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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.

Most quadrupedal mammals support a larger amount of body weight on their forelimbs compared with their hind limbs during locomotion, whereas most primates support more of their body weight on their hind limbs. Increased hind limb weight support is generally interpreted as an adaptation that reduces stress on primates' highly mobile forelimb joints. Thus, increased hind limb weight support was likely vital for the evolution of primate arboreality. Despite its evolutionary importance, the mechanism used by primates to achieve this important kinetic pattern remains unclear. Here, we examine weight support patterns in a sample of chimpanzees (Pan troglodytes) to test the hypothesis that limb position, combined with whole body center of mass position (COM), explains increased hind limb weight support in this taxon. Chimpanzees have a COM midway between their shoulders and hips and walk with a relatively protracted hind limb and a relatively vertical forelimb, averaged over a step. Thus, the limb kinematics of chimpanzees brings their feet closer to the COM than their hands, generating greater hind limb weight support. Comparative data suggest that these same factors likely explain weight support patterns for a broader sample of primates. It remains unclear whether primates use these limb kinematics to increase hind limb weight support, or whether they are byproducts of other gait characteristics. The latter hypothesis raises the intriguing possibility that primate weight support patterns actually evolved as byproducts of other traits, or spandrels, rather than as adaptations to increase forelimb mobility.

The striking contrast in positional behavior exhibited by lorisids (slow quadrupedalism/suspension) and galagids (leaping/quadrupedalism) is well reflected in their postcranial morphology, particularly in the limbs. Although they exhibit very different spinal postures and movements, vertebral adaptations have been less well explored in these taxa. This study addressed morphological and functional differentiation in the lumbar vertebrae of four species of lorisids and five species of galagids. Linear and angular measurements of lumbar vertebrae were compared among taxa using canonical variates analysis (CVA) in conjunction with pairwise comparisons among selected variables. The results were interpreted in the context of a broader comparative sample, including the addition of indriids to the CVA. Compared to galagids, lorisids have relatively shorter lumbar spinous processes that are more perpendicularly (to caudally) oriented relative to a coronal plane. Lorisids also have relatively wider laminae and more transversely oriented prezygapophyses. These features promote lumbar stability and reflect antipronogrady, multiplane spinal movements, and upside-down suspension. Within lorisids, vertebral body length and height vary with body size, reflecting the additional resistance to bending that is required for larger body sizes. Galagid lumbar shape is influenced by body size, but does not show strong variation in accordance with positional behavior differences as defined here. Galagids, indriids, and lorisids are distinct in lumbar morphology and function, but their similarities in lumbar length reduction are suggestive of antipronograde postures in the common ancestor of the galagids, including those who have shifted to a more quadrupedal locomotor repertoire.

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