The feet of squirrels allow them to climb trees and hang from almost vertical surfaces via sharp claws and swiveling ankle joints.

     
  "The squirrel is particularly well adapted for tree climbing. It has sharp claws, and instead of having backward-pointing toes like the climbing birds, it can swivel the whole back foot round at the ankle so that it points backwards. The squirrel can thus hang from an almost vertical surface provided there is enough irregularity on the tree trunk into which to hook its claws." (Foy and Oxford Scientific Films 1982:183)
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Livers of squirrels survive cold temperatures during hibernation by maintaining microcirculation.

     
  "Close investigation of hibernating states is giving scientists new insight into animal physiology and, as a bonus, human health and disease as well…Organ transplant. Harvested human organs kept at cold temperatures can only remain viable for a few days at most. So, how do squirrels' livers and intestines stay perfectly healthy at less than 40 degrees Fahrenheit for weeks on end? In one study, 'we found that [hibernating] squirrels maintain, in a much healthier state, the microcirculation of the liver,' Carey said [Hannah Carey, a professor in the School of Veterinary Medicine at the University of Wisconsin-Madison]. Insights into just how this happens could increase the viability and number of human organs available for transplant, she said." (Mundell 2007)
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All squirrels are characterized by their relatively primitive jaw structure. 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. The infraorbital foramen is not enlarged to transmit muscle as it is in myomorphous (mice and rats) and hystricomorphous (cavys and guinea pigs) rodents.

 

There are three basic body forms in squirrels: 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 (patagiuim) 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. Like many other rodents, all squirrels have five functional toes on the hindfeet and four on the forefeet, with a well-developed claw on each digit.

 

Squirrels occur throughout the world except for Australia, southern South America, and some desert regions. They inhabit a wide variety of habitats from tropical rain forest to arctic tundra and from the tops of trees to underground tunnels. Most flying squirrels are nocturnal, other squirrels are nocturnal or diurnal. Squirrels range in size from the mouse-sized African pygmy squirrel ( Myosciurus pumilio) to the robust marmots and woodchucks (genus Marmota).

 

The skulls of sciurids are short, with a short rostrum and arched profile. The postorbital processes are well developed, and the distance between the orbits is considerable. The jugals are long, and their anterior ends contact the frontals. The palate appears broad and relatively short, ending at about the same level as the molar row. Bullae are relatively large but not inflated. The dental formula of sciurids is 1/1, 0/0, 1-2/1, 3/3 = 20-22, with the anterior premolar sometimes small and peglike. The remaining cheekteeth, which are always rooted, usually have well-developed transverse ridges.

 

Squirrels generally eat nuts and seeds, but fungi, insects and young birds are sometimes taken. Some tree squirrels strip bark and feed on bark and sap; red squirrels can do considerable damage to forestry plantations by eating conifer shoots. Ground squirrels eat mostly green plant material. Squirrels that live in temperate areas often cache or bury nuts and seeds for use during the winter. This caching behavior makes squirrels an important disperser of plant seeds, as buried seeds are often not found and may germinate. Many ground squirrels hibernate and some store food in underground burrows.

 

Some northern species of squirrels are hunted for their soft, thick fur, and many of the larger species are hunted for food. The tail of grey squirrels is used in artist's brushes, and marmot fat is used as a chest cold remedy in the Alps. Squirrels can be host to the parasites that carry plague and Rocky mountain tick fever, but squirrels are generally considered playful and attractive denizens of woodlands and prairies.

   

  References and literature cited: 

 

Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.

 

MacDonald, D. (Ed.). 1987. The Encyclopedia of Mammals. Facts on File Pubs, New York.

 

McLaughlin, C. A. 1984. Protrogomorph, sciuromorph, castorimorph, myomorph (geomyoid, anomaluroid, pedetoid, and ctenodactyloid) rodents. Pp. 267-288 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.

 

Paradiso, J. L. 1975. Walker's Mammals of the World, Third Edition. Johns Hopkins University Press, Baltimore.

 

Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.

 

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.

 

Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.

 

Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.