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By Sharon Jansa and Phil Myers
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.
Contributors
Sharon Jansa 
(author), University of Minnesota. Phil Myers (author), Museum of Zoology, University of Michigan.
