The Superficial Masseter and Gape in Mammals

Abstract

The morphology of mammalian jaw systems is a product of selection for many sometimes conflicting functions, including prehension and mastication of food, agonistic behavior, respiration, and vocalization. One of the first and most challenging tasks of a functional cranial analysis is to resolve the complex resultant structure into its component parts. This paper has two purposes: first, to call attention to a hitherto rather neglected functional parameter of jaw morphology-the influence of muscle action line on gape; and second, to present a method of analyzing this influence using the quantifunctional approach (Gould 1970) of constructing a mathematical model to predict morphology and testing it by comparison with real animals. Masticatory muscle orientation (i.e., the direction of the action line and the positions of origin and insertion relative to the joint) is usually considered to reflect adaptation for mechanical advantage (i.e., moment arm) (e.g., Turnbull 1970, and references therein), joint protection, or special requirements for particular mandibular movements, as in rodents (Maynard Smith and Savage 1959). The effect of muscle orientation on gape is usually considered briefly and nonquantitatively or not at all. The ability of an animal to achieve wide jaw opening may, however, be an important element in its biology. We do not claim that the full range of mammalian variation can be explained by gape adaptation. As one of the numerous factors contributing to musculoskeletal morphology, the influence of gape may often be obscured by selection for other parameters. However, in certain extreme cases such as carnivores or the hippopotamus, which show particularly wide jaw opening, adaptation for gape may become a primary determinant of jaw form. The model presented below is simplistic in that it takes into account only muscle stretch and moment arm. The main advantage of modeling in biology is, however, the generation of predictions by which the model can be verified or disproved. The serviceability of the present model is indicated by the consistency of its predictions with observations on real animals. Although only one muscle, the superficial masseter, is analyzed in detail, the method is general and can be applied anywhere. Absolute proof that a given mammal is specialized for wide jaw opening would require, at the very least, analysis of all adductor muscles. As this paper deals more with methodology and broad implications, we have not performed such analysis.