Postnatal development in a specialized bird: Quantitative and qualitative analysis of pelvic girdle morphological changes in <i>Rhea americana</i> (Aves, Palaeognathae)

The greater rhea (Rhea americana) is the largest bird in South America. It is flightless and cursorial, with a precocial postnatal development. This study aims to describe the postnatal morphological changes in the pelvic girdle of Rhea americana, focusing on ontogenetic scaling and gross anatomy to provide insight into the potential relationship between structure and function in cursorial birds. The pelves of 18 specimens representing four postnatal stages, were studied. Gross anatomical observations and staining methods were used to identify bone and cartilage, providing detailed information about qualitative morphological changes. Quantitative changes were assessed through linear regression analysis using pelvic linear dimensions and body mass to determine allometric growth patterns. The pelvic girdles of immature specimens are characterized by unfused bones, including the vertebrae synsacrales and the medial borders of the ilia, and small cartilaginous areas at the caudal end of the ilium, ischium, pubis, antitrochanter and tuberculum preacetabulare. These immature traits persisted until the most advanced juvenile stages were studied, indicating the slow postnatal growth typical of precocial birds. Similar to other precocial birds, the greater rhea exhibits large bone areas in its pelvic girdle, providing the mechanical strength necessary for locomotion from hatching. The pelvic dimensions showed a combined pattern of allometric and isometric growth related to hindlimb function and body support. The positive allometric growth of the ilium contributed to its increasing verticalization and narrowing, providing an increased dorsoventral surface area for the attachment of the powerful proximal hindlimb muscles. In contrast, the isometric growth of the intertrochanteric width helps in the uniform distribution of loading caused by body mass, providing adequate stability and support.