Meeting Abstract

96.3  Thursday, Jan. 7  Isometric scaling of lever arm lengths in squirrel jaws leaves jaw shape free to meet diverse functional demands SWIDERSKI, D.L.*; ZELDITCH, M.L.; Univ of Michigan, Ann Arbor; Univ of Michigan, Ann Arbor dlswider@umich.edu

Allometric scaling appears to be a nearly universal feature of morphological diversity, with the conspicuous exception of the mammalian mandible. Some theoretical analyses suggest mandibular lever arm lengths should be isometric relative to each other, which some empirical studies confirm. Our analysis of 23 New World tree squirrel species likewise found that most lever arm lengths were isometric relative to each other, but were negatively allometric relative to jaw size. Despite these correlations, most jaw shape diversity was uncorrelated with jaw size differences. To determine whether this pattern is particular to one group of fairly homogeneous and recently radiated species, we also evaluated jaw morphology in chipmunks and ground squirrels. In these more diverse terrestrial species, relative jaw lengths were as tightly constrained as in tree squirrels, but unlike the tree squirrel pattern, some lengths exhibited strong positive allometry relative to jaw size. The terrestrial squirrels were also similar to tree squirrels in that overall jaw shape was much more diverse than indicated by these length correlations, and that shape differences were largely independent of size differences. Thus both groups appear to be under strong constraints to maintain lever arm ratios (mechanical advantages) across broad size ranges, but many different jaw shapes can meet those constraints, allowing great flexibility to meet other demands such as optimizing gape size or resistance to deformation. The contrast between the uniformity of tree squirrels and the diversity of terrestrial squirrels reflects the relative diversity of those other demands on jaw form and function.