3D Carapace Asymmetry in Tortoises: A Geometric Morphometric Analysis

Abstract

Bilateral symmetry is a defining feature of the tortoise carapace; however, perfect symmetry is rarely achieved in biological systems. This study aimed to quantify bilateral carapace asymmetry in two tortoise species, the Leopard tortoise (Stigmochelys pardalis) and the Greek tortoise (Testudo graeca), using three-dimensional (3D) landmark-based geometric morphometrics within an object-symmetry framework. High-resolution 3D surface models of intact carapaces from 19 adult individuals were acquired using a handheld scanner, and homologous landmarks were analyzed following Generalized Procrustes Analysis. Shape variation was decomposed into symmetric and asymmetric components to evaluate fluctuating asymmetry (FA) and directional asymmetry (DA). Procrustes-based shape ANOVA revealed significant among-individual variation in symmetric carapace shape and a strong side effect consistent with directional asymmetry across the combined dataset. Both species exhibited significant DA, indicating the presence of consistent left–right shape biases in the carapace. Fluctuating asymmetry was detectable in both species; although descriptive differences in FA magnitude were observed between species, these differences were not statistically supported with the current sample size. Multivariate analysis of asymmetric shape indicated a tendency toward species differentiation. These findings demonstrate that the tortoise carapace exhibits both random and consistent components of asymmetry that can be effectively quantified using 3D geometric morphometric methods. Beyond developmental and evolutionary implications, establishing species-informed reference patterns of carapace asymmetry has practical relevance for veterinary medicine, particularly for clinical assessment and symmetry-based digital reconstruction workflows used in shell repair and prosthetic design. The results highlight the value of object-symmetry approaches for advancing both biological understanding and clinically oriented applications in chelonian morphology.

Keywords

• Carapace asymmetry
• Directional asymmetry
• Fluctuating asymmetry
• Geometric morphometrics
• Tortoises
• 3D shape analysis

Ethical Statement

Ethical approval was not required for this study, as all procedures were non-invasive and exempt from ethical review according to the decision of the Istanbul University–Cerrahpaşa Animal Experiments Local Ethics Committee (Decision No. 2024/57).

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