# Mandibular Shape Variation, Allometry and Modularity in Adult Mesocephalic Dogs (Canis lupus familiaris): Insights into Morphological Integration and Animal Anatomy

## TL;DR

This study explores how the shape of dog mandibles varies, revealing modular patterns and limited size-related changes, which can aid veterinary anatomy and research.

## Contribution

The study introduces a reproducible geometric morphometric analysis of mandibular modularity in mesocephalic dogs.

## Key findings

- Mandibular shape variation is organized into modular regions like the ramus and coronoid process.
- Shape variation is only slightly influenced by size (2.34% allometric effect).
- Two statistically distinct mandibular configurations were identified in mesocephalic dogs.

## Abstract

Understanding how the dog’s mandible varies in shape is important for veterinary anatomy, since this bone plays a key role in chewing, bite strength, and dental alignment. In this study, we analyzed the shape of 168 mandibles from adult mesocephalic dogs—those with balanced head proportions—using geometric morphometrics, a method that captures subtle shape differences through digital landmarks. The results showed two main mandibular configurations that overlapped widely but differed in the height of the coronoid process and the thickness of the mandibular ramus. Shape variation was only slightly related to size, meaning that larger mandibules did not necessarily differ in form. The mandible also showed a modular structure, composed of regions that vary partly independently, such as the ramus and angular process. These findings help explain how the dog’s mandible can be both functionally stable and flexible, adapting to different breeds and feeding behaviors. This approach improves anatomical teaching, supports diagnosis of mandibular asymmetries, and provides a reproducible method for veterinary, archaeological, and evolutionary studies.

The mandible of domestic dogs represents a key structure in veterinary anatomy. This study tested the hypothesis that mandibular shape variation in adult mesocephalic dogs follows a non-random modular pattern with limited allometric influence. A total of 168 dry mandibles from academic osteological collections were analyzed using geometric morphometrics. Four anatomical landmarks and two curves of sliding semilandmarks were digitized and processed through Generalized Procrustes Analysis. Principal component analysis revealed that 62.7% of total variance was concentrated in the first two axes, associated with the coronoid height, ramus robustness, and curvature of the mandibular body. Cluster and Canonical Variate Analyses identified two overlapping but statistically distinct configurations, reflecting the intrinsic morphological diversity of mesocephalic dogs. Procrustes regression confirmed a significant yet low allometric effect (2.34%), while modularity tests based on RV coefficients supported a structured organization involving the ramus, coronoid, and angular processes (processus angularis mandibulae) as relatively independent modules. These results indicate that mandibular shape variation is hierarchically organized rather than random, highlighting the coexistence of integration and modular independence within the masticatory apparatus. Beyond its morphometric contribution, this study provides a reproducible anatomical baseline for veterinary and comparative research, facilitating future analyses of sexual dimorphism, functional adaptation, and surgical applications.