Determinants of Variation in Trunk Morphology in Anthropoid Primates

Abstract

Bones of the torso are often used to infer postural and locomotor adaptation in fossil primates, based on inferred patterns of variation in overall torso form in extant primates and largely untested assumptions about how individual bones reflect the various forms. Observed variability of the bony pelvis, rib cage and vertebrae among taxa is most frequently hypothesized to reflect mobility and function of the upper and lower limbs. However, the degree of covariation among these torso elements and their relationships to factors other than limb function are poorly understood. To explore the determinants of variation in torso form, we quantified 3D torso shape from reconstructed CT scans of 57 anthropoid primates. We compared these data with linear and landmark data from the vertebrae, sterna, ribs, pelves, clavicles, scapulae and humeri of 256 anthropoids to determine patterns of relationships among torso skeletal elements. Finally, we conducted detailed analysis of integration patterns in the rib cages of 103 chimpanzees and 352 humans. Results reveal that the lower rib cage loosely follows pelvic form and is not as tightly integrated with upper rib cage shape as expected. Stiffness and control of the spine appears to be a much stronger determinant of pelvic form than previously recognized. Upper rib cage shape appears to relate to posture and perhaps other variables rather than locomotion or shoulder girdle morphology. There is also an unexpectedly weak relationship between the morphology of the upper rib cage and shoulder girdle. All together, there is more variation in overall torso shape and in its various components than previously recognized, which has implications for the reconstruction of locomotor behavior in fossil taxa and patterns of evolution in anthropoid primates.Support or Funding InformationNSF BCS 0716244, Leakey Foundation, Wenner Gren Foundation