Takeshi Yamasaki

Studies on the evolution of avian flight have failed to clarify why wing shape is weakly correlated with flight styles and phylogenetically constrained. Birds seem to have achieved their diverse flight styles owing to changes in the shapes of localized parts of the wing, despite a highly conserved wing outline. The alula, which stems from the first digit of the wing, is a part of the wing that might have enabled various flight styles to develop. To test this hypothesis, we measured alula length and width using museum specimens of 187 species from 21 orders of birds and used a phylogenetic comparative method to investigate the relationship between alula morphology and flight style, body mass, habitat and migratory distance. We found that: (1) phylogenetic signals were weak for the alula width and aspect ratio but moderate for length; (2) alula morphology exhibited an allometric relationship with body mass; and (3) fewer flapping birds had large alulae. Alula morphology might have evolved in response to changes in body mass and flight styles. Variations in the shape of localized wing parts, such as the alula, which modifies airflow around a wing, might be key to the evolution of the diverse flight styles of birds.

Studies have suggested that the brain morphology and flight ability of Aves are interrelated; however, such a relationship has not been thoroughly investigated. This study aimed to examine whether flight ability, volant or flightless, affects brain morphology (size and shape) in the Rallidae, which has independently evolved to adapt secondary flightlessness multiple times within a single taxonomic group. Brain endocasts were extracted from computed tomography images of the crania, measured by 3D geometric morphometrics, and were analyzed using principal component analysis. The results of phylogenetic ANCOVA showed that flightless rails have brain sizes and shapes that are significantly larger than and different from those of volant rails, even after considering the effects of body mass and brain size respectively. Flightless rails tended to have a wider telencephalon and more inferiorly positioned foramen magnum than volant rails. Although the brain is an organ that requires a large amount of metabolic energy, reduced selective pressure for a lower body weight may have allowed flightless rails to have larger brains. The evolution of flightlessness may have changed the position of the foramen magnum downward, which would have allowed the support of the heavier cranium. The larger brain may have facilitated the acquisition of cognitively advanced behavior, such as tool‐using behavior, among rails.