Squirrels belong to a large family of small or medium-sized rodents called the Sciuridae. The family includes tree squirrels, ground squirrels, chipmunks, marmots (including woodchucks), flying squirrels, and prairie dogs. Squirrels are indigenous to the Americas, Eurasia, and Africa, and have been introduced to Australia. The earliest known squirrels date from the Eocene and are most closely related to the mountain beaver and to the dormouse among living rodent families.
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.
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.
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. One of those species, Sciurus carolinensis, is now believed to be extinct. However, a flourishing feral colony of Funambulus pennantii persists there. Squirrels are especially diverse in African and southeast Asian forests.
Biogeographic Regions: nearctic (Native ); palearctic (Native ); oriental (Native ); ethiopian (Native ); neotropical (Native ); australian (Introduced )
Other Geographic Terms: holarctic
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.
Other Physical Features: endothermic ; heterothermic ; homoiothermic; bilateral symmetry ; polymorphic
Sexual Dimorphism: sexes alike; female larger; male larger
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.
Habitat Regions: temperate ; tropical ; polar ; terrestrial
Terrestrial Biomes: tundra ; taiga ; desert or dune ; savanna or grassland ; chaparral ; forest ; rainforest ; scrub forest ; mountains
Other Habitat Features: urban ; suburban ; agricultural ; riparian
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.
Foraging Behavior: stores or caches food
Primary Diet: carnivore (Eats terrestrial vertebrates, Eats eggs, Insectivore ); herbivore (Frugivore , Granivore ); omnivore ; mycophage
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.
Ecosystem Impact: disperses seeds; pollinates; soil aeration ; keystone species
- mites and ticks
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.
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.
- 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)
Anti-predator Adaptations: cryptic
sporangiophore of Pilaira anomala is saprobic in/on dung or excretions of dung of Sciuridae
Animal / dung saprobe
sporangiophore of Pilobolus crystallinus var. crystallinus is saprobic in/on dung or excretions of dung of Sciuridae
Animal / dung saprobe
sporangiophore of Pilobolus crystallinus var. kleinii is saprobic in/on dung or excretions of dung of Sciuridae
Based on studies in:
USA: Alaska (Tundra)
This list may not be complete but is based on published studies.
Known prey organisms
Based on studies in:
USA: Alaska (Tundra)
This list may not be complete but is based on published studies.
Life History and Behavior
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.
Communication Channels: visual ; tactile ; acoustic ; chemical
Other Communication Modes: pheromones ; scent marks
Perception Channels: visual ; tactile ; acoustic ; chemical
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.
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.
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.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous
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.
Parental Investment: altricial ; female parental care ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female); inherits maternal/paternal territory
Evolution and Systematics
The feet of squirrels allow them to climb trees and hang from almost vertical surfaces via sharp claws and swiveling ankle joints.
"The squirrel is particularly well adapted for tree climbing. It has sharp claws, and instead of having backward-pointing toes like the climbing birds, it can swivel the whole back foot round at the ankle so that it points backwards. The squirrel can thus hang from an almost vertical surface provided there is enough irregularity on the tree trunk into which to hook its claws." (Foy and Oxford Scientific Films 1982:183)
Learn more about this functional adaptation.
Livers of squirrels survive cold temperatures during hibernation by maintaining microcirculation.
"Close investigation of hibernating states is giving scientists new insight into animal physiology and, as a bonus, human health and disease as well…Organ transplant. Harvested human organs kept at cold temperatures can only remain viable for a few days at most. So, how do squirrels' livers and intestines stay perfectly healthy at less than 40 degrees Fahrenheit for weeks on end? In one study, 'we found that [hibernating] squirrels maintain, in a much healthier state, the microcirculation of the liver,' Carey said [Hannah Carey, a professor in the School of Veterinary Medicine at the University of Wisconsin-Madison]. Insights into just how this happens could increase the viability and number of human organs available for transplant, she said." (Mundell 2007)
Learn more about this functional adaptation.
Molecular Biology and Genetics
Statistics of barcoding coverage
Specimens with Sequences:536
Specimens with Barcodes:522
Species With Barcodes:81
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.
Relevance to Humans and Ecosystems
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.
Negative Impacts: injures humans (carries human disease); crop pest; causes or carries domestic animal disease ; household pest
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).
Positive Impacts: food ; body parts are source of valuable material; research and education; pollinates crops
Squirrels are members of the family Sciuridae, consisting of small or medium-size rodents. The family includes tree squirrels, ground squirrels, chipmunks, marmots (including woodchucks), flying squirrels, and prairie dogs. Squirrels are indigenous to the Americas, Eurasia, and Africa, and have been introduced to Australia. The earliest known squirrels date from the Eocene and are most closely related to the mountain beaver and to the dormouse among living rodent families.
The word "squirrel", first specified in 1327, comes from Anglo-Norman esquirel from the Old French escurel, the reflex of a Latin word sciurus. This Latin word was borrowed from the Ancient Greek word σκίουρος, skiouros, which means shadow-tailed, referring to the bushy appendage possessed by many of its members.
The native Old English word for the squirrel, ācweorna, survived only into Middle English (as aquerne) before being replaced. The Old English word is of Common Germanic origin, and cognates used by other countries to name the squirrel include the German Eichhörnchen (diminutive of Eichhorn, which is not used), the Norwegian ikorn/ekorn, the Dutch eekhoorn, the Swedish ekorre and the Danish egern.
Squirrels are generally small animals, ranging in size from the African pygmy squirrel at 7–10 cm (2.8–3.9 in) in length and just 10 g (0.35 oz) in weight, to the Alpine marmot, which is 53–73 cm (21–29 in) long and weighs from 5 to 8 kg (11 to 18 lb). Squirrels typically have slender bodies with bushy tails and large eyes. In general, their fur is soft and silky, although much thicker in some species than others. The color of squirrels is highly variable between—and often even within—species.
In general, the hind limbs are longer than the fore limbs, and they have four or five toes on each paw. Their paws include an often poorly developed thumb, and have soft pads on the undersides. Unlike most mammals, Tree squirrels can descend a tree head-first. They do so by rotating their ankles 180 degrees so the hind paws are backward-pointing and can grip the tree bark.
Squirrels live in almost every habitat from tropical rainforest to semiarid desert, avoiding only the high polar regions and the driest of deserts. They are predominantly herbivorous, subsisting on seeds and nuts, but many will eat insects and even small vertebrates.
As their large eyes indicate, in general squirrels have an excellent sense of vision, which is especially important for tree-dwelling species. They also have very versatile and sturdy claws for grasping and climbing. Many also have a good sense of touch, with vibrissae on their heads and limbs.
The teeth of sciurids follow the typical rodent pattern, with large gnawing incisors that grow throughout life, and grinding cheek teeth set back behind a wide gap, or diastema. The typical dental formula for sciurids is 22.214.171.124
Most squirrels die in the first year of life. Adult squirrels can have a lifespan of 5 to 10 years in the wild. Some can survive 10 to 20 years in captivity.
Squirrels breed once or twice a year and give birth to a varying number of young after three to six weeks, depending on species. The young are born naked, toothless, and blind. In most species of squirrel, only the female looks after the young, which are weaned at around six to ten weeks of age and become sexually mature at the end of their first year. In general, ground-dwelling species are social animals, often living in well-developed colonies, but the tree-dwelling species are more solitary.
Ground and tree squirrels are typically diurnal or crepuscular, while flying squirrels tend to be nocturnal—except for lactating flying squirrels and their offspring, which have a period of diurnality during the summer.
Squirrels cannot digest cellulose, so they must rely on foods rich in protein, carbohydrates, and fats. In temperate regions, early spring is the hardest time of year for squirrels, because buried nuts begin to sprout and are no longer available for the squirrel to eat, and new food sources have not become available yet. During these times, squirrels rely heavily on the buds of trees. Squirrels' diets consist primarily of a wide variety of plants, including nuts, seeds, conifer cones, fruits, fungi, and green vegetation. However, some squirrels also consume meat, especially when faced with hunger. Squirrels have been known to eat insects, eggs, small birds, young snakes, and smaller rodents. Indeed, some tropical species have shifted almost entirely to a diet of insects.
Predatory behavior has been noted by various species of ground squirrels, in particular the thirteen-lined ground squirrel. For example, Bailey, a scientist in the 1920s, observed a thirteen-lined ground squirrel preying upon a young chicken. Wistrand reported seeing this same species eating a freshly killed snake. Whitaker examined the stomachs of 139 thirteen-lined ground squirrels and found bird flesh in four of the specimens and the remains of a short-tailed shrew in one; Bradley, examining white-tailed antelope squirrels' stomachs, found at least 10% of his 609 specimens' stomachs contained some type of vertebrate, mostly lizards and rodents. Morgart observed a white-tailed antelope squirrel capturing and eating a silky pocket mouse.
The living squirrels are divided into five subfamilies, with about 58 genera and some 285 species. The oldest squirrel fossil, Hesperopetes, dates back to the Chadronian (late Eocene, about 40–35 million years ago) and is similar to modern flying squirrels.
A variety of fossil squirrels, from the latest Eocene to the Miocene, could not be assigned with certainty to any living lineage. At least some of these probably were variants of the oldest basal "protosquirrels" (in the sense that they lacked the full range of living squirrels' autapomorphies). The distribution and diversity of such ancient and ancestral forms suggest the squirrels as a group may have originated in North America.
Apart from these sometimes little-known fossil forms, the phylogeny of the living squirrels is fairly straightforward. The three main lineages are the Ratufinae (Oriental giant squirrels), Sciurillinae and all other subfamilies. The Ratufinae contain a mere handful of living species in tropical Asia. The neotropical pygmy squirrel of tropical South America is the sole living member of the Sciurillinae. The third lineage, by far the largest, has a near-cosmopolitan distribution. This further supports the hypothesis that the common ancestor of all squirrels, living and fossil, lived in North America, as these three most ancient lineages seem to have radiated from there; if squirrels had originated in Eurasia, for example, one would expect quite ancient lineages in Africa, but African squirrels seem to be of more recent origin.
The main group of squirrels also can be split into three subgroups, which yield the remaining subfamilies. The Sciurinae contains the flying squirrels (Pteromyini) and the Sciurini, which among others contains the American tree squirrels; the former have often been considered a separate subfamily, but are now seen as a tribe of the Sciurinae. The pine squirrels (Tamiasciurus), on the other hand, are usually included with the main tree squirrel lineage, but appear to be about as distinct as the flying squirrels; hence, they are sometimes considered a distinct tribe, Tamiasciurini.
Two of the three subfamilies are of about equal size, containing between nearly 70 and 80 species each; the third is about twice as large. The Sciurinae contains arboreal (tree-living) squirrels, mainly of the Americas and to a lesser extent Eurasia. The Callosciurinae is most diverse in tropical Asia and contains squirrels that are also arboreal, but have a markedly different habitus and appear more "elegant", an effect enhanced by their often very colorful fur. The Xerinae—the largest subfamily—are made up from the mainly terrestrial (ground-living) forms and include the large marmots and the popular prairie dogs, among others, as well as the tree squirrels of Africa; they tend to be more gregarious than other squirrels, which do not usually live together in close-knit groups.
- Basal and incertae sedis Sciuridae (all fossil)
- Subfamily Cedromurinae (fossil)
- Subfamily Ratufinae – Oriental giant squirrels (1 genus, 4 species)
- Subfamily Sciurillinae – neotropical pygmy squirrel (monotypic)
- Subfamily Sciurinae
- Subfamily Callosciurinae – Asian ornate squirrels
- Subfamily Xerinae – terrestrial squirrels
- American red squirrel
- Animal track
- Fox squirrel
- Purple squirrel
- Red squirrel
- Squirrel relationship with humans
- Western gray squirrel
- Seebeck, J. H. "Sciuridae". Fauna of Australia. Retrieved 2013-11-24.
- "squirrel, n.". The Oxford English Dictionary (2nd. ed.). Oxford University Press. 1989. Retrieved 8 November 2010.
- Whitaker & Elman (1980): 370
- "Squirrel". Online Etymology Dictionary. Retrieved 7 February 2008.
- Tree Squirrels, Wildlife Online, 23 November 2010.
- Milton (1984)
- Thorington, Richard W.; Koprowski, John L.; Steele, Michael A.; Whatton, James F. (2012). Squirrels of the World. Johns Hopkins University Press. p. 8. ISBN 1421404699.
- Squirrel Place - squirrels.org - Retrieved 14 December 2010.
- "Squirrel" - HowStuffWorks
- Thorington, Richard W.; Koprowski, John L.; Steele, Michael A.; Whatton, James F. (2012). Squirrels of the World. Johns Hopkins University Press. p. 12. ISBN 1421404699.
- "Red & Gray Squirrels in Massachusetts". MassWildlife. Massachusetts Division of Fisheries and Wildlife. Retrieved 3 April 2012.
- Törmälä, Timo; Vuorinen, Hannu; Hokkanen, Heikki (1980). "Timing of circadian activity in the flying squirrel in central Finland". Acta Theriologica 25 (32–42): 461–474. doi:10.4098/at.arch.80-42. Retrieved 11 July 2007.
- Richard W. Thorington, Katie Ferrell - Squirrels: the animal answer guide, JHU Press, 2006, ISBN 0-8018-8402-0, ISBN 978-0-8018-8402-3, p. 75.
- Friggens, M. (2002). "Carnivory on Desert Cottontails by Texas Antelope Ground Squirrels". The Southwestern Naturalist 47 (1): 132–133. doi:10.2307/3672818. JSTOR 3672818.
- Bailey, B. (1923). "Meat-eating propensities of some rodents of Minnesota". Journal of Mammalogy 4: 129.
- Wistrand, E.H. (1972). "Predation on a Snake by Spermophilus tridecemlineatus". American Midland Naturalist 88 (2): 511–512. doi:10.2307/2424389. JSTOR 2424389.
- Whitaker, J.O. (1972). "Food and external parasites of Spermophilus tridecemlineatus in Vigo County, Indiana". Journal of Mammalogy 53 (3): 644–648. doi:10.2307/1379067. JSTOR 1379067.
- Bradley, W. G. (1968). "Food habits of the antelope ground squirrel in southern Nevada". Journal of Mammalogy 49 (1): 14–21. doi:10.2307/1377723. JSTOR 1377723.
- Morgart, J. R. (May 1985). "Carnivorous behavior by a white-tailed antelope ground squirrel Ammospermophilus leucurus". The Southwestern Naturalist 30 (2): 304–305. doi:10.2307/3670745. JSTOR 3670745.
- Wilson, D.E.; Reeder, D.M. (2011). "Class Mammalia Linnaeus, 1758. In: Zhang, Z.-Q. (Ed.) Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness". Zootaxa 3148: 56–60.
- Emry, R. J.; Korth, W. W. (2007). "A new genus of squirrel (Rodentia, Sciuridae) from the mid-Cenozoic of North America". Journal of Vertebrate Paleontology 27 (3): 693. doi:10.1671/0272-4634(2007)27[693:ANGOSR]2.0.CO;2.
- Steppan & Hamm (2006)
- Steppan et al. (2004), Steppan & Hamm (2006)
- Milton, Katherine (1984): [Family Sciuridae]. In: Macdonald, D. (ed.): The Encyclopedia of Mammals: 612–623. Facts on File, New York. ISBN 0-87196-871-1
- Steppan, Scott J. & Hamm, Shawn M. (2006): Tree of Life Web Project – Sciuridae (Squirrels). Version of 13 May 2006. Retrieved 10 December 2007.
- Steppan, S. J.; Storz, B. L.; Hoffmann, R. S. (2004). "Nuclear DNA phylogeny of the squirrels (Mammalia: Rodentia) and the evolution of arboreality from c-myc and RAG1". Molecular Phylogenetics and Evolution 30 (3): 703–719. doi:10.1016/S1055-7903(03)00204-5. PMID 15012949.
- Thorington, R.W. & Hoffmann, R.S. (2005): Family Sciuridae. In: Mammal Species of the World – A Taxonomic and Geographic Reference: 754–818. Johns Hopkins University Press, Baltimore.
- Whitaker, John O. Jr. & Elman, Robert (1980): The Audubon Society Field Guide to North American Mammals (2nd ed.). Alfred Knopf, New York. ISBN 0-394-50762-2
To request an improvement, please leave a comment on the page. Thank you!