Meeting Abstract
To understand the evolution of flight in the avian lineage, it is necessary to resolve the functional morphology of the flight stroke. Thus far, studies have focused only on the cortical bone in wings. Trabecular bone more rapidly adapts to joint loading, and can provide subtle and specific mechanical signals within bone that correlate with behavior. Though trabecular analyses have led to major discoveries in mammalian biomechanics and evolution, trabecular morphology in relation to bird flight has yet to be explored. To begin to assess structure-function relationships in the avian shoulder, we used existing kinematic and mechanical data from various species to model the loading regime for different flight styles (e.g. flapping, soaring). We collected microCT scans of the proximal humerus in a broad, comparative set of museum specimens and an ontogenetic series of chukars. Trabecular structure was related to flight style, in combination with body size and phylogeny. Both mean trabecular thickness (Tb.Th) and the degree of anisotropy (DA) in the humeral head appeared to be highest in raptors, which habitually soar or flap-glide. Comparatively, Tb.Th and DA tend to be low in ground birds, which engage only in short bursts of flapping flight. Additionally, we found that Tb.Th scales allometrically, proportional to the 0.185 power of mass, which is lower than has been reported in the femur of birds and closer to the scaling exponent found in mammals. In the ontogenetic series of chukars, it appears that an initial increase in Tb.Th was concurrent with the onset of lift production, while DA decreased with age. Elucidating the relationship between trabecular structure and flight mechanics in the avian shoulder provides significant insight into extant flight and a fundamental resource for fossil interpretation in the avian lineage.
