How does the inner structure of the limb bones of aquatic birds relate to their locomotor abilities?

Aquatic birds encompass diverse ecologies and locomotion types. Some of them are flightless; others lack effective terrestrial locomotion, whereas numerous species perform well in water, on land, and in the air. Bone microanatomy provides insight into bone functional adaptations to different locomotor strategies. Previous studies on aquatic birds have relied mainly on 2D transverse sections. In this study, we aim to qualitatively and quantitatively analyze the microanatomical characteristics of limb long bones using several virtual sectional planes and whole-bone measurements to investigate in more detail the link between inner structure and locomotor abilities. In the humerus, femur, and tibiotarsus, whole bone compactness is higher in species with better diving abilities, likely reflecting adaptations to counteract buoyancy and drag. In these bones, flightless penguins exhibit the highest compactness, consistent with the release from flight-related constraints. The tarsometatarsus shows a distinct pattern and is more compact in foot-propelled and less terrestrial birds, whereas non-foot-propelled, terrestrial penguins show a lower compactness than in their other bones. Trabecular structure and orientation in the humerus appear to reflect general wing kinematics and wing-propelled locomotion, while hindlimb bones show structural traits probably related to body weight support and foot-propelled swimming. These findings contribute to a better understanding of the adaptations of the birds' skeletal system to various lifestyles and could prove useful in inferring the ecology of extinct species.