Squirrel Squirrels belong to family Sciuridae of small
or medium-size rodents. The family includes tree squirrels, ground squirrels, chipmunks,
marmots , 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. Etymology
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.
The native Old English word for the squirrel, ācweorna, survived only into Middle English
(as aquerne) before being replaced. The Old English word is of Common Germanic origin,
and cognates used by other countries to name the squirrel include the German Eichhhörnchen
(diminuitive of Eichhorn, which is not used), the Norwegian ikorn/ekorn, the Dutch eekhoorn,
the Swedish ekorre and the Danish egern. Characteristics
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.
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. 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.
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.
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. Many also have a good sense of touch, with vibrissae on their
heads and limbs. 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
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.
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.
Ground and tree squirrels are typically diurnal or crepuscular, while flying squirrels tend
to be nocturnal—except for lactating flying squirrels and their offspring, which have
a period of diurnality during the summer. Feeding
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. 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. Predatory behavior has been noted by various
species of ground squirrels, in particular the thirteen-lined ground squirrel. For example,
Bailey, a scientist in the 1920s, observed a thirteen-lined ground squirrel preying upon
a young chicken. Wistrand reported seeing this same species eating a freshly killed
snake. 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; 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. Morgart observed
a white-tailed antelope squirrel capturing and eating a silky pocket mouse.
Taxonomy The living squirrels are divided into five
subfamilies, with about 58 genera and some 285 species. 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. 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. 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.
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. 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. Basal and incertae sedis Sciuridae (all fossil)
Hesperopetes Kherem
Lagrivea Oligosciurus
Plesiosciurus Prospermophilus
Sciurion Similisciurus
Sinotamias Vulcanisciurus
Hesperopetes Kherem
Lagrivea Oligosciurus
Plesiosciurus Prospermophilus
Sciurion Similisciurus
Sinotamias Vulcanisciurus
Subfamily Cedromurinae (fossil) Subfamily Ratufinae – Oriental giant squirrels
(1 genus, 4 species) Subfamily Sciurillinae – neotropical pygmy
squirrel (monotypic) Subfamily Sciurinae
Tribe Sciurini – tree squirrels (5 genera, about 38 species)
Tribe Pteromyini – true flying squirrels (15 genera, about 45 species)
Tribe Sciurini – tree squirrels (5 genera, about 38 species)
Tribe Pteromyini – true flying squirrels (15 genera, about 45 species)
Subfamily Callosciurinae – Asian ornate squirrels
Tribe Callosciurini (13 genera, nearly 60 species)
Tribe Funambulini palm squirrels (1 genus, 5 species)
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, about 50 species) Tribe Marmotini – ground squirrels, marmots,
chipmunks, prairie dogs, etc. (6 genera, about 90 species)
Tribe Xerini – spiny squirrels (3 genera, 6 species)
Tribe Protoxerini (6 genera, about 50 species) Tribe Marmotini – ground squirrels, marmots,
chipmunks, prairie dogs, etc. (6 genera, about 90 species)