Bones of Modern Waterbirds Could Help Us Better Understand the Lifestyle of Their Ancient Relatives
Many bird species are capable of swimming or diving, but those that spend most of their time in the water often lose their ability to fly (such as penguins) or can only walk awkwardly on land (such as grebes and loons). However, some species are excellent swimmers, flyers, and walkers, allowing them to move efficiently in three significantly different environments (such as ducks and some auk species). Martin Segesdi, a Ph.D. student at the Department of Paleontology at ELTE, supported by the Erasmus+ program, collaborated with the researchers from the Natural History Museum of Paris to explore the relationship between the shapes of limb bones of certain waterbird species and their specific types of movement, as well as the anatomical innovations associated with their locomotion. Their study was published in the journal The Anatomical Records.
For their analysis, the researchers digitized bird limb bones using a 3D scanner and examined the shapes of the bone models using a geometric morphometric approach. In this process, descriptive points (known as landmarks) that are anatomically or geometrically identical were identified on the surfaces of the 3D digital bone models, allowing for a multivariate analysis of their coordinates. The advantage of this method is that it provides an objective overview of the shape variability of the bones and allows for a quantitative comparison of bone characteristics across different species. It also enables researchers to distinguish the effects of phylogenetic relationships and size on bone shape from the effects related to lifestyle, revealing which changes are attributable, for example, to an aquatic lifestyle.
Illustration comparing the shape of ulnae from the study. Although phylogenetic relationships significantly influence bone shape, it was found that, in the case of the ulna, swimming styles also affected its shape (Source: Segesdi et al. 2024).
The study revealed that while phylogenetic relationships have a significant impact on bone shape, statistical analysis confirmed that the shape of the ulna is influenced by specific swimming styles, regardless of phylogenetic relationships. The wing bones of birds that swim underwater using their wings, for example, tend to have more robust joints and more developed triceps muscles, as these features enhance shoulder and elbow stability. In the hindlimbs, larger body size and more stable walking are characteristic of birds with stronger femurs (such as penguins and ducks), but this trait also appears in species that are best adapted for swimming with their hindlimbs (such as grebes). The structure of the leg bones of cormorants and ducks shows similarities that help them assume foot postures advantageous for both walking and swimming.
The study's results contribute to a better understanding of the diversity of bird anatomy and their successful adaptation to different lifestyles. They can also assist in the study of fossilized bird bones and in interpreting our knowledge of now-extinct species, as the lifestyle of extinct vertebrates is often inferred from the morphology of their bones.
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