Density and material orientation in ape craniofacial cortical bone

Objectives: Profound similarities exist among the genomes of great apes and humans. Yet, remarkable differences exist in craniofacial structure, suggesting that craniofacial adaptations played an important role in human evolution. The aim of this study is to examine patterns of density and material orientation in the cortical bone of ape craniofacial skeletons. We hypothesize that such patterns are similar to those found in humans.

Methods: Thirty-six cylinders of cortical bone were removed from the mid- and upper faces of adult female cadavers of a chimpanzee and a gorilla. Archimedes principal of buoyancy was used to determine relative density in the bone specimens. Longitudinal ultrasonic velocities were measured around the perimeters of the cylinders at 10„a intervals to determine the orientation of the maximum and minimum stiffness in the plane of the cortical plate.

Results: Density in the gorilla crania ranged from 1675-1958kg/m3 (X=1812;SD=87), which was slightly greater than similar values in the chimpanzee skull (range=1585-1855;X=1747;SD=88.8). These values fall within the range of similar measurements taken from large samples of human crania. Anisotropy varied between regions from highly oriented to isotropic. This pattern is also found in human crania. Orientations of maximum stiffness at highly oriented sites tended to be similar to those in humans.

Conclusion: Density and anisotropy in a limited sample of ape crania were similar to those found in human skulls. The rarity of these specimens precludes larger samples, but the variability found in human crania suggests that some differences may be apparent only after more data is available. (This work is supported by the Baylor Oral Health Foundation and NSF HOMINID grant 0523159).