Modularity of the Neck in Birds (Aves)

The neck connects the head and the trunk and is the key structure allowing all movements of the head. The neck morphology of birds is the most variable among living tetrapods, including significant differences in the number and shape of the cervical vertebrae. Despite these differences, according to the literature, three morphofunctional regions (i.e., modules) have been identified along the neck. However, this regionalization has not been quantitatively tested through a geometric morphometric approach applied to the cervical vertebrae. Based on the examination of 187 cervical vertebrae belonging to 16 species with various ecologies, we revealed a common modular structure of the neck using 3D surface geometric morphometrics. We adopted an approach without a priori clustering to identify modules along the neck. The phylogenetic influence on each module was tested. Then, each module was digitally reconstructed as a 3D vertebral model, and postural characteristics were studied. We characterized 9 modules: 7 are transpecific, being shared by at least 2 and up to 15 species. Two modules are specific to species with particularly long necks. The modularity pattern appears to be tightly linked to morphofunctional aspects and partially to phylogeny. In contrast, feeding ecology seems to be more closely related to the chaining of modules (the neck) than to the modules themselves. A study of postural properties revealed that each modular unit exhibits a characteristic curvature. Overall, the modular structure of the neck corresponds to the three traditional functional regions. However, the results also revealed unexpected pattern complexity, including subdivisions within these regions. The study of the patterns of modularity is therefore a relevant approach for challenging the three-functional-region hypothesis and allowed us to identify the structure of the diversity of the necks of birds.