Functional morphology of indrid lumbar vertebrae

Indrids are primarily vertical clingers and leapers but also engage to various extents in climbing, suspensory postures, bimanual movement, bipedal hopping, and quadrupedalism. It has been demonstrated that these behaviors are well reflected in the appendicular anatomy of these primates, while indrid vertebral anatomy has received relatively little attention. In this morphometric study, biomechanically relevant aspects of the lumbar vertebrae of Indri indri, Propithecus diadema, and Propithecus verreauxi were compared to those of Varecia variegata, a large-bodied predominantly pronograde and quadrupedal lemur. Results indicate that, compared to Varecia, the indrids have relatively shorter lumbar vertebral bodies, shorter lumbar regions, more dorsally projecting lumbar spinous processes, and more dorsally positioned lumbar transverse process tips. In addition, indrid lumbar spinous and transverse processes are oriented differently than those of Varecia. Overall, indrid lumbar vertebral morphology converges with that of hominoids, atelines, and possibly lorids, suggesting a lumbar adaptation to upright or "antipronograde" postures that require a reduction in spinal flexibility. The dorsally projecting spinous processes may be related to back muscle extensor power during leaping and/or the maintenance of upright postures. By contrast, the lumbar vertebrae of Varecia resemble those of primates and other mammals that have habitually pronograde postures and emphasize spinal flexibility in the sagittal plane during locomotion.