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To understand the mechanical effects of different modes of locomotion on the femoral neck of chimpanzees, we investigated the cross-sectional morphology of the femoral neck of 4 chimpanzees (Pan troglodytes schweinfurthii) collected from the Mahale Mountains, Tanzania. We performed serial computed tomography (CT) scans of the neck from the femoral head to the base of the neck perpendicular to the long axis of the neck. We measured the cortical thickness of the serial 5 cross sections of the neck region every 45°around the circumference, i.e., 8 points per section, and examined the cross-sectional properties of the mid-section. When we compared the superior and inferior parts of the cortical thickness of the femoral neck, the inferior part exhibited the greatest cortical thickness whereas the superior part had the smallest values in every specimen. Researchers have also observed such regional differences between superior and inferior cortical thicknesses in bipedal humans and other primates, although these differences are not as large in the chimpanzee as in bipedal hominini. The present study differed from the past study on hominini and chimpanzees in that the superior anterior (SA) part exhibited greater cortical thickness in chimpanzees. We believe these observations reflect the structural strengthening of parts of the chimpanzee femoral neck that is needed to accommodate the mechanical loads imposed by arboreal vertical climbing and terrestrial quadrupedal and bipedal locomotion.

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An osteometric approach was used to demonstrate the relationship between vertebral body morphology and bipedal standing using 17 rats, which had been divided into control and exercise groups. Only the latter group (n = 9) performed a series of bipedal standing exercises using operant conditioning. Statistical analysis was conducted to allow for inter-group comparisons with respect to six linear dimensions and five indices for each of the 24 vertebral bodies, from the third cervical through the last lumbar vertebrae. Detected effects of bipedal standing exercises on the vertebral body were as follows: (1) dorsal height decreased from the caudal thoracic to the lumbar vertebrae; (2) the dorsoventral diameter of the cranial surface increased at the thoracolumbar level; (3) the dorsal-to-ventral height ratio decreased in the lumbar vertebrae; (4) the dorsoventral-to-transverse diameter ratio of the cranial surface increased in the middle thoracic and lumbar vertebrae; and (5) the height-to-dorsoventral diameter ratio decreased in the fifth thoracic vertebra, and from the tenth thoracic to the fourth lumbar vertebrae. In vertebral bodies at the thoracic and lumbar levels, experimentally induced osteological changes such as wedging, dorsoventral elongation, and craniocaudal robusticity were discussed in terms of their comparative morphology in humans and quadrupedal mammals, and then compared with those of a bipedally trained Japanese macaque.

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