Ontogenetic development of limb bone microstructure in the king penguin, Aptenodytes patagonicus (Miller, 1778), with considerations for palaeoecological inferences in Sphenisciformes

Birds have colonized various habitats during their evolutionary history, including the aquatic environment. Several studies have investigated the gross morphological changes of the avian skeleton in response to increasing swimming capabilities, but few have documented in detail the microstructural specializations associated with this process. Bone microstructure is nevertheless often used to reconstruct the ecology of extinct aquatic birds, although this relationship is incompletely understood in modern forms, especially for the extent of intra-specific variability related, or not, to age. In an attempt to fill this gap, we investigated the change in limb bone microstructure from hatching to adulthood in 34 Aptenodytes patagonicus specimens. Our results reveal substantial intraspecific variability, regardless of the ontogenetic stage considered. Although most skeletal growth is achieved in the first few months after hatching, limb bone microstructure undergoes important maturation throughout ontogeny until fledging, which occurs in 1-year-old chicks. Architecture and compactness in hindlimb bones mature earlier than in forelimb bones, reflecting the time offset in the use of musculoskeletal elements involved in terrestrial locomotion in chicks (hindlimbs) and sub-aquatic locomotion in adults (forelimbs). These observations allow critical reassessment of previous palaeoecological reconstructions in extinct Sphenisciformes and set a good comparative framework for rigorous inferences.