JAW MUSCLES OF NEW-WORLD SQUIRRELS

The jaw, suprahyoid, and extrinsic tongue muscles are described for ei ght species of New World squirrels, spanning more than an order of mag nitude in body mass. Anatomical differences are discussed in the light of body size, natural history, and phylogeny. The relative sizes of d ifferent muscles, their orientations, and the shapes and positions of their areas of attachment vary but show few trends in relation to body size. The anatomical differences are likewise not readily explained b y the mechanical requirements of the animals' diets, which are similar . The most marked anatomical differences occur in Sciurillus (the pygm y tree squirrel), as well as those genera-Glaucomys (the flying squirr el) and Tamias (the chipmunk)-that are taxonomically most distinct fro m the tree squirrels. Sciurillus is noteworthy for its unusually small temporalis and an anterior deep masseter that is oriented to assist i n retraction of the jaw. Tamias has a more vertically oriented tempora lis and greater inclination in the anterior masseter muscles than the other squirrels, features that may be associated with its large diaste ma and relatively posteriorly situated cheek teeth, which in turn may relate to its having cheek pouches. Our results form a valuable databa se of information to be used in further studies of functional morpholo gy and phylogeny. (C) 1995 Wiley-Liss, Inc.