Ontogenetic shape changes in the pelvis of the Greater Rhea (Aves, Palaeognathae) and their relationships with cursorial locomotion: a geometric morphometric approach

Knowledge of the ontogenetic pattern of morphological features is essential to improve biological interpretations. The study of morphological features of the pelvic girdle and hind limb apparatus throughout growth is an excellent approach to understand how the skeletal morphology and muscles are interrelated during growth in a bird with a specialized mode of locomotion. The Greater Rhea (Rhea americana) is a large cursorial palaeognathous bird with long legs and powerful musculature. The postnatal shape changes of the pelvis of this bird were studied with geometric morphometric techniques, using landmarks and semilandmarks. In addition, regression analyses were used to explore the association between pelvic shape changes with muscle and body mass. The pelvises of 16 specimens of Rhea americana from 1 month old to adulthood were studied in dorsal and lateral views. Noticeable differences in pelvic shape were noted between ages, particularly in lateral view. In young birds, the pre‐ and post‐acetabular ilium was subequal in length, whereas in adults the pre‐acetabular ilium became shorter. In dorsal view, the main shape changes observed were the progressive thinning of both ilium portions and the elongation of the vertex craniolateralis ilii from chicks to adulthood. In this view, the only clear differentiation was between young and adult birds. Shape differences were influenced by body mass and pelvic muscles; the post‐acetabular muscle mass explained the highest percentage of the variation. The specialized locomotion of Greater Rhea is reflected in their pelvic musculoskeletal system, in which the change to a longer post‐acetabular ilium correlates with the growth of the powerful post‐acetabular muscles. The actions of these muscles provide the necessary strength to support the body mass, minimize the body swinging movements and propel the body forward during locomotion. Bone morphology is affected by the forces produced by body mass and the muscle activity, demonstrating the presence of common growth mechanisms, which are primordial and gave rise to a functional and properly proportioned adult. During postnatal growth the pelvis of Rhea americana undergoes marked changes, with widely differing morphologies between young birds and adults that are linked to the growth of the thigh muscles and with the increase in body mass.