Developmental differences in cortical bone structure in chimpanzee and human femora reflect early locomotor...

The cortical bone structure of long bone diaphyses changes throughout growth via skeletal modeling and has important implications for bone strength and structural integrity. Ontogenetic trends in diaphyseal structure have been identified in both chimpanzees and humans but it is not yet clear how these trends compare given notable differences in the timing of locomotor development, anatomical development, and life history events. This study presents an interspecific comparison using a sample of humans aged from birth to 19 years (N = 103) and a sample of chimpanzees (P. t. verus) aged from 2 weeks to 13 years (N = 20). Micro-CT scans were collected from the femur and cross-sectional metrics at the midsection were quantified including area measurements (total area, medullary area, and cortical area), cross-sectional shape (Ix/Iy), and intracortical porosity. Using piecewise polynomials, developmental trajectories were compared relative to chronological ages as well as normalized ages (dental development and age at female sexual maturity) and percentage of adult femur length to account for species-specific differences in development and life history. Results indicate a shared pattern of restructuring occurring during early postnatal life in both species, involving a rapid expansion of the medullary cavity in tandem with a transient increase in cortical porosity as a likely result of non-mechanical factors. Distinct differences in Ix/Iy patterns were recorded, which mirrored differences in the timing and pattern of locomotor development between species. Relative to humans, chimpanzees exhibited delayed development of skeletal parameters that is consistent with observational data on the timing of locomotor independence.