Ontogeny of locomotion and head movement of the African spurred tortoise (Geochelone sulcata)

Abstract

The aim of this research is to describe the changes of head and neck kinematics as they relate to the growth of the African spurred tortoise (Geochelone sulcata). The basics of tortoise locomotion have been described previously. These studies, however, have not investigated locomotor changes in head kinematics due to body size. Infra‐red motion capture was used to obtain detailed kinematics of the head, limbs, and carapace on six live tortoises ranging in body mass from 10.8 to 66.2 kg. The results of the study showed that while there is no correlation between body size and stride duration (p = 0.198) or duty factor (p = 0.109), there is a significant positive correlation between size of tortoise and stride length (p = 0.016). To assess head stability, we compared the average magnitude of the maximum positive and negative differences between the rotational acceleration of the head and body in the sagittal plane. Head instability, regardless of body size, increases as stride average linear velocity increases (all p < 0.004). This rate of head instability increase with stride velocity was less marked in the larger tortoises compared to smaller tortoises (p = 0.017). Thus, larger tortoises maintain a more constant head stability. Our data do not differentiate increases in muscle mass, larger moment of inertia because of increased head mass, or other possible control factors as the source of this increase in head stability.