Fig. 1. A photograph showing a typical example of the external dorsal curvature of seated macaques (Macaca sylvana). The flexed dorsal curvature of the animal on the right (the `groomer') was used to estimate the approximate total flexion angle of the lumbar column from tangents drawn to the upper and lower lumbar regions (indicated schematically in Fig. 3). This total flexion angle was then partitioned equally amongst all the lumbar—lumbar intervertebral joints. This photograph was reproduced with the kind permission of the photographer (John Bracegirdle).