The
sciurids or
squirrelA squirrel is one of many small or medium-sized rodents in the family Sciuridae. In the English-speaking world, squirrel commonly refers to members of this family's genera Sciurus and Tamiasciurus, which are tree squirrels with large bushy tails, indigenous to Asia, the Americas and Europe....
s are a large
familyIn biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus...
of rodents. The word Sciuridae means "shade-tail," and refers to the bushy appendage possessed by many of its members. It includes
tree squirrelAlthough the term tree squirrel can refer to any arboreal member of the family Sciuridae, it is generally in reference to the common and widely distributed members of the genus Sciurus and close kin, the tribe Sciurini. These genera contain most of the common, bushy-tailed squirrels in...
s,
ground squirrelThe ground squirrels are the members of the Sciuridae most closely related to the genus Marmota. They make up the tribe Marmotini in the large and mainly terrestrial squirrel subfamily Xerinae, and containing six living genera. Well-known members of this largely Holarctic group are the marmots ,...
s,
chipmunkChipmunks are small squirrel-like rodents of the genus Tamias. They are native to North America and Asia.- Etymology and taxonomy :Tamias is Greek for "storer," a reference to the animals' habit of collecting and storing food for winter use...
s, the
marmotMarmots are members of the genus Marmota, in the rodent family Sciuridae .Marmots are generally large ground squirrels...
s (which include woodchucks), and the true
flying squirrelFlying squirrels, scientifically known as Pteromyini or Petauristini, are a tribe of 44 species of squirrels .-Description:...
s. The African scaly-tailed flying squirrels, which belong to the family Anomaluridae, are not sciurids. Sciurids are found in all continents except
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
and
Antarctica| style="border-top:solid 1px #ccd2d9; padding:0.4em 1em 0.4em 0; vertical-align:top;" | 14,000,000 km
2 280,000 km
2 13,720,000 km
2 |-! style="border-top: solid 1px #ccd2d9; padding: 0.4em 1em 0.4em 0; vertical-align: top;...
.
Characteristics
Sciurids are generally small animals, ranging in size from the
African Pygmy SquirrelThe African Pygmy Squirrel is a species of rodent in the Sciuridae family. It is monotypic within the genus Myosciurus. It is found in Cameroon, Republic of the Congo, Equatorial Guinea, and Gabon. Its natural habitat is subtropical or tropical moist lowland forests. It is threatened by habitat...
, at in length, and just in weight, to the
Alpine MarmotThe Alpine Marmot is a species of marmot found in mountainous areas of central and southern Europe. Alpine Marmots live at heights between 800 and 3,200 metres, in the Alps, Carpathians, Tatras, the Pyrenees and Northern Apennines Italy...
, which is long, and weighs from . Sciurids typically have slender bodies with bushy tails and large eyes. Their
furFur is a synonym for hair, used more commonly in reference to non-human animals, usually mammals; particularly those with extensive body hair coverage. The term is sometimes used to refer to the body hair of an animal as a complete coat, also known as the "pelage". Fur is also used to refer to...
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
thumbThe thumb when a person is standing in the medical anatomical position is the lateral-most digit of the hand. The Medical Latin English adjective for thumb is pollical.-Bones:The thumb consists of three bones:* Distal phalanx...
, although this is often poorly developed. The feet of sciurids also have a soft pad on the underside.
Sciurids live in almost every habitat from tropical
rainforestRainforests are forests characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1750–2000 mm . The monsoon trough, alternately known as the intertropical convergence zone, plays a significant role in creating Earth's tropical rain forests.From 40 to 75%...
to semiarid
desertA desert is a landscape or region that receives almost no precipitation. Deserts are defined as areas with an average annual precipitation of less than per year, or as areas where more water is lost by evapotranspiration than falls as precipitation. In the Köppen climate classification system,...
, avoiding only the high polar regions and the driest of deserts. They are predominantly
herbivorousA herbivore is an animal that is adapted to eat plants and not meat.Herbivory is a form of predation in which an organism consumes principally autotrophs such as plants, algae and photosynthesizing bacteria....
, 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
incisorIncisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s that grow throughout life, and grinding cheek teeth set back behind a wide gap, or
diastemaDiastema is a gap or space between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars.-In humans:...
. The typical
dental formulaDentition is the development of teeth and their arrangement in the mouth.All mammals except the monotremes, the xenarthrans, the pangolins, and the cetaceans have up to four distinct types of teeth, with a maximum number for each. These are the incisor , the canine, the premolar, and the molar ...
for sciurids is:
As their large eyes indicate, sciurids generally have an excellent sense of
visionVisual perception is the ability to interpret information and surroundings from visible light reaching the eye. The resulting perception is also known as eyesight, sight or vision...
, which is especially important for tree-dwelling species. Many also have a good sense of
touchThe somatosensory system is a diverse sensory system comprising the receptors and processing centres to produce the sensory modalities such as touch, temperature, proprioception , and nociception . The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal...
, with vibrissae on their heads and limbs.
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
weanedWeaning is the process of gradually introducing a mammal infant, either human or animal, to what will be its adult diet and withdrawing the supply of its mother's milk.The process takes place only in mammals, as only mammals produce milk...
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-
Evolution and systematics
The living squirrels are divided into 5 subfamilies, with about 50
generaGenera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...
and nearly 280
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
. Though the oldest squirrel fossil,
Douglassciurus, dates back to the
PriabonianThe Priabonian is, in the ICS's geologic timescale, the latest age or the upper stage of the Eocene epoch or series. It spans the time between 37.2 ± 0.1 Ma and 33.9 ± 0.1 Ma...
(Late
EoceneThe Eocene epoch, lasting from 55.8 ± 0.2 to 33.9 ± 0.1 Ma , is a major division of the geologic timescale and the second epoch of the Palaeogene period in the Cenozoic era. The Eocene spans the time from the end of the Paleocene epoch to the beginning of the Oligocene epoch. The start of the...
, 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
SciurinaeThe Sciurinae is a subfamily of squirrels in the . It unites the flying squirrels with certain related tree squirrels. Older sources place the flying squirrels in a separate subfamily and unite all remaining sciurids into the subfamily Sciurinae, but this has been strongly refuted by genetic...
. Thus, the squirrel lineage may well have originated quite some time earlier.
During the latest Eocene to the
MioceneThe Miocene is a geological epoch of the Neogene period and extends from about 23.03 to 5.33 million years before the present . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the...
, 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'
autapomorphiesIn cladistics, an autapomorphy is a derived trait that is unique to a given terminal group. That is, it is found only in one member of a clade, but not found in any others or outgroup taxa, not even those most closely related to the group .An autapomorphy is not present in the closest relative of...
). The distribution and diversity of such ancient and ancestral forms suggests that the squirrels as a group might have originated in North America.
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
AsiaAsia is the world's largest and most populous continent, located in the eastern and northern hemispheres. It covers 8.6% of the earth's total surface area and with approximately 4 billion people, it hosts 60% of the world's current human population.Asia is traditionally defined as part of the...
but were more widespread across
EurasiaEurasia is a large landmass covering about 52,990,000 km
2 or about 10.6% of the Earth's surface...
in prehistoric times. The
Neotropical Pygmy SquirrelThe Neotropical Pygmy Squirrel, Sciurillus pusillus, is a very small squirrel species from South America. It is the only living member of the genus Sciurillus and the subfamily Sciurillinae. It is found in the lowland rainforests of Brazil, Colombia, Peru, French Guiana, Guyana and...
of tropical
South AmericaSouth America is the southern continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere...
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
radiatedAn evolutionary radiation is an increase in taxonomic diversity or morphological disparity, due to adaptive change or the opening of ecospace. Radiations may affect one clade or many, and be rapid or gradual; where they are rapid, and driven by a single lineage's adaptation to their environment,...
from there - if squirrels had originated in Eurasia for example, one would expect quite ancient lineages in
AfricaAfrica is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people in 61 territories, it accounts for about 14.8% of the...
, but African squirrels seem to be of more recent origin.
The main group of squirrels also can be split up in three, which yields the remaining subfamilies. The
SciurinaeThe Sciurinae is a subfamily of squirrels in the . It unites the flying squirrels with certain related tree squirrels. Older sources place the flying squirrels in a separate subfamily and unite all remaining sciurids into the subfamily Sciurinae, but this has been strongly refuted by genetic...
are the only squirrel group for which there is significant uncertainty as regards
taxonomyTaxonomy is the practice and science of classification. The word finds its roots in the Greek , taxis and , nomos...
. 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
tribeIn biology, a tribe — or infrafamily — is a taxonomic rank between family and genus. It is sometimes subdivided into subtribes.-See also:* Rank * Rank *International Code of Botanical Nomenclature...
of the Sciurini. The
pine squirrelPine squirrels are squirrels of the genus Tamiasciurus. Currently only two species are classified in this genus, the American Red Squirrel T. hudsonicus and the Douglas Squirrel T. douglasii...
s 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.
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
CallosciurinaeCallosciurinae is an Asiatic subfamily of squirrels. It contains about 60 species, most of them in South East Asia. It is named after the Callosciurus genus, which means 'Beautiful Squirrels'.-Classification:*Family Sciuridae...
on the other hand is most diverse in tropical Asia and contains squirrels which are also arboricolous, but have a markedly different
habitusHabitus is a complex concept, but in its simplest usage could be understood as a structure of the mind characterized by a set of acquired schemas, sensibilities, dispositions and taste . The particular contents of the habitus are the result of the objectification of social structure at the level of...
and appear more "elegant", an effect enhanced by their often very colorful fur. The
XerinaeXerinae is a subfamily of squirrels, many of which are highly terrestrial.-References:* Thorington, R. W. and R. S. Hoffmann. 2005. Family Sciuridae. Pp 754-818 in Wilson, E. D. and Reeder, D. M. , Mammal Species of the World A Taxonomic and Geographic Reference, Vol. 2. Johns Hopkins University...
- the largest subfamily - are made up from the mainly terrestrial (ground-living) forms and include the large
marmotMarmots are members of the genus Marmota, in the rodent family Sciuridae .Marmots are generally large ground squirrels...
s and the popular
prairie dogPrairie dogs are burrowing rodents native to the grasslands of North America. There are five different species of prairie dogs: black-tailed, white-tailed, Gunnison, Utah, and Mexican prairie dogs. They are a type of ground squirrel. On average, these stout-bodied rodents will grow to be between ...
s among others; they tend to be more gregarious than other squirrels which do not usually live together in close-knit groups.
- Basal and incertae sedis
Incertae sedis , abbreviated "inc. sed.", is a term used to define a taxonomic group where its broader relationships are unknown or undefined.-Examples:The taxonomy for humans is usually recognized as follows:...
Sciuridae (all fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
)
- 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
In biology, a monotypic taxon is a taxonomic group with only one type. The usage differs slightly between botany and zoology:In botany, a monotypic taxon is a taxon that has only one species: Ginkgo is a monotypic genus, while Ginkgoaceae is a monotypic family...
)
- Subfamily Sciurinae
The Sciurinae is a subfamily of squirrels in the . It unites the flying squirrels with certain related tree squirrels. Older sources place the flying squirrels in a separate subfamily and unite all remaining sciurids into the subfamily Sciurinae, but this has been strongly refuted by genetic...
- 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
Callosciurinae is an Asiatic subfamily of squirrels. It contains about 60 species, most of them in South East Asia. It is named after the Callosciurus genus, which means 'Beautiful Squirrels'.-Classification:*Family Sciuridae...
- Asian ornate squirrels
- Tribe Callosciurini (13 genera, nearly 60 species)
- Tribe Funambulini palm squirrels (1 genus, 5 species)
- Subfamily Xerinae
Xerinae is a subfamily of squirrels, many of which are highly terrestrial.-References:* Thorington, R. W. and R. S. Hoffmann. 2005. Family Sciuridae. Pp 754-818 in Wilson, E. D. and Reeder, D. M. , Mammal Species of the World A Taxonomic and Geographic Reference, Vol. 2. Johns Hopkins University...
- terrestrial squirrels
- Tribe Xerini - spiny squirrels (3 genera, 6 species)
- Tribe Protoxerini
Protoxerini is a tribe of squirrels found in Africa.-References:*Thorington, R. W. and R. S. Hoffmann. 2005. Family Sciuridae. Pp 754–818 in Mammal Species of the World A Taxonomic and Geographic Reference. Johns Hopkins University Press, Baltimore....
(6 genera, c.50 species)
- Tribe Marmotini - ground squirrels, marmots, chipmunks, prairie dogs, etc (6 genera, c.90 species)
External links