Squirrels are a diverse group consisting of approximately 279 species and 51 genera that are broken into five subfamilies (Ratufinae, Sciurillinae, Sciurinae, Xerinae, and Callosciurinae). The family Sciuridae includes tree squirrels, ground squirrels, and flying squirrels. Tree squirrels have long, bushy tails, sharp claws and large ears. Some have well-developed ear tufts. Flying squirrels have a furred membrane (patagium) extending between the wrist and ankle that allows them to glide between trees. Ground squirrels are generally more robust than tree squirrels and often have short, sturdy forelimbs that are used for digging. Their tails, while fully furred, generally are not as bushy as those of tree squirrels. ("The IUCN Red List of Threatened Species", 2008; Gurnell, 1987; Jansa and Myers, 2000; Lurz, 2011; Steppan and Hamm, 2006; Thorington and Ferrell, 2006)
Sciurids range in body size from mouse-sized African pygmy squirrels to robust red giant flying squirrels of Asia, weighing up to 3 kilograms. They vary greatly in geographic range and habitat. Squirrels are native throughout the world, with the exception of Antarctica, Australia, southern South America, and some desert regions. They occupy habitats ranging from tundra to rainforest. Some squirrels live solitary lives such as woodchucks, while others, such as prairie dogs, live in communities of hundreds of individuals with complex social structures. Squirrels are largely herbivorous, eating seeds, nuts, fruits, fungi, and other plant matter; however, insects, eggs and the occasional small vertebrate may be part of the diverse diet of these animals. ("The IUCN Red List of Threatened Species", 2008; Gurnell, 1987; Jansa and Myers, 2000; Lurz, 2011; Steppan and Hamm, 2006; Thorington and Ferrell, 2006)
Squirrels are found worldwide, native to all terrestrial regions with the exception of Australia, Madagascar, southern South America, Antarctica, Greenland, many oceanic islands, and certain desert regions such as the Sahara. Two species of squirrels were introduced to Australia in the 19th century, Sciurus carolinensis and Funambulus pennantii, but only F. pennantii has persisted there. Squirrels are especially diverse in African and southeast Asian forests. (Anderson and Jones, 1984; Macdonald, 1984; Matthews, 1971; Nowak, 1991; Seebeck, 1989; Steppan and Hamm, 2006; Thorington and Ferrell, 2006)
- Biogeographic Regions
- Other Geographic Terms
From trees to burrows underground, sciurids are found in a vast array of habitats, including rainforests, arid grasslands, arctic tundras, forests, suburban areas, and cities. Sciurids can be found at high elevations, such as the Himalayan marmots (Marmota himalaya), which are found at elevations up to 5000 meters. (Steppan and Hamm, 2006; Thorington and Ferrell, 2006)
- Terrestrial Biomes
- desert or dune
- savanna or grassland
- scrub forest
Squirrels are characterized by their long bodies, soft fine hair (although some have very thick hair), and large eyes. The hindfeet have five digits while their forefeet have four digits. Claws are found on all terminal phalanges except the thumb, which has a nail. Vibrissae, which are important for tactical stimuli, are found all over the body by the nose, cheek, eye, chin, wrist, feet, and outside of the legs. Sciurids vary in size from very small, like African pygmy squirrels (Myosciurus pumilio, approximately 10 g), to substantially large, such as Alpine marmots (Marmota marmota, 3 to 8 kg). They also vary substantially in fur color including black, white, red, and brown. Sciurids have three basic morphologies: ground squirrel, tree squirrel, and flying squirrel forms. Ground squirrels tend to have large broad forefeet, with the middle digit being the longest, short, stout limbs used for digging, and short tails. Tree squirrels have long muscular legs, long arms, large ears, and long bushy tails. Flying squirrels are characterized by their gliding membrane, a furred patagium which attaches to the forelimbs via the styliform cartilage at the wrist and extends down to the heel of the hindlimbs. Additionally, flying squirrels have the longest limbs relative to body size of all squirrels.
What links all squirrels is their skull architecture and relatively primitive jaw structure. Their skulls are short, with a short rostrum and arched profile. The skull has an broad, tilted zygomatic plate that serves as the attachment point for the lateral branch of the masseter muscle. The superficial branch of masseter muscle originates on a prominent bump of bone of the side of rostrum called the masseteric tubercle. They have small infraorbital foramena that is not enlarged to transmit muscle as it is in myomorphous (mice and rats) and hystricomorphous (cavys and guinea pigs) rodents. Squirrels have long jugals, well-developed postorbital processes, and large bullae that are not inflated. The anterior ends of the jugals contact the frontals and the palate is broad and relatively short, ending at the same level as the molar row. The zygomasseteric architecture of skulls is sciuromorphous (the lateral branch of the masseter muscle has shifted to the rostrum).
The teeth of sciurids is characterized by four chisel-like incisors covered in enamel that grow continuously and have roots that extend well back into the maxilla and mandible. Since they are used for gnawing, these teeth are kept short and sharp. The incisors are followed by a diastema and cheek teeth which are rooted and brachydont or hypsodont. The dental formula of sciurids is 1/1, 0/0, 1-2/1, 3/3 = 20-22. (Anderson and Jones, 1984; Hayssen, 2008a; Jansa and Myers, 2000; Michaux, et al., 2008; Nowak, 1991; Steppan and Hamm, 2006; Thorington and Ferrell, 2006)
- Sexual Dimorphism
- sexes alike
- female larger
- male larger
Sciurid mating systems are polygynandrous; multiple males may mate with multiple females in a single breeding season. Generally, females are widely dispersed and males do not defend a territory, but this behavior varies. Some species of ground squirrels live in groups, forcing males to defend a small territory to attract females. In North America, many sciurids have two peak times of reproduction each year: one from December to January and the other from May to June. The sciurid breeding season is marked by the development of male testes and sexual swelling of the females, as is common in many mammals. Males may follow the scent left by estrous females or they may simply chase females. Groups of males, ranging from 4 to 9 individuals, chase a female from branch to branch at top speeds in what is known as a “mating chase.” Typically, one dominant male stands out from the rest. His dominance is determined by his ability to keep close proximity both during and between mating chases. After the female accepts a particular male, copulation ensues, and the mating chase continues, allowing for multiple copulations for both males and females. Ground squirrels, however, have a different mating system because they hibernate through the winter. Females go into estrous soon after hibernation, marking the beginning of breeding season. Female receptivity varies between species. Shorter estrous allows a single male to mate a single female, while longer estrous allows a single female to mate with multiple males. Ground squirrel mating rituals take place underground or in the nest, therefore much of this behavior is unknown. (Hayssen, 2008b; Thorington and Ferrell, 2006; Vernes, 2004)
- Mating System
- polygynandrous (promiscuous)
Gestation ranges from 29 to 65 days, depending on the size of the species, with smaller squirrels having shorter gestation periods. For squirrels that hibernate, mothers must wean their young in enough time to gain winter weight for hibernation. All sciurids give birth to their young in a nest. Although a single male can fertilize an entire litter, usually a litter has varying paternity, so a single litter could have multiple fathers. A typical litter consists of four offspring that are born naked, with closed eyes and ears. Development and sexual maturity varies from species to species, with some squirrels being able to leave the nest after 26 days, and reaching sexual maturity by 87 days, to squirrels that are fully developed after 42 days, but don’t reach sexual maturity until they are 3 years old. Hibernating squirrels tend to develop more quickly, with lactation times averaging 38 days. In tree and flying squirrels, lactation averages 70 days, longer than most other squirrel groups. African tree squirrels tend to be born at larger birth weights and have relatively shorter times to independence. (Hayssen, 2008b; Thorington and Ferrell, 2006; Vernes, 2004)
- Key Reproductive Features
- seasonal breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
All squirrels are altricial at birth. Parental investment involves the female providing food and care for the young. In species in which females aggregate, they may bequeath natal territory to juveniles, which increases survivorship. In some African sciurids, females share in the care of their young. (Hayssen, 2008b; Thorington and Ferrell, 2006)
Squirrels have been found to live a maximum of 8 to 14 years in the wild and up to 16 or more years in captivity. However, many squirrels do not live past their first year of life in the wild. (Macdonald, 1984; Thorington and Ferrell, 2006)
Ground squirrels tend to be the most social squirrels. These squirrels play, groom, and vocalize. Males and juveniles may help care for infants along with females. Group living allows these squirrels to benefit from alarm calling and mobbing predators. Living in groups also has its disadvantages, such as increased competition and aggression between individuals and disease risk. Ground squirrels make burrows underground which they use as nests for raising young and as a way to escape more extreme ambient temperatures above ground (heat during the day, cold at night or in the winter).
Tree squirrels are generally solitary, although some do form groups when nesting. Others may form travel pairs or loose aggregations around resources. Nesting tree squirrels use hollow trees and fill them with leaves and needles. Tree squirrels are also very agile in maneuvering through the trees. They are able to navigate and memorize aspects of their complex habitats. They are also known for their caching and hoarding behaviors, which requires learning and remembering landmarks in complex environments.
Flying squirrels are the only nocturnal sciurids and little is known about their behavior. These squirrels are not capable of true flight, instead they use a specialized membrane to glide from one place to another after they jump from a height. They raise young and aggregate to stay warm in tree cavities. (Bordignon, 2000; Thorington and Ferrell, 2006)
Communication and Perception
Squirrels have distinct vocalizations for particular situations, such as infants calling to their mothers and adults vocalizing during aggression. Males vocalize during the mating season to attract mates. Many squirrel species use distinctive alarm calls to warn conspecifics of dangers, including alarm calls that warn of specific threats, distinguishing between aerial and terrestrial predators. Squirrels also use posture and movement as a form of communication, with messages carried by tail position, stomping of the feet, or body posture. As in most mammals, olfaction is very important in communication. Females indicate sexual receptiveness through pheromones and social position or relatedness may also be inferred through chemical cues. (Thorington and Ferrell, 2006)
Most squirrels eat mainly tree seeds and fruit, but their diet is diverse, including insects, eggs, fungi, lichens, and small vertebrates. While some squirrels consume fungi as a secondary component of their diet, it makes up nearly half of the diet of other species. Many species opportunistically take animal prey, such as the young and eggs of birds or other mammals. Foods such as buds, shoots, flowers, bark, lichens, and green plant material have generally low energy content per unit weight and make up a smaller portion of the diet. But the amount of each type of food consumed is determined mainly by its availability and accessibility. For this reason, diet composition changes from region to region, season to season, and year to year. Many squirrel species cache or hoard food as well. (Gurnell, 1987; Thorington and Ferrell, 2006)
- Primary Diet
- eats terrestrial vertebrates
- eats eggs
- Foraging Behavior
- stores or caches food
Sciurids are a typical meal for many opportunistic domestic and wild predators. The most common predators may be birds of prey, such as eagles and hawks, falcons, owls, and cats, canids, and weasels. But many predators have been observed eating squirrels, including large snakes, bigmouth bass, and chimpanzees . Squirrels are also hunted for food and fur by humans. (Gurnell, 1987; Thorington and Ferrell, 2006)
The most common tactics of sciurids for avoiding predators is camouflage and escape. Squirrels typically have coats with color that matches their surroundings. Tree squirrels often have lighter coloration on the ventral side compared to the dorsal, allowing them to blend in with the light sky to a predator that is looking at the squirrel from below and at to blend in with the dark ground when being stalked by an aerial predator. Squirrels also avoid capture by quickly darting away from predators, remaining vigilant, biting, clawing, hiding in burrows or nests and sounding alarm calls. Squirrels that are commonly attacked by snakes take on a completely different defense tactic known as mobbing. Especially common in communal prairie dogs (Cynomys), these squirrels will attack snakes, pouncing, biting and scratching until the snake leaves the area near the burrow or is killed. (Thorington and Ferrell, 2006)
- Anti-predator Adaptations
- Known Predators
- birds of prey (Accipitridae)
- cats (Felidae)
- foxes (Vulpes)
- owls (Strigiformes)
- snakes (Serpentes)
- martens (Martes)
- bears (Ursidae)
- African wild dogs (Lycaon pictus)
- coyotes (Canis latrans)
- badgers (Taxidea taxus)
- weasels and ferrets (Mustela)
- great herons (Ardea)
- raccoons (Procyon lotor)
- crows and ravens (Corvus)
- chimpanzees (Pan troglodytes)
- domestic dogs (Canis lupus familiaris)
- domestic Cats (Felis catus)
- ermine (Mustela erminea)
- wolves (Canis lupus)
- humans (Homo sapiens)
- wolverines (Gulo gulo)
- snow leopards (Uncia uncia)
- wild cats (Felis silvestris)
- bigmouth bass (Micropterus salmoides)
Squirrels are important sources of prey for a vast array of predators including predators that are threatened or endangered. For rare species such as snow leopards and northern spotted owls, squirrels are an essential part of the diet. Black-footed ferrets eat almost exclusively prairie dogs. Tree and flying squirrels are also essential in the regeneration of forests around the world through their seed dispersal activities. This is not only because seeds are left in the feces of these animals but also because of the caching habits of many squirrels. Squirrels are also important in dispersing the spores of fungi that they eat, including ecologically important underground endorhyzal fungi. Squirrels serve as host to a number of parasites such as fleas, mites and ticks. These parasites are known for causing the transfer of a number of diseases, such as plague, from squirrel to squirrel and to other mammals, including humans. Some species have also been identified as playing the role of a keystone species in their ecosystem. One study conducted by Kotiliar et al. (1999) confirmed that prairie dogs acted as a keystone species in the Great Plains of the United states. Not only were prairie dogs an important food source for predators such as golden eagles and swift foxes, but their burrows were very important for a number of animals as well. Abandoned burrows were used as housing for animals ranging from raccoons and cottontail rabbits to shrews and voles. Others were found to feed on the vegetation that had been disturbed by the construction of prairie dog colonies. The digging of these burrows were also found to be beneficial for a number of plants because of the aeration of soil and fertilizing properties in their feces. (Gurnell, 1987; Thorington and Ferrell, 2006)
- mites and ticks
Economic Importance for Humans: Positive
Squirrels are hunted for their meat and pelts. Squirrel pelts were also once used as a form of currency. The modern word for money in Finland actually comes from a root word that means squirrel skins. Since their diets consist of mainly fruits and seeds, squirrels become very useful in seed dispersal. Squirrels that eat flowers or drink nectar may also aid in pollination. Squirrels are used in medical and scientific research. A research project at the Institute of Arctic Biology at the University of Alaska, Fairbanks has been studying hibernation in arctic ground squirrels to learn more about strokes, heart attacks, and neurodegenerative diseases in humans caused by reduced blood flow. Groundhogs suffer from a virus very similar to Hepatitis B in humans and exhibit similar disease progression such as cancer of the liver and liver disease. For this reason, these animals are used as models for studying the disease, treatments, and advances in liver transplant techniques (summarized in Thorington and Ferrell, 2006). (Gurnell, 1987; Thorington and Ferrell, 2006)
- Positive Impacts
- body parts are source of valuable material
- research and education
- pollinates crops
Economic Importance for Humans: Negative
The majority of problems sciurids cause humans are a result of their voracious appetites. These animals dig up seeds planted by farmers as well as devouring crops. Many ground and tree squirrels commonly participate in “bark stripping” where they pull the bark off of trees to get to the tissue underneath. This may stunt the growth of the tree, cause a reduction in fruit production, or cause the tree to die. Ranchers see prairie dogs as a threat, although those threats are generally not substantiated. Squirrels can be seen as a nuisance to homeowners as well. They may chew through electrical and telephone wiring, insulation, and house siding. Squirrels sometimes climb into and cache food in transformers and generators, causing power outages in surrounding communities. Squirrels may carry diseases, such as plague, which can be transmitted to humans and domestic animals. In western North America, rock squirrels and prairie dog species are the most common and frequent source of transmission to humans. ("Information on Plague", 2005; Gurnell, 1987; Thorington and Ferrell, 2006)
- Negative Impacts
- carries human disease
- crop pest
- causes or carries domestic animal disease
- household pest
Of the 279 species of Sciuridae, two are listed as critically endangered while another 15 are listed as endangered and 16 are listed as vulnerable. The most endangered sciurid is the Vancouver Marmot (Marmota vancouverensis), consisting of only an estimated 35 individuals in the wild as of 2004. Common factors leading to these marmots being listed as endangered include destruction of habitat and human encroachment. Lack of accurate information on populations and threats is another important factor in sciurid conservation generally. ("North American Mammals", 2009; "The IUCN Red List of Threatened Species", 2008; Thorington and Ferrell, 2006)
- IUCN Red List [Link]
- Not Evaluated
Squirrels are known from the end of the Eocene in Wyoming and the early Oligocene in southern France. (Thorington and Ferrell, 2006)
Emily McBride Brown (author), University of Michigan-Ann Arbor, Alexandra Michelle Peri (author), University of Michigan-Ann Arbor, Nicole Ann Santarosa (author), University of Michigan-Ann Arbor, Tanya Dewey (editor), University of Michigan-Ann Arbor, Phil Myers (editor), University of Michigan-Ann Arbor.
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
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.
living in the southern part of the New World. In other words, Central and South America.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
uses sound to communicate
living in landscapes dominated by human agriculture.
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.
- bilateral symmetry
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.
an animal that mainly eats meat
- causes or carries domestic animal disease
either directly causes, or indirectly transmits, a disease to a domestic animal
Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.
uses smells or other chemicals to communicate
used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
- desert or dunes
in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.
- active during the day, 2. lasting for one day.
- dominance hierarchies
ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates
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.
- female parental care
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.
Referring to a burrowing life-style or behavior, specialized for digging or burrowing.
an animal that mainly eats fruit
an animal that mainly eats seeds
An animal that eats mainly plants or parts of plants.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.
a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.
Found in northern North America and northern Europe or Asia.
An animal that eats mainly insects or spiders.
referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
- keystone species
a species whose presence or absence strongly affects populations of other species in that area such that the extirpation of the keystone species in an area will result in the ultimate extirpation of many more species in that area (Example: sea otter).
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.
an animal that mainly eats fungus
- native range
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
an animal that mainly eats all kinds of things, including plants and animals
found in the oriental region of the world. In other words, India and southeast Asia.
chemicals released into air or water that are detected by and responded to by other animals of the same species
the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.
rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
- scent marks
communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them
- scrub forest
scrub forests develop in areas that experience dry seasons.
- seasonal breeding
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
associates with others of its species; forms social groups.
- soil aeration
digs and breaks up soil so air and water can get in
- stores or caches food
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
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
- tropical savanna and grassland
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
- temperate grassland
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.
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.
2005. "Information on Plague" (On-line). CDC: Division of Vector-Borne Infectious Diseases. Accessed February 17, 2009 at http://www.cdc.gov/ncidod/dvbid/plague/info.htm.
2009. "North American Mammals" (On-line). Smithsonian National Museum of Natural History. Accessed February 01, 2009 at http://www.mnh.si.edu/mna/image_menu.cfm?family=33.
Net Industries. 2009. "Squirrels and Relatives - Sciuridae" (On-line). Animal Life Resource - Mammals and Other Warm-Blooded Animals. Accessed February 03, 2009 at http://animals.jrank.org/pages/3363/Squirrels-Relatives-Sciuridae.html.
2008. "The IUCN Red List of Threatened Species" (On-line). Accessed February 01, 2009 at http://www.iucnredlist.org/documents/mammals/gma_redlist_by_famil_v1223563405.xls.
Anderson, S., J. Jones. 1984. Orders and Families of Recent Mammals of the World. United States of America: Wiley-Interscience Publication.
Asdell, S. 1964. Patterns of Mammalian Reproduction. Ithaca: Cornell University Press.
Bordignon, M. 2000. Behaviour and daily activity of the squirrel Sciurus ingrami in a secondary aracaria forest in southern brazil. Canadian Journal of Zoology, 10: 1732-1739.
Dewey, T. 2007. "Sciuromorpha" (On-line). Animal Diversity Web. Accessed February 02, 2009 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Sciuromorpha.html.
Gurnell, J. 1987. The Natural History of Squirrels. New York: Facts on File Inc.
Hayssen, V. 2008. Reproductive effor in squirrels: Ecological, phylogenetic, allometic, and latitudinal patterns. Journal of Mammalogy, 3: 89-91.
Hayssen, V. 2008. Patterns of Body and Tail Length and Body Mass in Sciuridae. Journal of Mammalogy, 89/4: 852-873. Accessed February 03, 2009 at http://proquest.umi.com.proxy.lib.umich.edu/pqdweb?did=1559964161&sid=1&Fmt=6&clientId=17822&RQT=309&VName=PQD.
Jansa, S., P. Myers. 2000. "Family Sciuridae" (On-line). Animal Diversity Web. Accessed February 12, 2009 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Sciuridae.html.
Long, J. 2003. Introduced Mammals of the World. Australia and United Kingdom: CSIRO Publishing and CABI Publishing.
Lurz, P. 2011. "Squirrels and Relatives III: Tree Squirrels" (On-line). Grzimek's Animal Life. Accessed April 16, 2011 at http://animals.galegroup.com.proxy.lib.umich.edu.
Macdonald, D. 1984. The Encyclopedia of Mammals. New York, NY: Facts on File, Inc..
Matthews, L. 1971. The Life of Mammals: Volume II. New York City: Universe Books.
Michaux, J., L. Hautier, T. Simonin, M. Vianey-Liaud. 2008. Phylogeny, adaptation, and mandible shape in Sciuridae (Rodentia, Mammalia). Mammalia, 72/4: 286-296. Accessed February 03, 2009 at http://www.reference-global.com.proxy.lib.umich.edu/doi/pdf/10.1515/MAMM.2008.049.
Nowak, R. 1991. Walker's Mammals of the World 5th Edition, Volume I. Baltimore and London: The Johns Hopkins University Press.
Seebeck, J. 1989. "Fauna of Australia 46. Sciuridae" (On-line). Accessed March 08, 2009 at http://www.environment.gov.au/biodiversity/abrs/publications/fauna-of-australia/pubs/volume1b/46-ind.pdf.
Steppan, S., S. Hamm. 2006. "Sciuridae. Squirrels. Version 13 May 2006" (On-line). The Tree of Life Web Project. Accessed January 28, 2009 at http://tolweb.org/Sciuridae/16456/2006.05.13.
Steppan, S., B. Storz, R. Hoffmann. 2004. Nuclear DNA Phylogeny of the Squirrels (Mammalia:Rodentia) and the Evolution of Arboreality from c-myc and RAG1. Molecular Phylogenetics and Evolution, 30: 703-719. Accessed February 15, 2009 at http://bio.fsu.edu/~steppan/Steppan_et_al_Sciuridae.pdf.
Thorington, R., K. Ferrell. 2006. Squirrels - The Animal Answer Guide. Baltimore, Maryland: The Johns Hopkins University Press.
Vernes, K. 2004. Breeding biology and seasonal capture success of northern flying; squirrels and red squirrels in southern new brunswick. Northeastern Naturalist, 11: 123-137.
Wilson, D., D. Reeder. 2005. Mammal Species of the World: 3rd Edition, Volume 2. Baltimore: The Johns Hopkins University Press.