Possible link between brain size and flight mode in birds: Does soaring ease the energetic limitation of the brain?

Sociality, ecology and developmental constraints predict variation in brain size across birds

Conflicting theories have been proposed to explain variation in relative brain size across the animal kingdom. Ecological theories argue that the cognitive demands of seasonal or unpredictable environments have selected for increases in relative brain size, whereas the 'social brain hypothesis' argues that social complexity is the primary driver of brain size evolution. Here, we use a comparative approach to test the relative importance of ecology (diet, foraging niche and migration), sociality (social bond, cooperative breeding and territoriality) and developmental mode in shaping brain size across 1886 bird species. Across all birds, we find a highly significant effect of developmental mode and foraging niche on brain size, suggesting that developmental constraints and selection for complex motor skills whilst foraging generally imposes important selection on brain size in birds. We also find effects of social bonding and territoriality on brain size, but the direction of these effects do not support the social brain hypothesis. At the same time, we find extensive heterogeneity among major avian clades in the relative importance of different variables, implying that the significance of particular ecological and social factors for driving brain size evolution is often clade- and context-specific. Overall, our results reveal the important and complex ways in which ecological and social selection pressures and developmental constraints shape brain size evolution across birds.

Digest: Brain or brawn: Trade-offs between brain size and flight mode in birds

Brain size is extremely variable across species, and its evolution depends upon the calorific trade-offs between it and other organs and activities. Shiomi investigated potential brain size trade-offs with different flight modes in birds. Flight can be energetically expensive, and costs are especially high with powered flight. This comparative study indicated that migratory birds employing less energetic modes of flight had relatively larger brains than migratory birds using powered flight, suggesting that brain size is impacted by the energetic costs of flight.

Relationship between flightlessness and brain morphology among Rallidae

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.