Lateral bending of the lumbar spine during quadrupedalism in strepsirhines

Much research has been devoted to spinal kinematics of nonmammalian vertebrates, while comparatively little is known about the locomotor role of spinal movements in mammals, especially primates. This study, conducted at the Duke University Primate Center, examines the function of lateral spinal bending during quadrupedal walking among a diverse sample of strepsirhines. The taxa studied include Loris tardigradus (1), Nycticebus coucang (1), N. pygmaeus (1), Cheirogaleus medius (2), Varecia variegata (2), Eulemur fulvus (2), and a total sample size of 261 strides. Lateral bending varies among the taxa with respect to both magnitude and effects of velocity, and does not appear to be correlated with body size. In addition, the timing of lateral bending during a stride appears to differ from that reported for other (nonmammalian) tetrapods. On average, maximum lateral flexion occurs just after ipsilateral foot touchdown, which may be functionally associated with touchdown of the contralateral forelimb during diagonal sequence gait. For some of the taxa, lateral flexion coincides more closely with foot touchdown as velocity increases, suggesting a functional role in increasing hindlimb stride length. Both of these timing patterns contrast with those reported for lizards. Finally, although lorids as a group have been described as having a "sinuous" gait, this study shows more pronounced lateral flexion in Nycticebus than in Loris.