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Shrews are a morphologically and ecologically diverse group of small-bodied mammals, ranging in body mass from under 2 grams to around 100 grams. Their diversity and size makes them an excellent model for investigating the effects of small body size on the ecomorphology of the skeleton. We want to investigate if there is ecologically informative variation in skeletal morphoogy of these tiny mammals, and if so, does the relationship between shape and ecology vary across body size? Using μCT scans of skeletons from the Field Museum of Natural History, we quantified morphology of the middle lumbar vertebra in 25 species of shrews via 3D geometric morphometrics. Because the lumbar spine is heavily involved in mammalian locomotion, we analyzed the correlation between locomotor mode and vertebral shape, as well as between centroid size and vertebral shape. We found no statistically significant association between centroid size and locomotor group,or between size and shape. However, we found variation in vertebral morphology across locomotor groups, with notable shape differences in centrum aspect ratio, height of the neural arch, and location of the transverse processes. Fossorial and scansorial species diverge the most from the overall mean shape. We also recovered phylogenetic signals associated with shape. Future work will refine the relationship between shape and bone performance under various loading scenarios and enable us to better separate functional from phylogenetic effects. 

The relative contributions of trabecular (spongy) and cortical (compact) bone to bone strength and stiffness are poorly understood across mammalian body size. In mammals, some small species have notably reduced their vertebral trabecular bone structure, resulting in mostly hollow medullary cavities. To assess the importance of trabecular structure to the mechanical properties of small mammalian vertebrae, we conducted finite element analysis on the lumbar vertebrae of 25 species of shrews (Soricidae) weighing 2-100g. We analyzed two sets of models: vertebrae with the trabecular structure intact (full), and vertebrae with all trabeculae excised from the centrum (hollow). All models were scaled to the same ratio of load to surface area. The cranial end of the centrum was immobilized, and a 5N craniocaudally-oriented load was applied to the caudal end of the centrum. We measured mean von Mises stress (MVMS) to capture strength, and total strain energy to capture stiffness. MVMS and total strain energy both decrease as body size increases, and hollow models experience higher stresses and strains than full models. With increasing body size, the difference in total strain energy between full and hollow models decreases, but the difference in MVMS slightly increases. This suggests a difference in the functional advantage conferred by trabeculae among small mammals, as well as a possible selective pressure for different functional emphasis in very small and larger mammalian bones. 

The relative contributions of trabecular (spongy) and cortical (compact) bone to bone strength and stiffness, although investigated in humans, is mostly unclear. As a result, we do not understand how the skeleton of small animals, especially the axial skeleton, has evolved to deal with the particular challenges of life at tiny size. In mammals, some small species have notably reduced their vertebral trabecular bone structure, resulting in mostly hollow medullary cavities. To assess the importance of trabecular structure to the mechanical properties of small mammalian vertebrae, and incorporate the effects of both trabecular and cortical bone structure, we conducted finite element analysis on the lumbar vertebrae of 15 species of shrews (Mammalia: Soricidae). We analyzed two sets of models: vertebrae with the trabecular structure intact, and vertebrae with all trabeculae excised from the centrum. In all models, the cranial end of the centrum was immobilized, and a 5N load was applied to the caudal end of the centrum, parallel to the craniocaudal axis. Results indicate higher peak stresses and larger displacements in models lacking trabeculae. Although smaller body size constrains the number of trabeculae that small mammals develop, we expect that these trabeculae contribute disproportionately to bone strength and stiffness. Ongoing work will validate these analyses with empirical materials testing and assess how bone morphofunctional characteristics change as body size increases.