The ontogeny of the femur and tibia during post-hatching development in the tortoise Chelonoidis chilensis (Chelonia: Testudinidae)

Abstract As animals grow, their bones adapt to changes in size and anatomy, preventing organ failure as body mass increases. This process, known as allometry, is particularly intriguing in terrestrial turtles, where the development of limb bones during ontogeny remains poorly understood. This study examines the ontogeny of the femur and tibia in the terrestrial tortoise Chelonoidis chilensis , focusing on changes in bone proportions, shape, and density from juveniles to adults. Using X-ray imaging, allometric analysis, and geometric morphometrics, the study found that both the femur and tibia exhibit proportional growth in relation to body mass, maintaining consistent allometric relationships throughout ontogeny. Bone density in the femur increases significantly in adult tortoises, likely due to mechanical stress and the need for enhanced structural support. However, the geometric morphometric analysis did not reveal significant changes in bone shape across the ontogenetic stages. These findings suggest that the growth patterns and density adjustments in Chelonoidis chilensis are adaptations to the biomechanical demands of supporting a heavy body in a terrestrial environment. This research provides valuable insights into the developmental biology of terrestrial tortoises, with implications for understanding their evolutionary adaptations.