Penguin osteohistology: the key role of the inner circumferential layer in sex determination

Abstract

In this study, we investigate the osteohistological variation in Sphenisciformes, focussing on the microstructural patterns associated with medullary bone (MB) and the inner circumferential layer (ICL) across several species, including Aptenodytes forsteri, Pygoscelis antarctica, P. papua and Spheniscus magellanicus. Histological sections of both appendicular and axial skeletons were analysed, revealing significant variations in the presence of MB and the ICL across taxa. We aimed to expand the understanding of microanatomy and histology in Sphenisciformes by describing these variations and testing a hypothesis regarding sexual dimorphism based on ICL presence. Key findings include the identification of MB in all specimens, while S. magellanicus exhibited a more limited distribution of MB. Additionally, females of P. antarctica and A. forsteri had an ICL, although this layer was partially remodelled due to osteoclastic activity linked to egg laying. These findings contribute to the ongoing discussion of sex discrimination in extant and extinct penguin taxa and highlight the importance of MB and the ICL in understanding reproductive and developmental histories. Our results suggest that, although the ICL may not be an absolute indicator of sex, its absence or remodelling in adult individuals may serve as a useful tool to differentiate females, even in the fossil record. Furthermore, the variability in MB distribution in S. magellanicus suggests dietary influences, specifically calcium intake, which warrants further investigation under controlled conditions.