The goal of this study was to understand how variability in mechanical loading across ontogeny and between species is reflected in the trabecular and cortical morphology of nonhuman great ape limb bones. Specifically, this study sought to determine whether trabecular and cortical bone responds to changes in substrate use and positional behavior from infancy through adulthood, using chimpanzees as a model. Chimpanzees are an ideal species for this investigation because they have well documented age-related locomotor shifts.

The second chapter used microCT and pQCT data from the proximal femur and humerus of chimpanzees across ontogeny to identify patterns of trabecular morphological change. It then assessed how trabecular and cortical bone correlate across ontogeny. Trabecular and cortical bone operate as a unit, but the degree to which they both respond to locomotor change is unknown in chimpanzees. Results indicated that femoral bone volume fraction (BV/TV) in infants and juveniles was lower than that in adults and adolescents. In the humerus infants had significantly lower BV/TV than all other ages. There were few predictive relationships between cortical and trabecular bone. These findings indicate that the two bone compartments respond differently to locomotor variation.

The third chapter used pQCT data from the femur and humerus of chimpanzees, gorillas, and orangutans to determine whether changes in the proportion of arboreal and terrestrial locomotion were reflected in cortical bone during development. Infant and juvenile chimpanzees, who move in the trees frequently, were predicted to be more similar to arboreal orangutans, while adult and adolescent chimpanzees were predicted to resemble terrestrial gorillas. Relative strength ratios were different between infant and adult Pan, matching predicted changes. These results tracked changes found in Pongo (relatively stronger humerus) and Gorilla (relatively stronger femur) . Infant chimpanzees had rounder bones compared to adults who had relatively elliptical bones. These results suggest that cortical bone adapts to changes in behavior across ontogeny.

The fourth chapter assessed whether intraspecific variability in cross-sectional properties of long bones differed in populations of gorillas and chimpanzees. These two species are closely related and live in similar habitats, but are different in terms of their locomotion, body size, and levels of sexual dimorphism. Overall levels of intraspecific variation were similar within each species, with females tending to be more variable than males. Gorillas tended to be more variable than chimpanzees. There were few differences between bones, indicating that intraspecific variation in the morphology of bones is quite constrained.

This research demonstrated the importance of studying both cortical and trabecular bone, and how the complex relationship between the two bone types and locomotion. Future research should be devoted to understanding the differences in magnitude of response to locomotion in cortical and trabecular bone, and how these responses may contribute to overall skeletal variability. This research will allowed for insights into the degree of plasticity of cortical and trabecular bone in response to locomotor variability across ontogeny. These data are critical for understanding how bone responds to varying loads across the lifespan. Creating trabecular proxies for locomotor variability allows for novel applications of modern skeletal data to understand the locomotor evolution of fossil apes and humans. Developing new tools to interpret the preserved internal skeletal anatomy of modern and fossil taxa will allow for better understanding of locomotor evolution in apes, including hominins to be attained.