Squirrel

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

Squirrels
Temporal range: Late Eocene—Recent
Various members of the family Sciuridae
Callosciurus prevostii Tamias sibiricus Tamiasciurus hudsonicus
Sciurus niger Spermophilus columbianus
Xerus inauris Cynomys ludovicianus
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Class: Mammalia
Order: Rodentia
Suborder: Sciuromorpha
Family: Sciuridae
Fischer de Waldheim, 1817
Subfamilies and tribes

and see text

Squirrels

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

That 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, 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] 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.[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]

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.

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.[10]

Behavior

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

Squirrel eating a fruit in Manyara National Park, Tanzania
Squirrel eating a peanut.
Squirrel from the jungle of Alwar, Rajasthan
Squirrel eating grains
A domestic squirrel.

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

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]

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

Notes

  1. ↑ Seebeck, J. H. "Sciuridae" (PDF). 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. 1 2 3 Milton (1984)
  7. ↑ 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.
  8. 1 2 Squirrel Place. squirrels.org. Retrieved 14 December 2010.
  9. ↑ "Squirrel" - HowStuffWorks
  10. ↑ 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.
  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. doi:10.1093/jmammal/4.2.129.
  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" (PDF). 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): 693–698. doi:10.1671/0272-4634(2007)27[693:ANGOSR]2.0.CO;2.
  22. 1 2 3 Steppan & Hamm (2006)
  23. ↑ Steppan et al. (2004), Steppan & Hamm (2006)

References

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