Prezygapophyseal articular surface morphology in the galagid tail : the influence of tail use and vertebral level

Tail use among primates is diverse, with both behavioral and mechanical functions. Primates with tails use them during locomotion to maintain balance on unstable substrates and to direct body motion while leaping. Anatomically, the caudal vertebrae vary based on tail use, although the majority of tail anatomy studies have focused on prehensile tails, with all other forms of tail use lumped into 'non-prehensile'. This distinction conceals the diversity of tail use in the 'non-prehensile' category. For example, vertical clinging and leaping primates use their tails to counterbalance the forces of their body during a leap, which likely impacts their caudal vertebral anatomy in comparison with other primates. The zygapophyses are likely functionally important in tail movements, as they form the synovial joints between the caudal vertebrae. This study evaluates variation in zygapophyseal articular surface morphology between vertical clinging and leaping primates and those that are primarily arboreal quadrupeds. [Micro] CT scans of eight species of galagids were segmented and the proximal caudal vertebrae (those bearing zygapophyses) were extracted as 3D surfaces. The prezygapophyseal articular surfaces were cropped from the 3D surfaces, and measurements of overall curvature, orientation relative to the sagittal plane, and surface area were quantified. Variation was evaluated based on degree of vertical clinging and leaping specialization and vertebral level. Results showed no significant difference in prezygapophyseal articular surface morphology among locomotor specialization categories but significant differences across vertebral levels. These results suggest that locomotor behavior is potentially not the main driver of variation in caudal vertebral anatomy, or that galagids use their tails in similar ways regardless of their locomotor specialization. The results also suggest that there may be variation in movement patterns along the proximal tail. Future analyses of prezygapophyseal articular surface anatomy would benefit from a more diverse, larger sample, and further research into the kinematics of tail use during locomotion.