The wordsquirrel, first attested in 1327, comes from theAnglo-Normanesquirel which is from theOld Frenchescurel, the reflex of aLatin wordsciurus, which was taken from theAncient Greek wordσκίουρος (skiouros; fromσκία-ουρος) 'shadow-tailed', referring to the long bushy tail which many of its members have.[3][4]Sciurus is also the name of one of its genuses.[4]
Agroup of squirrels is called a "dray"[6] or a "scurry".[7]
Characteristics
Reaching out for food on a garden bird feeder, this squirrel can rotate its hind feet, allowing it to descend a tree headfirst.Skull of anOriental giant squirrel (genusRatufa)—note the classicsciuromorphous shape of the anteriorzygomatic region.
Squirrels are generally small animals, ranging in size from theAfrican pygmy squirrel andleast pygmy squirrel at 10–14 cm (3.9–5.5 in) in total length and just 12–26 g (0.42–0.92 oz) in weight,[8][9] to theBhutan giant flying squirrel at up to 1.27 m (4 ft 2 in) in total length,[10] and severalmarmot species, which can weigh 8 kg (18 lb) or more.[11][12] Squirrels typically have slender bodies with very long very bushy tails and large eyes. In general, theirfur 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.[13]
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 developedthumb, have soft pads on the undersides[14] and versatile, sturdyclaws for grasping andclimbing.[15]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.[16]
Head
As their large eyes indicate, squirrels have excellentvision, which is especially important for the tree-dwelling species. Many also have a good sense oftouch, withvibrissae on their limbs as well as their heads.[14]
The teeth of sciurids follow the typical rodent pattern, with largeincisors (for gnawing) that grow throughout life, and cheek teeth (for grinding) that are set back behind a wide gap, ordiastema. The typicaldental formula for sciurids is1.0.1.31.0.1.3.[17]
Tail
The purposes of squirrels' tails, to benefit the squirrel, include:[18]
To keep rain, wind, or cold off itself.
To cool off when hot, by pumping more blood through its tail.
The hairs from squirrel tails are prized infly fishing when tyingfishing flies.[19] Squirrel hair is very fine, making it better for tying fishing flies.[20]
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 tropicalrainforest to semiariddesert, avoiding only the highpolar regions and the driest of deserts. They are predominantlyherbivorous, subsisting on seeds and nuts, but many will eatinsects and even small vertebrates.[21]
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.[22] 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 professionalwildlife rehabilitator until they could be safely returned to the wild,[23] 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 routinelyeuthanized instead.
Squirrels mate either once or twice a year and, following agestation period of three to six weeks, give birth to a number of offspring that varies by species. The young arealtricial, being born naked, toothless, and blind. In most species of squirrel, the female alone looks after the young, which areweaned 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.[14]
Ground squirrels and tree squirrels are usually eitherdiurnal orcrepuscular,[24] while the flying squirrels tend to benocturnal—except for lactating flying squirrels and their young, which have a period of diurnality during the summer.[25]
During hot periods, squirrels have been documented tosploot, or lay their stomachs down on cool surfaces.[26]
Squirrels, like other rodents, employ species-specific strategies to store food, buffering against periods of scarcity.[27] In temperate regions, squirrels commonly cache nuts beneath leaf litter, inside hollow trees, or underground.[28] However, in subtropical and humid environments, traditional caching can lead to mold growth, decomposition, or premature germination.[29] To counteract these challenges, some squirrels, particularly in subtropical zones, hang nuts or mushrooms on tree branches.[29] This behavior, believed to minimize fungal infections and reduce the risk of food loss, also inadvertently aids certain trees, likeCyclobalanopsis, in expanding their range, with forgotten or dislodged nuts sprouting in new locations, influencing forest ecology.[30] Two species of flying squirrel, theparticolored flying squirrel andHainan 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.[30]
Because squirrels cannot digestcellulose, they must rely on foods rich inprotein,carbohydrates, andfats. Intemperate regions, early spring is the hardest time of year for squirrels because the nuts theyburied 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 primarilyherbivores, eat a wide variety of plants, as well asnuts,seeds,conifer cones,fruits,fungi, and greenvegetation. Some squirrels, however, also consume meat, especially when faced with hunger.[21][32] Squirrels have been known to eat smallbirds, youngsnakes, and smaller rodents, as well asbird eggs andinsects. Sometropical squirrel species have shifted almost entirely to a diet of insects.[33]
Squirrels, likepigeons and other fauna, aresynanthropes, 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 anurban environment.[34] When squirrels were almost completely eradicated during theIndustrial Revolution inNew 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 thethirteen-lined ground squirrel.[35] For example, Bernard Bailey, a scientist in the 1920s, observed a thirteen-lined ground squirrel preying upon a youngchicken.[36] Wistrand reported seeing this same species eating a freshly killedsnake.[37] 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 straydog inLazo, Russia.[38] Squirrel attacks on humans are exceedingly rare, but do occur.[39][40]
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-tailedshrew in one;[41] Bradley, examining the stomachs ofwhite-tailed antelope squirrels, found at least 10% of his 609 specimens' stomachs contained some type of vertebrate, mostlylizards and rodents.[42] Morgart observed a white-tailed antelope squirrel capturing and eating asilky pocket mouse.[43]
A variety of fossil squirrels, from the latestEocene to theMiocene, have not been assigned with certainty to any living lineage. At least some of these probably were variants of the oldestbasal "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.[46]
Apart from these sometimes little-known fossil forms, thephylogeny of the living squirrels is fairly straightforward. The three main lineages are theRatufinae (Oriental giant squirrels), Sciurillinae and all other subfamilies. The Ratufinae contain a mere handful of living species in tropicalAsia. Theneotropical pygmy squirrel of tropicalSouth 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 haveradiated from there; if squirrels had originated inEurasia, for example, one would expect quite ancient lineages inAfrica, but African squirrels seem to be of more recent origin.[46]
^Payne, J.; C.F. Francis (1998).A Field Guide to the Mammals of Borneo (3 ed.). The Sabah Society. p. 243.ISBN967-99947-1-6.
^Choudhury, A. (2002). "Petaurista nobilis singhei: First record in India and a note on its taxonomy".The Journal of the Bombay Natural History Society.99 (1):30–34.
^Kryštufek, B.; B. Vohralík (2013). "Taxonomic revision of the Palaearctic rodents (Rodentia). Part 2. Sciuridae: Urocitellus, Marmota and Sciurotamias".Lynx, N. S. (Praha).44:27–138.
^Armitage, K.B.; Blumstein, D.T. (2002). "Body-mass diversity in marmots. Holarctic marmots as a factor of biodiversity". In K.B. Armitage; V.Yu. Rumiantsev (eds.).Holarctic Marmots as a Factor of Biodiversity. ABF Publishing House. pp. 22–32.
^"Rodents". How Stuff Works. 22 April 2008. Retrieved30 December 2016.
^Thorington, Richard W.; Koprowski, John L.; Steele, Michael A.; Whatton, James F. (2012).Squirrels of the World. Johns Hopkins University Press. p. 8.ISBN978-1-4214-0469-1.
^Thorington, Richard W.; Koprowski, John L.; Steele, Michael A.; Whatton, James F. (2012).Squirrels of the World. Johns Hopkins University Press. p. 12.ISBN978-1-4214-0469-1.
^Peiman, Kathryn (June 2016). "Sublethal consequences of urban life for wild vertebrates".Environmental Reviews.24 (4):416–425.doi:10.1139/er-2016-0029.hdl:1807/74036.
^Whitaker, J.O. (1972). "Food and external parasites ofSpermophilus tridecemlineatus in Vigo County, Indiana".Journal of Mammalogy.53 (3):644–648.doi:10.2307/1379067.JSTOR1379067.
^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.JSTOR1377723.
Whitaker, John O. Jr.; Elman, Robert (1980).The Audubon Society Field Guide to North American Mammals (2nd ed.). New York: Alfred A. Knopf, Jr.ISBN0-394-50762-2.
Further reading
Milton, Katherine (1984): "Family Sciuridae". In: Macdonald, D. (ed.):The Encyclopedia of Mammals: 612–623. Facts on File, New York.ISBN0-87196-871-1.
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.Bibcode:2004MolPE..30..703S.doi:10.1016/S1055-7903(03)00204-5.PMID15012949.
Thorington, R.W. and Hoffmann, R.S. (2005): "Family Sciuridae". In:Mammal Species of the World: A Taxonomic and Geographic Reference: 754–818. Johns Hopkins University Press, Baltimore.
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