Hind limb scaling in birds and other theropods: Implications for terrestrial locomotion

An analysis of hind limb skeletal elements of non-avian theropods and ground-dwelling birds was performed to reveal patterns of change in shape and proportion with size. When femora of equal length are compared, birds exhibit a significantly larger midshaft diameter than non-avian theropods. As total limb length increases, avian femora become relatively shorter (negative allometry), while those of non-avian theropods become relatively longer (positive allometry). Avian femoral/tibiotarsal ratios are all below 0.8 and decrease with limb size, whereas ratios of non-avian theropods are well above 0.8 and tend to increase with limb size. In addition, avian femora exhibit a unique diameter/length relationship not seen in other theropod hind limb bones. Several studies have shown that within the avian limb, the short, robust femur resists bending to a far greater degree than the relatively longer, slimmer tibiotarsus. This is to be expected, as analyses of running birds show that the femur is oriented relatively perpendicular to the ground reaction force throughout the stride, which would subject it to high bending moments. When compared to birds, non-avian theropods have relatively long, slender femora that do not seem to be built to withstand the forces associated with such an orientation. Reconstructing all non-avian theropods in avian-like poses (subhorizontal femur, knee well flexed) with avian locomotor kinematics (relatively little hip extension at most speeds) ignores major differences in scaling between these groups of organisms.