Meeting Abstract
Though birds have long been admired by biologists and engineers alike for having lightweight bones with specialized “reinforcements,” very little work has been done to investigate this internal substructure (i.e. trabecular bone). Trabecular bone, a complex 3D matrix, mechanically adapts to an organism’s behavior over its lifetime. This has facilitated success in using trabecular structure to interpret function in fossil mammals, though no attempt has been made in birds. We thus collected high-resolution microCT scans of the humerus across a broad, comparative set of 51 species which vary in flight mode on a continuum from flapping to soaring. Whole bones were segmented and trabecular matrix parameters were measured for the humeral head. We developed a new parameter (Trabecular Extent, Tb.Ex) to holistically assess the extent of reinforcing structures in the bone. Across corvids, increases in trabecular thickness, ellipsoid factor, and the degree of anisotropy significantly covaried with increases in gliding/soaring behavior, while volume fraction did not vary. Similar patterns were found in a preliminary analysis across the phylogeny. Tb.Ex scaled allometrically within, but not across clades, and also varied with flight mode and ecology. Preliminary comparison of cross-sectional geometry and Tb.Ex suggests a mechanical tradeoff between trabecular and cortical bone. Our results support that trabecular bone in the wing maximizes volume while minimizing mass, but the specific architecture and extent relates to more nuanced differences in kinematics and loading across flight modes and ecologies. Ongoing work will explore the mechanical role of trabecular bone as well as apply our results to fossil interpretation, and overall provides both crucial insight into flight mechanics as well as a robust, novel approach to understanding the evolution of avian flight.
