Flightlessness and long bone allometry in Palaeognathiformes and Sphenisciformes

Palaeognathiformes and Sphenisciformes are the only two major taxa that have lost the capacity for flying. Nevertheless, the consequences of this flightlessness are completely different. While Palaeognathiformes are all running birds, Sphenisciformes are adapted to swimming. To study the morphology of leg and wing bones, length, sagittal and transverse diameters and second moment of area of humerus, ulna, radius, femur, tibiotarsus and tarsometatarsus were measured. Thirty-three specimens from fourteen species of Palaeognathiformes and eleven specimens from seven species of Sphenisciformes were studied. Several non-flying species from other avian orders were also considered. Regressions of all these parameters were calculated with body mass as the independent variable. For each bone, the ratio sagittal diameter/transverse diameter was also calculated. Most of the correlations are significantly different from the isometric condition. From a mechanical point of view, the most interesting result is that the second moment of area of the leg long bones always scales with strong positive allometry, whereas in the case of the wing long bones, the scaling shows high negative allometry. Biometrically, Sphenisciformes display a constant wing long bone morphology, with transverse diameters much larger than sagittal. In contrast, Palaeognathiformes present a highly variable morphology of the wing bones, affecting length and diameters.