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Squirrels Temporal range: Late Eocene—Recent PreЄ Є O S D C P T J K Pg N
Various members of the family Sciuridae Callosciurus prevostii Tamias sibiricus Tamiasciurus hudsonicus Sciurus niger Spermophilus columbianus Xerus inauris Marmota sp. Cynomys ludovicianus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Rodentia
Suborder:	Sciuromorpha
Family:	Sciuridae Fischer de Waldheim, 1817
Subfamilies and tribes
Subfamily Ratufinae Subfamily Sciurillinae Subfamily Sciurinae Tribe Sciurini Tribe Pteromyini Subfamily Callosciurinae Tribe Callosciurini Tribe Funambulini Subfamily Xerinae Tribe Xerini Tribe Protoxerini Tribe Marmotini and see text

Squirrels are members of the family Sciuridae, a family that includes small or medium-size rodents. The squirrel family includes tree squirrels, ground squirrels, chipmunks, marmots (including woodchucks), flying squirrels, and prairie dogs amongst other rodents. Squirrels are indigenous to the Americas, Eurasia, and Africa, and were introduced by humans to Australia.^[1] The earliest known squirrels date from the Eocene period and are most closely related to the mountain beaver and to the dormouse among other living rodent families.

Etymology

The word "squirrel", first attested in 1327, comes from the Anglo-Norman esquirel which is 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.^[2][3]

The native Old English word for the squirrel, ācweorna, survived only into Middle English (as aquerne) before being replaced.^[4] The Old English word is of Common Germanic origin, cognates of which are still used in other Germanic languages, including the German Eichhörnchen (diminutive of Eichhorn, which is not as frequently used), the Norwegian ikorn/ekorn, the Dutch eekhoorn, the Swedish ekorre and the Danish egern.

Characteristics

Reaching out for food on a garden bird feeder, this squirrel can rotate its hind feet, allowing it to descend a tree head-first.

Skull of an Oriental giant squirrel (genus Ratufa)—note the classic sciuromorphous shape of the anterior zygomatic region.

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 Laotian giant flying squirrel at 1.08 m (3 ft 7 in) in length and the Alpine marmot, which 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, though much thicker in some species than others. The coat color of squirrels is highly variable between—and often even within—species.^[5]

In most squirrel species, the hind limbs are longer than the fore limbs, while all species have either four or five toes on each paw. The paws, which include an often poorly developed thumb, have soft pads on the undersides^[6] and versatile, sturdy claws for grasping and climbing.^[7]Tree squirrels, unlike most mammals, can descend a tree head-first. They do so by rotating their ankles 180 degrees, enabling the hind paws to point backward and thus grip the tree bark from the opposite direction.^[8]

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.^[9]

As their large eyes indicate, squirrels have an excellent sense of vision, which is especially important for the tree-dwelling species. Many also have a good sense of touch, with vibrissae on their limbs as well as their heads.^[6]

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 1.0.1.31.0.1.3.^[10]

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.^[11] Premature death may be caused 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,^[12] 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.

Behavior

Young squirrels

Squirrels mate either once or twice a year and, following a gestation period of three to six weeks, give birth to a number of offspring that varies by species. The young are 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.^[6]

Ground squirrels and tree squirrels are usually either diurnal or crepuscular,^[13] while the flying squirrels tend to be nocturnal—except for lactating flying squirrels and their young, which have a period of diurnality during the summer.^[14]

Feeding

Squirrel eating a fruit in Manyara National Park, Tanzania

Because squirrels cannot digest cellulose, 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 the nuts they buried are beginning to sprout (and thus are no longer available to eat), while many of the usual food sources have not yet become available. During these times, squirrels rely heavily on the buds of trees. Squirrels, being primarily herbivores, eat a wide variety of plants, as well as nuts, seeds, conifer cones, fruits, fungi, and green vegetation. Some squirrels, however, also consume meat, especially when faced with hunger.^[9][15] Squirrels have been known to eat small birds, young snakes, and smaller rodents, as well as bird eggs and insects. Indeed, some tropical squirrel species have shifted almost entirely to a diet of insects.^[16]

Predatory behavior has been observed in various species of ground squirrels, in particular the thirteen-lined ground squirrel.^[17] For example, Bailey, a scientist in the 1920s, observed a thirteen-lined ground squirrel preying upon a young chicken.^[18] Wistrand reported seeing this same species eating a freshly killed snake.^[19] 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;^[20] Bradley, examining the stomachs of white-tailed antelope squirrels, found at least 10% of his 609 specimens' stomachs contained some type of vertebrate, mostly lizards and rodents.^[21] Morgart observed a white-tailed antelope squirrel capturing and eating a silky pocket mouse.^[22]

Taxonomy

The living squirrels are divided into five subfamilies, with about 58 genera and some 285 species.^[23] 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.^[24]

A variety of fossil squirrels, from the latest Eocene to the Miocene, have 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.^[25]

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.^[25]

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.^[26]

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.^[25]

• 
  Basal and incertae sedis Sciuridae (all fossil)
  
  • 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)


• 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)
  
  

See also

• American red squirrel


• Animal track


• Fox squirrel


• Blue squirrel


• Red squirrel


• Squirrel relationship with humans


• Western gray squirrel

References

Further reading

• 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
  

External links

Wikimedia Commons has media related to Sciuridae.

Wikispecies has information related to Sciuridae

Look up squirrel in Wiktionary, the free dictionary.

• Tree of Life: Sciuridae


• 
  Squirrel Tracks: How to identify squirrel tracks in the wild


• National Geographic link on Squirrels


• List of names of squirrel taxa