Species
Sciuridae
IUCN
NCBI
EOL Text
Squirrels are known from the end of the Eocene in Wyoming and the early Oligocene in southern France.
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Barcode of Life Data Systems (BOLD) Stats
Specimen Records:736
Specimens with Sequences:536
Specimens with Barcodes:522
Species:96
Species With Barcodes:81
Public Records:301
Public Species:52
Public BINs:68
Squirrels are a diverse group consisting of approximately 279 species and 51 genera that are broken into five subfamilies (Ratufinae, Sciurillinae, Sciurinae, Xerinae, and Callosciurinae). The family Sciuridae includes tree squirrels, ground squirrels, and flying squirrels. Tree squirrels have long, bushy tails, sharp claws and large ears. Some have well-developed ear tufts. Flying squirrels have a furred membrane (patagium) extending between the wrist and ankle that allows them to glide between trees. Ground squirrels are generally more robust than tree squirrels and often have short, sturdy forelimbs that are used for digging. Their tails, while fully furred, generally are not as bushy as those of tree squirrels.
Sciurids range in body size from mouse-sized African pygmy squirrels to robust red giant flying squirrels of Asia, weighing up to 3 kilograms. They vary greatly in geographic range and habitat. Squirrels are native throughout the world, with the exception of Antarctica, Australia, southern South America, and some desert regions. They occupy habitats ranging from tundra to rainforest. Some squirrels live solitary lives such as woodchucks, while others, such as prairie dogs, live in communities of hundreds of individuals with complex social structures. Squirrels are largely herbivorous, eating seeds, nuts, fruits, fungi, and other plant matter; however, insects, eggs and the occasional small vertebrate may be part of the diverse diet of these animals.
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Squirrels are found worldwide, native to all terrestrial regions with the exception of Australia, Madagascar, southern South America, Antarctica, Greenland, many oceanic islands, and certain desert regions such as the Sahara. Two species of squirrels were introduced to Australia in the 19th century. One of those species, Sciurus carolinensis, is now believed to be extinct. However, a flourishing feral colony of Funambulus pennantii persists there. Squirrels are especially diverse in African and southeast Asian forests.
Biogeographic Regions: nearctic (Native ); palearctic (Native ); oriental (Native ); ethiopian (Native ); neotropical (Native ); australian (Introduced )
Other Geographic Terms: holarctic
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Of the 279 species of Sciuridae, two are listed as critically endangered while another 15 are listed as endangered and 16 are listed as vulnerable. The most endangered sciurid is the Vancouver Marmot (Marmota vancouverensis), consisting of only an estimated 35 individuals in the wild as of 2004. Common factors leading to these marmots being listed as endangered include destruction of habitat and human encroachment. Lack of accurate information on populations and threats is another important factor in sciurid conservation generally.
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Squirrels are characterized by their long bodies, soft fine hair (although some have very thick hair), and large eyes. The hindfeet have five digits while their forefeet have four digits. Claws are found on all terminal phalanges except the thumb, which has a nail. Vibrissae, which are important for tactical stimuli, are found all over the body by the nose, cheek, eye, chin, wrist, feet, and outside of the legs. Sciurids vary in size from very small, like African pygmy squirrels (Myosciurus pumilio, approximately 10 g), to substantially large, such as Alpine marmots (Marmota marmota, 3 to 8 kg). They also vary substantially in fur color including black, white, red, and brown. Sciurids have three basic morphologies: ground squirrel, tree squirrel, and flying squirrel forms. Ground squirrels tend to have large broad forefeet, with the middle digit being the longest, short, stout limbs used for digging, and short tails. Tree squirrels have long muscular legs, long arms, large ears, and long bushy tails. Flying squirrels are characterized by their gliding membrane, a furred patagium which attaches to the forelimbs via the styliform cartilage at the wrist and extends down to the heel of the hindlimbs. Additionally, flying squirrels have the longest limbs relative to body size of all squirrels.
What links all squirrels is their skull architecture and relatively primitive jaw structure. Their skulls are short, with a short rostrum and arched profile. The skull has an broad, tilted zygomatic plate that serves as the attachment point for the lateral branch of the masseter muscle. The superficial branch of masseter muscle originates on a prominent bump of bone of the side of rostrum called the masseteric tubercle. They have small infraorbital foramena that is not enlarged to transmit muscle as it is in myomorphous (mice and rats) and hystricomorphous (cavys and guinea pigs) rodents. Squirrels have long jugals, well-developed postorbital processes, and large bullae that are not inflated. The anterior ends of the jugals contact the frontals and the palate is broad and relatively short, ending at the same level as the molar row. The zygomasseteric architecture of skulls is sciuromorphous (the lateral branch of the masseter muscle has shifted to the rostrum).
The teeth of sciurids is characterized by four chisel-like incisors covered in enamel that grow continuously and have roots that extend well back into the maxilla and mandible. Since they are used for gnawing, these teeth are kept short and sharp. The incisors are followed by a diastema and cheek teeth which are rooted and brachydont or hypsodont. The dental formula of sciurids is 1/1, 0/0, 1-2/1, 3/3 = 20-22.
Other Physical Features: endothermic ; heterothermic ; homoiothermic; bilateral symmetry ; polymorphic
Sexual Dimorphism: sexes alike; female larger; male larger
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The majority of problems sciurids cause humans are a result of their voracious appetites. These animals dig up seeds planted by farmers as well as devouring crops. Many ground and tree squirrels commonly participate in “bark stripping” where they pull the bark off of trees to get to the tissue underneath. This may stunt the growth of the tree, cause a reduction in fruit production, or cause the tree to die. Ranchers see prairie dogs as a threat, although those threats are generally not substantiated. Squirrels can be seen as a nuisance to homeowners as well. They may chew through electrical and telephone wiring, insulation, and house siding. Squirrels sometimes climb into and cache food in transformers and generators, causing power outages in surrounding communities. Squirrels may carry diseases, such as plague, which can be transmitted to humans and domestic animals. In western North America, rock squirrels and prairie dog species are the most common and frequent source of transmission to humans.
Negative Impacts: injures humans (carries human disease); crop pest; causes or carries domestic animal disease ; household pest
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From trees to burrows underground, sciurids are found in a vast array of habitats, including rainforests, arid grasslands, arctic tundras, forests, suburban areas, and cities. Sciurids can be found at high elevations, such as the Himalayan marmots (Marmota himalaya), which are found at elevations up to 5000 meters.
Habitat Regions: temperate ; tropical ; polar ; terrestrial
Terrestrial Biomes: tundra ; taiga ; desert or dune ; savanna or grassland ; chaparral ; forest ; rainforest ; scrub forest ; mountains
Other Habitat Features: urban ; suburban ; agricultural ; riparian
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Squirrels are hunted for their meat and pelts. Squirrel pelts were also once used as a form of currency. The modern word for money in Finland actually comes from a root word that means squirrel skins. Since their diets consist of mainly fruits and seeds, squirrels become very useful in seed dispersal. Squirrels that eat flowers or drink nectar may also aid in pollination. Squirrels are used in medical and scientific research. A research project at the Institute of Arctic Biology at the University of Alaska, Fairbanks has been studying hibernation in arctic ground squirrels to learn more about strokes, heart attacks, and neurodegenerative diseases in humans caused by reduced blood flow. Groundhogs suffer from a virus very similar to Hepatitis B in humans and exhibit similar disease progression such as cancer of the liver and liver disease. For this reason, these animals are used as models for studying the disease, treatments, and advances in liver transplant techniques (summarized in Thorington and Ferrell, 2006).
Positive Impacts: food ; body parts are source of valuable material; research and education; pollinates crops
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