ground squirrel

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Sciurids Fossil range: Late Eocene - Recent
Eastern Gray Squirrel, Sciurus carolinensis - a tree squirrel of tribe Sciurini
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Eutheria
Superorder:	Euarchontoglires
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

The sciurids or squirrels are a large family of rodents. The word Sciuridae means "shade-tail," and refers to the bushy appendage possessed by many of its members^[1]. It includes tree squirrels, ground squirrels, chipmunks, the marmots (which include woodchucks), and the true flying squirrels. The African scaly-tailed flying squirrels, which belong to the family Anomaluridae, are not sciurids. Sciurids are found in all continents except Australia and Antarctica.

Characteristics

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

Sciurids 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). Sciurids typically have slender bodies with bushy tails and large eyes. Their fur is generally soft and silky, although much thicker in some species than others. The colour of sciurids is highly variable between - and often even within - species.

The hindlimbs of sciurids are generally longer than the forelimbs, and they have four or five toes on each foot. Their paws on their forefeet include a thumb, although this is often poorly developed. The feet of sciurids also have a soft pad on the underside.^[2]

Sciurids 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. Indeed, some tropical species have shifted almost entirely to a diet of insects. 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:

As their large eyes indicate, sciurids generally have an excellent sense of vision, which is especially important for tree-dwelling species. Many also have a good sense of touch, with vibrissae on their heads and limbs.^[2]

Sciurids 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, blind, and helpless. In almost all species, 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. Ground dwelling species are generally social animals, often living in well-developed colonies, but the tree-dwelling species are more solitary-^[2]

Evolution and systematics

Grizzled Giant Squirrel (Ratufa macroura) of the Ratufinae

Southern Flying Squirrel (Glaucomys volans) of the Pteromyini

Prevost's Squirrel (Callosciurus prevosti) of the Callosciurini

Unstriped Ground Squirrel (Xerus rutilus) of the Xerini

Alpine Marmot (Marmota marmota) of the Marmotini

The living squirrels are divided into 5 subfamilies, with about 50 genera and nearly 280 species. Though the oldest squirrel fossil, Douglassciurus, dates back to the Priabonian (Late Eocene, around 37.5 - 35 million years ago), this animal was apparently a nearly-modern tree squirrel, albeit with a primitive skull, and it is in fact usually placed in the Sciurinae. Thus, the squirrel lineage may well have originated quite some time earlier.^[3]

During the latest Eocene to the Miocene, there were a variety of squirrels which cannot be assigned with certainty to any living lineage. At least some of these probably were variants of the oldest, basal "proto-squirrels" (in the sense that they lacked the full range of living squirrels' autapomorphies). The distribution and diversity of such ancient and ancestral forms suggests that the squirrels as a group might have originated in North America.^[3]

Apart from these sometimes little-known fossil forms, the phylogeny of the living squirrels is fairly straightforward. There are three main lineages, one comprising the Ratufinae (Oriental giant squirrels). These contain a mere handful of living species in tropical Asia but were more widespread across Eurasia in prehistoric times. The Neotropical Pygmy Squirrel of tropical South America is the sole living member of the Sciurillinae. The third lineage is by far the largest and contains all other subfamilies; it 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.^[3]

The main group of squirrels also can be split up in three, which yields the remaining subfamilies. The Sciurinae are the only squirrel group for which there is significant uncertainty as regards taxonomy. Namely, it is not altogether clear how distant the true flying squirrels (Pteromyini) are from the tree squirrels (Sciurini); the former have often been considered a separate subfamily but are now seen as a tribe of the Sciurini. The pine squirrels 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.^[4]

Be that as it may, the three-way split of the main squirrel lineage is rather neat from a biogeographical and ecological perspective. Two of the three subfamilies are of about equal size, containing between nearly 70 to some 80 species each; the third is about twice as large. The Sciurinae contains arboricolous (tree-living) squirrels, mainly of the Americas and to a lesser extent Eurasia. The Callosciurinae on the other hand is most diverse in tropical Asia and contains squirrels which are also arboricolous, 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; they tend to be more gregarious than other squirrels which do not usually live together in close-knit groups.^[3]

• Basal and incertae sedis Sciuridae (all fossil)
  • Getuloxerus
  • Kherem
  • Oligosciurus
  • Plesiosciurus
  • Prospermophilus
  • Sciurion
  • Similisciurus
  • Sinotamias
  • Vulcanisciurus
• Subfamily Cedromurinae (fossil)
• Subfamily Ratufinae - Oriental giant squirels (1 genus, 4 species)
• Subfamily Sciurillinae - Neotropical Pygmy Squirrel (monotypic)
• Subfamily Sciurinae
  • Tribe Sciurini - tree squirrels (5 genera, c.38 species; includes Tamiasciurini which may be a distinct tribe)
  • Tribe Pteromyini - true flying squirrels (15 genera, c.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, c.50 species)
  • Tribe Marmotini - ground squirrels, marmots, chipmunks, prairie dogs, etc (6 genera, c.90 species)

See also

• Squirrel

Footnotes

References

• 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 2006-MAY-13. Retrieved 2007-DEC-10.
• Steppan, Scott 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" (pdf). Mol. Phyl. Evol. 30(3): 703-719. doi:10.1016/S1055-7903(03)00204-5
• 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

• Tree of Life: Sciuridae
• Andrew's Squirrel Encyclopaedia
• List of names of squirrel taxa