Squirrels are members of the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Sciuridae (), a family that includes small or medium-sized

rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and Mandible, lower jaws. About 40% of all mammal specie ...

s. The squirrel family includes

tree squirrel Tree squirrels are the members of the squirrel Family (biology), family (Sciuridae) commonly just referred to as "squirrels". They include more than 100 arboreal species native to all continents except Antarctica and Oceania. They do not form a ...

s, ground squirrels (including chipmunks and prairie dogs, among others), and

flying squirrel Flying squirrels (scientifically known as Pteromyini or Petauristini) are a tribe (biology), tribe of 50 species of squirrels in the family (biology), family Squirrel, Sciuridae. Despite their name, they are not in fact capable of full flight i ...

s. Squirrels are indigenous to the Americas, Eurasia, and Africa, and were introduced by humans to Australia. The earliest known fossilized squirrels date from the

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

epoch, and among other living rodent families, the squirrels are most closely related to the

mountain beaver The mountain beaver (''Aplodontia rufa'')Other names include boomer, mountain boomer, ground bear, giant mole, gehalis, sewellel, suwellel, showhurll, showtl, and showte, as well as a number of other Native American terms. "Mountain beaver" is a ...

and

dormice A dormouse is a rodent of the family (biology), family Gliridae (this family is also variously called Myoxidae or Muscardinidae by different taxonomists). Dormice are nocturnal animals found in Africa, Asia, and Europe. They are named for their ...

Etymology

The word ''squirrel'', first attested in 1327, comes from the Anglo-Norman which is from the

Old French Old French (, , ; ) was the language spoken in most of the northern half of France approximately between the late 8th

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

word , which was taken from the Ancient Greek word (; from ) 'shadow-tailed', referring to the long bushy tail which many of its members have. ''Sciurus'' is also the name of one of its genuses. The native Old English language, Old English word for the squirrel, , only survived into Middle English language, Middle English (as ) before being replaced. The Old English word is of

Common Germanic Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the reconstructed proto-language of the Germanic branch of the Indo-European languages. Proto-Germanic eventually developed from pre-Proto-Germanic into three Germanic bra ...

origin,

cognate In historical linguistics, cognates or lexical cognates are sets of words that have been inherited in direct descent from an etymological ancestor in a common parent language. Because language change can have radical effects on both the s ...

s of which are still used in other

Germanic languages The Germanic languages are a branch of the Indo-European languages, Indo-European language family spoken natively by a population of about 515 million people mainly in Europe, North America, Oceania, and Southern Africa. The most widely spoke ...

, including the German (diminutive of , which is not as frequently used); the Norwegian /; the Dutch ; the Swedish and the Danish . A group of squirrels is called a "dray" or a "scurry".

Characteristics

Squirrels are generally small animals, ranging in size from the African pygmy squirrel and least pygmy squirrel at in total length and just in weight, to the Bhutan giant flying squirrel at up to in total length, and several marmot species, which can weigh or more. Squirrels typically have slender bodies with long, bushy tails and large eyes. In general, their fur is soft and silky, though much thicker in some species than others. The coat color of squirrels is highly variable between—and often even within—species. In most squirrel species, the hind limbs are longer than the forelimbs, while all species have either four or five toes on each foot. The feet, which include an often poorly developed

thumb The thumb is the first digit of the hand, next to the index finger. When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for thumb ...

, have soft pads on the undersidesMilton (1984) and versatile, sturdy

claw A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or Arthro ...

s for grasping and climbing. Tree squirrels, unlike most mammals, can descend a tree headfirst. They do so by rotating their ankles 180 degrees, enabling the hind feet to point backward and thus grip the tree bark from the opposite direction.

Head

As their large eyes indicate, squirrels have excellent

vision Vision, Visions, or The Vision may refer to: Perception Optical perception * Visual perception, the sense of sight * Visual system, the physical mechanism of eyesight * Computer vision, a field dealing with how computers can be made to gain und ...

, which is especially important for the tree-dwelling species. Many also have a good sense of

touch The somatosensory system, or somatic sensory system is a subset of the sensory nervous system. The main functions of the somatosensory system are the perception of external stimuli, the perception of internal stimuli, and the regulation of bo ...

, with vibrissae on their limbs as well as their heads. The teeth of sciurids follow the typical rodent pattern, with large incisors (for gnawing) that grow throughout life, and cheek teeth (for grinding) that are set back behind a wide gap, or diastema. The typical dental formula for sciurids is .

Tail

The purposes of squirrels' tails, to benefit the squirrel, include: * To keep rain, wind, or cold off itself. * To cool off when hot, by pumping more blood through its tail. * As a counterbalance when jumping about in trees * As a

parachute A parachute is a device designed to slow an object's descent through an atmosphere by creating Drag (physics), drag or aerodynamic Lift (force), lift. It is primarily used to safely support people exiting aircraft at height, but also serves va ...

when jumping. * To signal with. The hairs from squirrel tails are prized in fly fishing when tying fishing flies. Squirrel hair is very fine, making it better for tying fishing flies. When the squirrel sits upright, its tail folded up its back may stop predators looking from behind from seeing the characteristic shape of a small mammal.

Lifetime

Squirrels live in almost every habitat, from tropical

rainforest Rainforests are forests characterized by a closed and continuous tree Canopy (biology), canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforests can be generally classified as tropi ...

to semiarid

desert A desert is a landscape where little precipitation occurs and, consequently, living conditions create unique biomes and ecosystems. The lack of vegetation exposes the unprotected surface of the ground to denudation. About one-third of the la ...

, 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 Insects (from Latin ') are hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed ...

and even small vertebrates.Squirrel Place
. squirrels.org. Retrieved 14 December 2010. Many juvenile 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. Premature death may occur when a nest falls from the tree, in which case the mother may abandon her young if their body temperature is not correct. Many such baby squirrels have been rescued and fostered by a professional wildlife rehabilitator until they could be safely returned to the wild, although the density of squirrel populations in many places and the constant care required by premature squirrels means that few rehabilitators are willing to spend their time doing this and such animals are routinely euthanized instead. File:Squirrel enjoying in the sun.jpg, Squirrel in sunlight File:Squirrel in Chandigarh.jpg, Squirrel in

Chandigarh Chandigarh is a city and union territory in northern India, serving as the shared capital of the states of Punjab and Haryana. Situated near the foothills of the Shivalik range of Himalayas, it borders Haryana to the east and Punjab in the ...

File:Squirrel near Chandigarh.jpg, Squirrel near Chandigarh File:Squirrel on mango tree.jpg, Squirrel on mango tree

Behavior

Squirrels mate either once or twice a year and, following a

gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregn ...

period of three to six weeks, give birth to a number of offspring that varies by species. The young are

altricial Precocial species in birds and mammals are those in which the young are relatively mature and mobile from the moment of birth or hatching. They are normally nidifugous, meaning that they leave the nest shortly after birth or hatching. Altricial ...

, being born naked, toothless, and blind. In most species of squirrel, the female alone looks after the young, which are weaned at six to ten weeks and become sexually mature by the end of their first year. In general, the ground-dwelling squirrel species are social, often living in well-developed colonies, while the tree-dwelling species are more solitary. Ground squirrels and tree squirrels are usually either diurnal or

crepuscular In zoology, a crepuscular animal is one that is active primarily during the twilight period, being matutinal (active during dawn), vespertine (biology), vespertine/vespertinal (active during dusk), or both. This is distinguished from diurnalit ...

, while the flying squirrels tend to be

nocturnal Nocturnality is a ethology, behavior in some non-human animals characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnality, diurnal meaning the opposite. Nocturnal creatur ...

—except for lactating flying squirrels and their young, which have a period of diurnality during the summer. During hot periods, squirrels have been documented to sploot, or lay their stomachs down on cool surfaces. Squirrels, like other rodents, employ species-specific strategies to store food, buffering against periods of scarcity. In temperate regions, squirrels commonly cache nuts beneath leaf litter, inside hollow trees, or underground. However, in subtropical and humid environments, traditional caching can lead to mold growth, decomposition, or premature germination. To counteract these challenges, some squirrels, particularly in subtropical zones, hang nuts or mushrooms on tree branches. This behavior, believed to minimize fungal infections and reduce the risk of food loss, also inadvertently aids certain trees, like Cyclobalanopsis, in expanding their range, with forgotten or dislodged nuts sprouting in new locations, influencing forest ecology. Two species of flying squirrel, the particolored flying squirrel and Hainan flying squirrel aid such cacheing by carving grooves into the nuts to fix the nuts tightly between small intersecting twigs, akin to the mortise-tenon joint in carpentry.

Feeding

Because squirrels cannot digest

cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

, they must rely on foods rich in

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

carbohydrates A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ma ...

, and

fat In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food. The term often refers specif ...

s. In

temperate In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ran ...

regions, early spring is the hardest time of year for squirrels because the nuts they buried are beginning to sprout (and thus are no longer available to eat), while many of the usual food sources are not yet available. During these times, squirrels rely heavily on tree buds. Squirrels, being primarily herbivores, eat a wide variety of plants, as well as nuts,

seed In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be Sowing, sown, which may include seed and husk or tuber. Seeds ...

conifer cone A conifer cone, or in formal botanical usage a strobilus, : strobili, is a seed-bearing organ on gymnosperm plants, especially in conifers and cycads. They are usually woody and variously conic, cylindrical, ovoid, to globular, and have sc ...

fruit In botany, a fruit is the seed-bearing structure in flowering plants (angiosperms) that is formed from the ovary after flowering. Fruits are the means by which angiosperms disseminate their seeds. Edible fruits in particular have long propaga ...

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

, and green

vegetation Vegetation is an assemblage of plants and the ground cover they provide. It is a general term, without specific reference to particular Taxon, taxa, life forms, structure, Spatial ecology, spatial extent, or any other specific Botany, botanic ...

. Some squirrels, however, also consume meat, especially when faced with hunger. Squirrels have been known to eat small

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s, young

snake Snakes are elongated limbless reptiles of the suborder Serpentes (). Cladistically squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales much like other members of the group. Many species of snakes have s ...

s, and smaller rodents, as well as

bird egg Bird eggs are laid by the females and range in quantity from one (as in condors) to up to seventeen (the grey partridge). Avian clutch size, Clutch size may vary latitudinally within a species. Some birds lay eggs even when the eggs have not been ...

s and

insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

s. Some

tropical The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

squirrel species have shifted almost entirely to a diet of insects. Squirrels, like pigeons and other fauna, are synanthropes, in that they benefit and thrive from their interaction in human environments. This gradual process of successful interaction is called synurbanization, wherein squirrels lose their inherent fear of humans in an urban environment. When squirrels were almost completely eradicated during the

Industrial Revolution The Industrial Revolution, sometimes divided into the First Industrial Revolution and Second Industrial Revolution, was a transitional period of the global economy toward more widespread, efficient and stable manufacturing processes, succee ...

in New York, they were later re-introduced to "entertain and remind" humans of nature. The squirrel blended into the urban environment so efficiently that when synanthropic behavior stops (i.e. people do not leave trash outside during particularly cold winters), they can become aggressive in their search for food. Aggression and predatory behavior has been observed in various species of ground squirrels, in particular the thirteen-lined ground squirrel. For example, Bernard Bailey, a scientist in the 1920s, observed a thirteen-lined ground squirrel preying upon a young

chicken The chicken (''Gallus gallus domesticus'') is a domesticated subspecies of the red junglefowl (''Gallus gallus''), originally native to Southeast Asia. It was first domesticated around 8,000 years ago and is now one of the most common and w ...

. Wistrand reported seeing this same species eating a freshly killed

. There has also been at least one report of squirrels preying on atypical animals, such as an incident in 2005 where a pack of black squirrels killed and ate a large stray dog in Lazo, Russia. Squirrel attacks on humans are exceedingly rare, but do occur. 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 Shrews ( family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to dif ...

in one; Bradley, examining the stomachs of white-tailed antelope squirrels, found at least 10% of his 609 specimens' stomachs contained some type of vertebrate, mostly

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

s and rodents. Morgart observed a white-tailed antelope squirrel capturing and eating a silky pocket mouse.

Taxonomy

The living squirrels are divided into five subfamilies, with about 58

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

and some 285

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

. The oldest squirrel fossil, ''Hesperopetes'', dates back to the Chadronian (late

, about 40–35

million years ago Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a unit of time equal to (i.e. years), or approximately 31.6 teraseconds. Usage Myr is in common use in fields such as Earth science and cosmology. Myr is also used w ...

) and is similar to modern flying squirrels. A variety of fossil squirrels, from the latest

to the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

, have not been 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.Steppan & Hamm (2006) Apart from these sometimes little-known fossil forms, the

phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...

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 Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...

. The neotropical pygmy squirrel of tropical

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

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 Eurasia ( , ) is a continental area on Earth, comprising all of Europe and Asia. According to some geographers, Physical geography, physiographically, Eurasia is a single supercontinent. The concept of Europe and Asia as distinct continents d ...

, for example, one would expect quite ancient lineages in

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

, but African squirrels seem to be of more recent origin. The main group of squirrels can be split into five subfamilies: the Callosciurinae, 60 species mostly found in

South East Asia Southeast Asia is the geographical United Nations geoscheme for Asia#South-eastern Asia, southeastern region of Asia, consisting of the regions that are situated south of China, east of the Indian subcontinent, and northwest of the Mainland Au ...

; the Ratufinae, 4 cat-sized species found in

south South is one of the cardinal directions or compass points. The direction is the opposite of north and is perpendicular to both west and east. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Proto-Germanic ''*sunþa ...

and

southeast Asia Southeast Asia is the geographical United Nations geoscheme for Asia#South-eastern Asia, southeastern region of Asia, consisting of the regions that are situated south of China, east of the Indian subcontinent, and northwest of the Mainland Au ...

; the Sciurinae, which contains the

s (Pteromyini) and the tree squirrels, 83 species found worldwide; Sciurillinae, a single

n species; and Xerinae, which includes three tribes of mostly terrestrial squirrels, including the Marmotini ( marmots, chipmunks, prairie dogs, and other

Holarctic The Holarctic realm is a biogeographic realm that comprises the majority of habitats found throughout the continents in the Northern Hemisphere. It corresponds to the floristic Boreal Kingdom. It includes both the Nearctic zoogeographical reg ...

ground squirrels), Xerini (African and some Eurasian ground squirrels), and Protoxerini (African tree squirrels).

Taxonomy list

* Basal and ''

incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

'' Sciuridae (all

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

) ** '' Hesperopetes'' ** '' Kherem'' ** '' Lagrivea'' ** '' Oligosciurus'' ** '' Plesiosciurus'' ** '' Prospermophilus'' ** '' Sciurion'' ** '' Similisciurus'' ** '' Sinotamias'' ** '' Vulcanisciurus'' * Subfamily Cedromurinae (fossil) * Subfamily Ratufinae – Oriental giant squirrels (1 genus, 4 species) * Subfamily Sciurillinae – neotropical pygmy squirrel (

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...

) * Subfamily Sciurinae ** Tribe Sciurini – tree squirrels (5 genera, about 38 species) ** Tribe Pteromyini – true flying squirrels (15 genera, about 45 species) * Subfamily Callosciurinae – Asian ornate squirrels ** Tribe Callosciurini (13 genera, nearly 60 species) ** Tribe Funambulini palm squirrels (1 genus, 5 species) * Subfamily Xerinae – terrestrial squirrels ** Tribe Xerini – spiny squirrels (3 genera, 6 species) ** Tribe Protoxerini (6 genera, about 50 species) ** Tribe Marmotini – ground squirrels, marmots, chipmunks, prairie dogs, etc. (6 genera, about 90 species)

Relationship with humans

References

Sources

External links

*
Tree of Life: Sciuridae

How to identify squirrel tracks in the wild

{{Authority control Sciuromorpha Articles containing video clips Extant Eocene first appearances Rodents by common name Taxa named by Gotthelf Fischer von Waldheim