Sternal morphology can indicate modes of locomotion in extant birds

Extant birds are highly diverse in both morphology and mode of locomotion. Locomotion in birds is intimately associated with the sternum, which serves as both an attachment site for major flight muscles and as a component of the ventilatory apparatus. While the importance of the sternum suggests that variations in its morphology can have functional consequences, the relationship between sternal morphology and locomotor mode remains unclear. We assessed the variation in sternal morphology by sampling seven linear measurements of the sternum from 62 extant species of birds. Measurements were standardized by a geometric mean of femur and synsacrum length as a proxy for body size. For locomotion, flight and swimming abilities were coded separately, with six flight and four swimming modes used in total to capture the full range of variation represented in the sample. We applied phylogenetic principal component analysis and phylogenetic flexible discriminant analysis to the standardized measurements in order to compare the variation in sternal morphology between the locomotor modes. The correlation between the sternal variables and the locomotor modes was estimated using phylogenetic multiple analysis of variance. Despite the presence of shared variation, average sternal morphology differed significantly between locomotor modes. All seven sternal variables contributed to group separation, with different values of these variables being respectively associated with flightless terrestrial birds, passerine-type fliers, swimming birds, and continuous-flapping non-swimmers. The sternal features identified in our analyses serve as attachment sites for muscles involved in locomotion, alterations of which can influence locomotor performance by changing the muscular architecture. The correlation between sternal morphology and locomotion may benefit indirect estimations of locomotor mode in extinct taxa and ultimately provide new insights into the drivers of morphological variation.