Overview

Brief Summary

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.

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Ecology

Associations

Animal / dung saprobe
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

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Known predators

Sciuridae is prey of:
fox
Strigiformes
Mustelinae
Stercorarius

Based on studies in:
USA: Alaska (Tundra)

This list may not be complete but is based on published studies.
  • J. Brown, Ecological investigations of the Tundra biome in the Prudhoe Bay Region, Alaska, Special Report, no. 2, Biol. Pap. Univ. Alaska (1975), from p. xiv.
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Known prey organisms

Sciuridae preys on:
willows
sedges
grasses
Colaptes auratus
Reithrodontomys megalotis
Ictinia mississippiensis

Based on studies in:
USA: Alaska (Tundra)

This list may not be complete but is based on published studies.
  • J. Brown, Ecological investigations of the Tundra biome in the Prudhoe Bay Region, Alaska, Special Report, no. 2, Biol. Pap. Univ. Alaska (1975), from p. xiv.
  • Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2006. The Animal Diversity Web (online). Accessed February 16, 2011 at http://animaldiversity.org. http://www.animaldiversity.org
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Evolution and Systematics

Functional Adaptations

Functional adaptation

Feet good for climbing: squirrel
 

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.
  • Foy, Sally; Oxford Scientific Films. 1982. The Grand Design: Form and Colour in Animals. Lingfield, Surrey, U.K.: BLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, London. 238 p.
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Functional adaptation

Liver survives cold temperatures: squirrel
 

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.
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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:845Public Records:200
Specimens with Sequences:502Public Species:36
Specimens with Barcodes:492Public BINs:47
Species:89         
Species With Barcodes:72         
          
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Barcode data

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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Sciuridae

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Wikipedia

Squirrel

This article is about the squirrel family (Sciuridae) as a whole. For other uses, see Squirrel (disambiguation).

Squirrels belong to family Sciuridae 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.[1] 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.

Etymology[edit]

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.[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, and cognates used by other countries to name the squirrel include the German Eichhhörnchen (diminutive of Eichhorn, which is not used), the Norwegian ikorn/ekorn, the Dutch eekhoorn, the Swedish ekorre and the Danish egern.

Characteristics[edit]

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

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.[5]

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.[6] The eastern gray squirrel is one of very few mammalian species that can descend a tree head-first. It does this by turning its feet so the claws of its hind paws are backward-pointing and can grip the tree bark.[7]

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.[8]

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.[9] Many also have a good sense of touch, with vibrissae on their heads and limbs.[6]

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

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 1.0.1.31.0.1.3[citation needed]

The lifespan of the gray squirrel is approximately six years. Most urban squirrels do not reach their first birthday. This is due not to predators but rather to automobiles. Compare this to its rural counterpart, which often perishes from lack of food.[10][dubious ]

Behavior[edit]

Several species of squirrels have melanistic phases. In large parts of United States and Canada, the most common variety seen in urban areas is the melanistic form of the eastern gray squirrel.

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.[6]

Ground and tree squirrels are typically diurnal or crepuscular,[11] while flying squirrels tend to be nocturnal—except for lactating flying squirrels and their offspring, which have a period of diurnality during the summer.[12]

Feeding[edit]

Squirrel eating a fruit in Manyara National Park, Tanzania
Squirrel eating a peanut.
The Indian palm squirrel is the most common type of squirrel found in India.
The Squirrel from the Jungle of Alwar, Rajasthan

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.[8] 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.[13]

Predatory behavior has been noted by various species of ground squirrels, in particular the thirteen-lined ground squirrel.[14] For example, Bailey, a scientist in the 1920s, observed a thirteen-lined ground squirrel preying upon a young chicken.[15] Wistrand reported seeing this same species eating a freshly killed snake.[16] 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;[17] 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.[18] Morgart observed a white-tailed antelope squirrel capturing and eating a silky pocket mouse.[19]

Taxonomy[edit]

A domestic squirrel.

The living squirrels are divided into five subfamilies, with about 58 genera and some 285 species.[20] 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.[21]

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.[22]

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.[22]

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.[23]

Squirrel eating grains

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.[22]

See also[edit]

Notes[edit]

  1. ^ Seebeck, J. H. "Sciuridae". Fauna of Australia. Retrieved 2013-11-24. 
  2. ^ "squirrel, n.". The Oxford English Dictionary (2nd. ed.). Oxford University Press. 1989. Retrieved 8 November 2010. 
  3. ^ Whitaker & Elman (1980): 370
  4. ^ "Squirrel". Online Etymology Dictionary. Retrieved 7 February 2008. 
  5. ^ Tree Squirrels, Wildlife Online, 23 November 2010.
  6. ^ a b c Milton (1984)
  7. ^ Jenkins, Farish (1974). Primate Locomotion. New York: Academic Press. p. 61. ISBN 0123840503. 
  8. ^ a b Squirrel Place - squirrels.org - Retrieved 14 December 2010.
  9. ^ "Squirrel" - HowStuffWorks
  10. ^ Squirrel History, squirrels.org
  11. ^ "Red & Gray Squirrels in Massachusetts". MassWildlife. Massachusetts Division of Fisheries and Wildlife. Retrieved 3 April 2012. 
  12. ^ 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. 
  13. ^ 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.
  14. ^ 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. 
  15. ^ Bailey, B. (1923). "Meat-eating propensities of some rodents of Minnesota". Journal of Mammalogy 4: 129. 
  16. ^ Wistrand, E.H. (1972). "Predation on a Snake by Spermophilus tridecemlineatus". American Midland Naturalist 88 (2): 511–512. doi:10.2307/2424389. JSTOR 2424389. 
  17. ^ 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. 
  18. ^ 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. 
  19. ^ 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. 
  20. ^ 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. 
  21. ^ 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. 
  22. ^ a b c Steppan & Hamm (2006)
  23. ^ Steppan et al. (2004), Steppan & Hamm (2006)

References[edit]

  • 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 ProjectSciuridae (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
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