Using DiceCT to evaluate the functional correlates of masticatory muscle architecture

Abstract

Diffusible iodine-based contrast-enhanced computed tomography (DiceCT) is a process in which the radiopacity of soft-tissues is enhanced by binding them to a dense molecule (in this case, Lugol's iodine) so that they can be viewed using x-ray-based visualization. Although DiceCT has previously been used to examine masticatory muscle fiber architecture, including in some lemurs, for the first time we have applied this technique to a large enough interspecific sample size to evaluate the functional adaptations of these muscles in three-dimensional space. We performed this analysis on 5 lemurids (Hapalemur, Lemur, Varecia,and two species of Eulemur) and three other strepsirrhines (Propithecus, Daubentonia,and Otolemur) as increasingly distant phylogenetic brackets. While our analysis confirms the findings of previous studies based on traditional gross dissection (i.e., more folivorous species have relatively shorter fascicle lengths and higher PCSA than do frugivorous taxa, giving them relatively higher bite force though lower linear gape abilities) these new analyses reveal more about the arrangement of these muscles. Namely, this approach shows, for the first time, 1) how these muscles are angled in three dimensional space - a necessary step to fully resolving the effects of pennation in these complex muscles, 2) that the jaw adductor muscles, in near occlusion, are curved and in a state of gross compression - thus changing our understanding of the force potential of these muscles in this state, and 3) the detailed gross and fascicular morphology of these muscles, particularly the deep muscles, in greater detail than can be accomplished using traditional methods. While these advantages improve our understanding of the function of these muscles, the technique is still extremely expensive in both material costs and scan time and the data collection is very time consuming. While these disadvantages currently preclude the application of this approach to large enough samples to explore broad functional questions across multiple lineages, if the costs in time and materials can be reduced, this three-dimensional visualization approach (or cheaper/easier approaches that can accomplish the same level of analysis) has the potential to yield new insights into the evolution of the functional anatomy of soft-tissues in three-dimensions.