Biochemical significance of cross-sectional geometry of avian long bones

Cortical area, maximum second moment of area and polar moment were calculated for the long bones of 39 species of birds. Regressions of all these parameters to body mass were established. At the same time, the orientations of the maximum second moment of area were statistically tested. The parameters calculated on humerus and ulna scaled according to the predictions derived from the geometric similarity hypothesis, while those calculated for the long bones of the leg showed higher exponents, very close to the predictions of the elastic similarity hypothesis. Confidence intervals calculated for radius parameters appeared to agree with both predictions. Only for the tarsometarsus was it impossible to establish a global orientation pattern for the maximum second moment of area. In the other cases, the orientation was: sagittal in the radius, posteromedial-anterolateral in the proximal long bones (humerus and femur) and posterolateral-anteromedial in the distal long bones (ulna and tibiotarsus). The implications of the present findings are discussed in terms of the possible correlations between the orientation patterns produced in the cross-sectional geometry of avian long bones and the load carried.