The comparative method is widely used to understand brain-behaviour relationships in comparative psychology. Such studies have demonstrated functional relationships between the brain and behaviour as well as how the brain and behaviour evolve in concert with one another. Here, the authors illustrate with their data on tool use and cerebellar morphology in birds that such comparisons can be further extended to (a) relate the morphology of a brain region to a behaviour, and (b) provide insight

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... o the function of an often overlooked brain region in comparative cognitive studies, the cerebellum. Their results indicate that tool-using species have a significantly more folded cerebellar cortex, but not a larger cerebellum than non-tool-using species. This marks the first demonstration of an empirical relationship between the folding of a neural structure and a cognitive behaviour and in so doing, provides critical insight into the neural basis of tool use and the role of the cerebellum in cognitive processes.