2: 34-40.
Sullivan, T., H. Coates, L. Jozsa, P. Diggle. 1993. Influence of feeding damage by small mammals on tree growth and wood quality in young lodgepole pine. Canadian Journal of Forest Research, 23: 799-809.
Vache, M., J. Ferron, P. Gouat. 2001. The ability of red squirrels (Tamiasciurus hudsonicus) to discriminate conspecific olfactory signatures. Canadian Journal of Zoology, 79/7: 1296-1300. Accessed January 03, 2012 at http://www.nrcresearchpress.com/doi/abs/10.1139/z01-085?journalCode=cjz.
Walton, M. 1903. A hermit's wild friends. Boston, Massachusetts: D. Estes, 304 p..
Wilson, M., T. De Santo, K. Sieving. 2003. Red squirrels and predation risk to bird nests in northern forests. Canadian Journal of Zoology, 81/7: 1202-1208.
Wirsing, A., T. Steury, D. Murray. 2002. Relationship between body condition and vulnerability to predation in red squirrels and snowshoe hares. Journal of Mammalogy, 83/3: 707-715. Accessed November 03, 2011 at http://www.asmjournals.org/doi/abs/10.1644/1545-1542(2002)083%3C0707%3ARBBCAV%3E2.0.CO%3B2.
Wrigley, R. 1969. Ecological notes on the mammals of southern Quebec. Canadian Field-Naturalist, 83: 201-211.