Comparative Fluctuating Asymmetry and Directional Asymmetry in Four Cattle Skulls.

This study investigates fluctuating asymmetry (FA) and directional asymmetry (DA) in the skulls of four cattle breeds: Eastern Anatolian Red, Southern Anatolian Red, Holstein and Simmental by using geometric morphometrics methods. A total of 89 skulls were analysed (Eastern Anatolian Red, n = 16; Southern Anatolian Red, n = 21; Simmental, n = 24; Holstein, n = 28) using 3D landmark based methods, including Procrustes superimposition, principal component analysis (PCA) and Procrustes ANOVA. PCA showed that the first two axes explained 32.39% of DA variation and 26.2% of FA variation, reflecting the multidimensional nature of cranial asymmetry. The results show that DA accounts for a greater proportion of total shape variation than FA, indicating a consistent asymmetry pattern across breeds. PCA revealed overlapping FA among breeds, with Holstein and Eastern Anatolian Red exhibiting greater variation, indicating higher developmental instability. FA, a marker of developmental instability, was most prominent along the horizontal (x-axis) and vertical (y-axis) planes, particularly in the nasal and orbital regions, suggesting mechanical loading as a possible influence. Procrustes ANOVA confirmed significant breed effects for skull shape (p < 0.0001), but asymmetry was not breed dependent. The analysis revealed that DA is primarily influenced by biomechanical factors, such as head posture, muscle attachments and habitual head movements, particularly in the occipital region. In contrast to masticatory structures, nasal asymmetry was linked to respiratory efficiency and environmental adaptation. These findings provide new insights into the role of genetic and environmental influences on cattle skull development and demonstrate the value of geometric morphometrics methods in capturing subtle morphological variation beyond traditional linear measurements.