Microstructural Variations in the Bone of Pygoscelis antarctica (Aves, Sphenisciformes) During the Postnatal Ontogeny

Ontogenetic changes in bone microstructure provide key information on growth patterns and functional differentiation in birds. This study describes microanatomical and histological variation in the appendicular and axial skeleton across an ontogenetic series of Pygoscelis antarctica. Fifty-two thin sections were analyzed, representing five chicks of different ages, a juvenile, and an adult. Early stages are characterized by thin, highly vascularized cortices and extensive trabecular bone surrounding a large medullary cavity. The chick sequence shows progressive periosteal deposition, trabecular expansion, and increasing osteosclerosis. In the juvenile, secondary compaction and the initial development of the inner circumferential layer appear together with the onset of extensive remodeling. The adult specimen exhibits fully developed outer and inner circumferential layers, lamellar cortical tissue, and marked secondary remodeling, with differential medullary reduction among skeletal elements. Compared with Aptenodytes patagonicus, P. antarctica shows earlier and more pronounced medullary reduction and secondary trabecular compaction in the forelimbs, likely related to its shorter dependency period and earlier initiation of marine locomotion. In contrast, hindlimbs retain a medullary cavity and follow sex-linked pathways of medullary modification: males develop an inner circumferential layer, whereas females show trabecular bone followed by secondary compaction.