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1 Introducing Squirrels
- Johns Hopkins University Press
- Chapter
- Additional Information
What are squirrels? Squirrels are among the most widely known and recognized mammals, inhabiting all continents except Antarctica and Australia. In many parts of the world they occupy human habitats, gladly sharing our lunches in a city park, helping to empty our bird feeders, or feeding on our crops. Squirrels belong to a family of rodents, the Sciuridae, whose common ancestor lived some 30 to 40 million years ago and gave rise to the 278 species we currently assign to the family. All squirrels share a number of anatomical features —teeth, jaw musculature, skull, and other bones—that scientists use to identify recent and fossil squirrels. There are also important differences in anatomy between the main groups of squirrels. Below we discuss some basic components of squirrel anatomy, including similarities and differences between tree squirrels, flying squirrels, and ground squirrels. Teeth and skull. Squirrels, like all rodents, have four chisel-like incisors at the front of the mouth—two above and two below—which they use for gnawing. These incisors grow continuously and have roots that extend well back into the maxilla and mandible, unlike human teeth, which have short roots and grow only so long. Constant gnawing keeps rodent incisors short and sharp, because the enameled layer on the outside of the tooth wears much more slowly than the inner layers of dentine. Squirrels use their incisors to gnaw almost anything, but under natural conditions they use them most commonly to cut vegetation and to gnaw hard objects like nuts. If for some reason a squirrel cannot gnaw, its incisors will continue to grow until the animal dies because it can no longer eat. Chapter 1 Introducing Squirrels Cheek teeth are separated from the incisors by a space called the diastema . A squirrel’s cheek teeth consist of one or two premolars and three molars in the upper jaw, and one premolar and three molars in the lower jaw. These have short roots and determinate growth, meaning they grow for a short while and then stop. Squirrels use their molars and premolars for grinding up food before they swallow it, just as you do. The molars and premolars have cusps (called cones on the upper teeth and conids on the lower teeth) connected by ridges (called lophs on the upper teeth and lophids on the lower teeth). The differences and similarities in patterns of these cusps and ridges on teeth are used extensively by paleontologists to identify fossils and to recognize which fossil mammals are closely related to which others. A squirrel must be able to move its jaw forward to bring its incisors together to gnaw, but it also must be able to move its jaw backward to bring the molars together to grind. To move the jaw forward to gnaw, some of the jaw muscles must attach farther forward on the skull than they do on the lower jaw. Different kinds of rodents have evolved jaw muscles that attach farther forward on the skull in several different ways. In all but the most ancient fossil squirrels, these muscles attach below and in front of the orbit of the eye, and also (in most squirrels) on the side of the nose. This type of jaw muscle attachment is called sciuromorphy. It is also found in a handful of other rodents, like beavers, pocket gophers, and kangaroo rats. Hands and feet. The hands and feet of squirrels are relatively unspecialized , with five digits on each, although the thumb ranges from small to diminutive. Like you, they have three phalanges, or bones, on each finger 2 Squirrels: The Animal Answer Guide Eastern gray squirrels (Sciurus carolinensis ) are perhaps the most widely recognized squirrel species in the world. Photo © Donald Reeve, www.scarysquirrel.org [3.230.128.106] Project MUSE (2024-03-19 10:00 GMT) and toe, except for the thumb and the first toe, each of which has two. The relative proportions of the phalanges of squirrels, particularly of ground squirrels, are different from yours, and they have claws instead of nails on the terminal phalanges of all digits except for the thumb. There is a small nail on the thumb, but it may not be large enough to be seen. The hands of burrowing squirrels tend to be wider than those of tree squirrels, with the middle digit the longest, presumably because a wider hand is more useful for digging. In tree squirrels, the fourth digit is the longest and serves as an effective grappling iron when the...