Furcula Diversity Within the Avian Flight Apparatus

Abstract

A major unknown in avian biology is the morphological basis of flight style diversity. While certain aspects of avian pectoral anatomy, such as an enlarged sternum and hypertrophied pectoral muscles, clearly relate to aerial locomotion in general, the finer details of form-function correspondence are less apparent. Nevertheless, furcula shape has been used to infer flight style in extinct avians because the furcula is the cranial origin of the largest flight muscle, m. pectoralis, in most extant birds. However, previous studies, which examined the furcula in isolation, indicate only a modest correlation between furcula shape and flight style. We hypothesized that the relationship of the furcula to flight style may be heavily mediated by other aspects of pectoral anatomy, particularly furcula orientation and flight muscle morphology. We used computed tomography and digital dissection techniques to collect in situ morphological data on a large (n=108) and diverse (21 orders) sample of formalin-fixed specimens. While some specimens required the addition of Lugol's iodine to visualize soft tissue, most specimens did not require contrast enhancement to permit bone and muscle reconstruction. We found moderate correspondence between flight style and furcula shape consistent with previous studies (e.g., soaring birds have flatter, more V-shaped furculae), however the magnitude of shape differences between flight style and taxonomic categories was smaller, perhaps due to different measurement techniques. Novel measures of furcular orientation in the pectoral girdle revealed that larger, slower-flapping birds tend to have more cranially-positioned furculae, which may relate to a greater need for wing protraction via m. pectoralis. Further, the centers-of-mass of the main pectoral muscles tend to be more cranial in large birds with more laterally-bowed furculae. In most measures, hummingbirds did not group with birds of similar mass or furcula shape, likely owing to their kinematically-unique flight style. Overall, our findings recommend caution in inferring flight style from furcula shape alone. Rather, furcula shape may be better viewed as part of a larger functional unit including the sternum, coracoids, and major flight muscles.