The functional morphology of feather vanes was examined by combining morphological examination with mechanical tests. A geometrical model was derived which related the in-plane behaviour of the vane to the orientation of the barbs and barbules. This predicted that the small branching angles of both barbs and proximal barbules should result in a vane which is easier to move distally than proximally. These predictions were verified by mechanical tests on primary and secondary feathers of the pigeon Columba livia. A further prediction, that the inclination of the barbs' cross section should make the vanes more resistant to forces from below than those from above, was also confirmed by mechanical tests. Differences in the mechanical behaviour of feathers are related to differences in their morphology and function. The vanes of outer primaries are more resistant to out-of-plane forces than those of the inner primaries and secondaries, particularly towards their tip, a property which will help them withstand the larger aerodynamic forces to which they may be subjected in flight. The outermost primary vane also showed the least asymmetry to out-of-plane forces as a result of the more vertical orientation of its barbs. This may help it to act as a reversible aerofoil during take-off.
One key evolutionary innovation that separates vertebrates from invertebrates is the notochord, a central element that provides the stiffness needed for powerful movements. Later, the notochord was further stiffened by the vertebrae, cartilaginous and bony elements, surrounding the notochord. The ancestral notochord is retained in modern vertebrates as intervertebral material, but we know little about its mechanical interactions with surrounding vertebrae. In this study, the internal shape of the vertebrae—where this material is found—was quantified in sixteen species of fishes with various body shapes, swimming modes, and habitats. We used micro-computed tomography to measure the internal shape. We then created and mechanically tested physical models of intervertebral joints. We also mechanically tested actual vertebrae of five species. Material testing shows that internal morphology of the centrum significantly affects bending and torsional stiffness. Finally, we performed swimming trials to gather kinematic data. Combining these data, we created a model that uses internal vertebral morphology to make predictions about swimming kinematics and mechanics. We used linear discriminant analysis (LDA) to assess the relationship between vertebral shape and our categorical traits. The analysis revealed that internal vertebral morphology is sufficient to predict habitat, body shape, and swimming mode in our fishes. This model can also be used to make predictions about swimming in fishes not easily studied in the lab, such as deep sea and extinct species, allowing the development of hypotheses about their natural behavior.
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