Eigenshape and Biomechanical Analysis of the Phyllostomid Mandibular Symphysis

NICOLAY, C.W.*; SHERWOOD, R.J.: Eigenshape and Biomechanical Analysis of the Phyllostomid Mandibular Symphysis

Bats of the family Phyllostomidae are characterized by considerable dietary diversity, which may be reflected in the morphology of the mandibular symphysis. Symphyseal outline shapes and biomechanical properties of the symphysis were compared using bone densitometer (CT) scans (midline sagittal sections) of dry skulls. The diets of the 9 species (71 individuals) sampled range from fig specialists to dedicated nectarivores. Eigenshape analysis was performed to quantify symphyseal outline shape. CT sections were digitized and converted PHI* coordinates, which were then subjected to a singular value decomposition using the covariance matrix. Biomechanical properties calculated from bitmap images included cortical bone area, area moments of inertia, and angular orientation of the symphysis. Both techniques yield broadly similar (but not identical) results, effectively separating nectarivorous from frugivorous species, with omnivorous species falling intermediately. The second eigenshape factor (E2) is positively correlated with all biomechanical parameters (r = 0.52 – 0.59) and symphyseal orientation (r = 0.75), but other eigenshape factors (E1 and E3) are not well correlated with any mechanical variables or linear measurements. Both E2 and mechanical parameters are strongly correlated with palate length/breadth ratios and measures of mandible size (r = 0.63 – 0.87). When mapped onto published phylogenies, outline shape functions show a relatively strong phylogenetic signal, while biomechanical properties appear to be associated with finer patterns of dietary and allometric variation.

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