Abstract
Birds possess a unique balance organ, the lumbosacral organ (LSO), located in the lumbosacral region of the synsacrum. This organ surrounds the spinal cord and leaves distinct traces of its size and shape on the endocast of the vertebral canal. To date, many questions about the function of the LSO and its implications in bird biology remain. Here, we investigate whether the shape of the synsacral vertebral canal endocast, influenced by the LSO, is related to locomotor habits, pelvic morphology, and phylogeny. We used 2D and 3D geometric morphometrics to characterise the shape of the digital synsacral vertebral canal cast and to test whether its morphology is indicative of locomotor behaviour and pelvic morphology. We also quantified the phylogenetic signal to determine whether phylogeny has an impact on morphology. Our results suggest that the vertebral canal endocast is shaped by the LSO, particularly in predominantly perching birds, where it is proportionally larger than in other locomotor groups. We also show that the pelvic morphology covaries significantly with the vertebral canal morphology. A proportionally larger LSO corresponds to a shorter, wider pelvis, while a smaller LSO corresponds to a longer, more slender pelvis. Finally, in addition to a strong phylogenetic signal in vertebral canal morphology, we identify allometry, indicating that body size also influences LSO morphology.