Living small: Body size and bone function
How does being small change the way that an animal uses its body, and how does that affect the selective pressures that shape its morphology?
Finite element modeling of stress and displacement in shrew vertebrae. A, sagittal section through third lumbar vertebra of Suncus etruscus, the smallest extant mammal by mass. B, von Mises stress, and C, displacement, of S. etruscus third lumbar vertebra under 5N axial compression, with trabeculae intact (left) and with trabeculae removed (right). Note higher peak stresses and larger magnitude displacement in models with trabeculae removed.
The smallest of mammals, the shrew Suncus etruscus, has vertebrae with almost no trabeculae; how does this lack of trabeculae influence the way that their tiny vertebrae withstand force? Is there a cutoff size at which it is a waste of energy to produce trabeculae, and does that have to do only with body size, or with ecology as well? Small mammals face a unique set of biomechanical challenges when moving through their environments; furthermore, they often exhibit a wide variety of habitats and locomotor modes within a single clade, providing an abundance of natural experiments to understand how exceptionally small mammals adapt to various selective pressures. Of particular interest is the tradeoff between strength and stiffness at small body size: because of high deflection under muscular force, do small mammals have stiffer bones relative to body mass than larger mammals?
I am the lead PI of a recently funded NSF Integrative Organismal Systems collaborative proposal with Ken Angielczyk (Field Museum) and Tristan Stayton (Bucknell University) aimed at addressing several of the above questions. Using the ecological and functional diversity within the mammalian clade Eulipotyphla, which includes shrews, hedgehogs, moles, and solenodons, we will investigate i) the contribution of trabecular and cortical bone tissues to the mechanical properties of small mammal vertebrae, and ii) how body size affects the relative importance of strength and stiffness to efficient skeletal function. Eulipotyphla is the ideal group for these studies, because it includes the smallest living mammals (S. etruscus, as noted above), and a selection of species with various ecologies at a range of body sizes. This research plan is based on comparative finite element modeling (FEA) for modeling bone performance; and performance surface analysis, which I will use to incorporate biomechanical and morphological data to test hypotheses about evolutionary selective pressure. I recently led a study on the use of performance surface analysis on structures that are difficult to model (e.g., trabecular bone structure; Smith et al. 2021), which demonstrated that satisfactory performance landscapes can be produced using only morphological shapes that exist in natural populations. This finding greatly expands the feasible applications of performance surface analysis and will enable me and my collaborators to uncover new information about the fundamental selective pressures affecting extremely small vertebrate skeletons.
NSF PUBLIC ABSTRACT
NSF Division of Integrative Organismal Systems, Physiological Mechanisms and Biomechanics: Collaborative Research: RUI: The challenges of living small: functional tradeoffs in the vertebral bone structure of diminutive mammals.
Publications
Smith, S.M., Stayton, C.T., Angielczyk, K.D. 2021. How many trees to see the forest? Assessing the effects of morphospace coverage and sample size in performance surface analysis. Methods in Ecology and Evolution 00: 1– 14. doi: 10.1111/2041-210X.13624