Comparative morphology of the avian cerebellum: II. Size of folia

Despite the highly conserved circuitry of the cerebellum, its overall shape varies significantly among and within vertebrate classes. In birds, one of the most prominent differences among orders is the relative size of the cerebellar folia. The enlargement/reduction of individual folia is thought to relate to specific behavioral differences among taxa, but this has not been adequately tested. Here, we survey variation in cerebellar folia size among 96 species of birds and test for phylogenetic effects and correlations with behavior using a combination of conventional and phylogeny-based statistics. Overall, we found that phylogenetic history accounts for a significant amount of variation in the relative size of individual folia. Order membership, in particular, accounted for more than half of the interspecific variation in folia size. There are also complex relationships among folia such that the expansion of one folium is often accompanied by a reduction in other folia. With respect to behavioral correlates: (1) we did not find any significant correlations between folia size and reliance on trigeminal input; (2) there was some evidence supporting a correlation between strong hindlimbs and an expansion of the anterior lobe; and (3) there were significant reductions in folia I-III and expansions in folia VI and VII in species classified as strong fliers. This expansion likely reflects increased visual processing requirements in species with rapid and/or agile flight. It therefore appears that folium size is a product of both phylogenetic history and behavior in birds.