Morphological Diversity Despite Isometric Scaling of Lever Arms

Abstract

In the absence of a substantial functional shift, morphological evolution is usually expected to follow an allometric trajectory, however, studies of tree squirrel jaws have found isometry across most of their size range. This isometry appears to reflect the integration of a small number of lever arm lengths that are critical for generating bite force. To test whether this integration constrains only the ratios of these lengths, or jaw shape in general, we analyzed jaw shapes and a set of lengths comparable to those used in previous studies for 23 species of sciurine tree squirrels (Sciurus, Tamiasciurus and Microsciurus), a lineage that is both functionally uniform and spans a large size range. We found that the measured lengths were highly correlated and isometric with respect to each other, but negatively allometric with respect to jaw size. Shape differences are generally small, but shape diversity was still greater than the diversity of mechanical advantages (input lever lengths scaled by output lever length). In addition, phylogenetic analyses demonstrated that only a minute fraction of shape evolution is correlated with size evolution. This contrast between the diversity of shape and the stability of proportions among a suite of functionally relevant lengths suggests that constraints on those lengths and the associated mechanical parameters have little or no ability to restrict changes in other aspects of jaw form.