Evolutionary Variation and Biomechanics of the Avian Furcula

Abstract

The avian wishbone, or furcula, serves as the cranial‐most attachment of the m. pectoralis in most birds, variably connects to the coracoid and sternum, and is morphologically diverse across the clade. Furcula shape and microstructure covary with flight style, suggesting its functional importance to flight. Several overlapping hypotheses have been proposed for furcular morphology, such as aiding wing protraction or increasing ventilation efficiency, but the bone remains an enigmatic part of the avian pectoral girdle. In particular, we have little understanding of how furcula morphology and material properties relates to other pectoral structures, including flight muscles such as m. sternocoracoideus. Despite the hypothesized significance of the furcula in the origin of flight and diversification of flight styles, further work needs to be done to understand its functional properties among extant taxa.In order to explore variation in the furcula and pectoral structures relative to flight style, we studied members of 12 bird families using a three‐pronged approach. Specifically, we examined 1) the shape of the furcula in relation to its in situ orientation using non‐contrast computed tomography (CT), 2) the relative size of m. pectoralis and its subdivisions using contrast‐enhanced CT (DiceCT), and 3) the functional potential of the m. sternocoracoideus to effect or respond to furcula deformation using muscle lever analysis, material properties testing, and muscle spindle histology.We found substantial differences in furcular orientation, muscular proportions, and furcula material properties across the sample. Whereas earlier studies found a relatively weak correlation between flight style and furcula shape, we found that accounting for in situ furcula orientation improves the correlation between shape and flight style. Birds with more anteriorly‐curved furculae also show a relatively larger cranial division of m. pectoralis. These results support the hypothesis that furcula curvature mediates the protractive capability of m. pectoralis during downstroke. Notably, muscular differences between furculate and afurculate species of parrots (Psittaciformes) were insignificant despite similar flight styles, suggesting that other musculoskeletal structures may take on the functional role of the furcula in certain clades. We also found little variation in m. sternocoracoideus regarding spindle density and muscle moments about the coracosternal joint. The small size and suboptimal location of m. sternocoracoideus suggests that it serves a proprioceptive role, perhaps necessary to assess interglenoid distance during flight, rather than to laterally spread the furcula via the coracoids. Nevertheless, its morphological disparity across our sample remains unexplained.Support or Funding InformationUniversity of Missouri Life Sciences Fellowship