Three-dimensional (3D) Geometric Morphometrics of the Carnivora Axis: Shape variation, Allometry

Abstract

Objective: The axis is an important bone due to its adjacency to various structures and its contribution to the bony structure of the neck, as well as its responsibility for supporting the neck. Given the absence of similar studies in the literature, we sought to investigate the three-dimensional geometric morphometry of the axis. For this purpose, 17 axis bones from 8 different domestic species belonging to the carnivora family were analysed. Materials and Methods: CT images of the bones were converted into three-dimensional (3D) bone surfaces. A total of 102 landmarks were applied to the images recorded with the generalized Procrustes method. Principal Component Analysis (PCA) was performed with SlicerMorph programme. As a result of this analysis, shape variations of the bones were analysed. In addition, Procrustes Distance and Centroid size values of the bones were measured with the same programme. These values were processed into SPSS programme and the effect of age and weight on size was investigated by regression analysis. Results: The shape changes of the bones were analysed from five different perspectives: cranial, caudal, lateral, dorsal, and ventral. These analyses revealed shape variations, which were subsequently explained. Principal component analysis (PCA) showed that PC1 accounted for 21.04% of the total variation, PC2 explained 13.6%, and PC3 contributed 12%. Furthermore, regression analysis demonstrated that age and weight were statistically significant factors influencing centroid size. Conclusion: Principal component analysis revealed significant shape variations in the axis bones of carnivora, particularly in key anatomical features such as the processus transversus and dens, with notable bilateral differences. The allometric analysis showed a significant relationship between centroid size, shape, age and weight, emphasizing the influence of allometry on axis morphology. These findings enhance our understanding of the morphological diversity and functional adaptations of the axis bone in carnivorous mammals.