Research on the conditions favoring the evolution of complex cognition and its underlying neural structures has increasingly stressed the role of environmental variability. These studies suggest that the ability to learn, behave flexibly and innovate would be favored under unpredictable variations in the availability of resources, as it would enable organisms to adjust to novel conditions. Despite the growing number of studies based on the idea that larger-brained organisms would be better prepared to cope with environmental challenges, direct testing of the association between brain size and environmental variability per se remains scant. Here we focus on Neotropical parrots as our model group and test the hypothesis that if relatively larger brains were favored in climatically variable environments, larger-brained species should currently tolerate a higher degree of environmental uncertainty. Although we show that there are also other factors underlying the dynamics of brain size variation in this group, our results support the hypothesis that proportionally larger-brained species are more tolerant to climatic variability, both on a temporal and spatial scale. Additionally, they suggest that the differences in relative brain size among Neotropical parrots represent multiple, recent events in the evolutionary history of the group, and are particularly tied to an increased dependence on more open and climatically unstable habitats. As this is the first study to present evidence of the link between brain size and climatic variability in birds, our findings provide a step towards understanding the potential benefits underlying variation in brain size and the maintenance of highly enlarged brains in this and other groups.

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