Comparative intralimb coordination in avian bipedal locomotion.

Analyses of how intralimb coordination during locomotion varies within and across different taxa are necessary for understanding the morphological and neurological basis for locomotion in general. Previous findings suggest that intralimb proportions are the major source of kinematic variation that governs intralimb coordination across taxa. Also, independence of kinematics from habitat preference and phylogenetic position has been suggested for mammals. This leads to the hypothesis that among equally sized bird species exhibiting equal limb proportions, similar kinematics can be observed. To test this hypothesis, the bipedal locomotion of two distantly related ground-dwelling bird species (Eudromia elegans and Coturnix coturnix) and of a less terrestrial species (Corvus monedula) was investigated by means of a biplanar high-speed X-ray videographic analysis. Birds exhibited similar intralimb proportions and were filmed over a broad range of speed while moving on a treadmill. Joint and limb element angles, as well as pelvic rotations, were quantified. Regarding fore-aft motions of the limb joints and elements, a congruent pattern of intralimb coordination was observed among all experimental species. The sample of species suggests that this is largely independent of their habitat preference and systematic position and seems to be related to demands for coping with an irregular terrain with a minimum of necessary control. Hence, the initial hypothesis was confirmed. However, this congruence is not found when looking at medio-lateral limb motions and pelvic rotations, showing distinct differences between ground-dwellers (e.g. largely restricted to a parasagittal plane) and C. monedula (e.g. increased mobility of the hip joint).