Comparative morphology of the avian cerebellum: I. Degree of foliation.

Despite the conservative circuitry of the cerebellum, there is considerable variation in the shape of the cerebellum among vertebrates. One aspect of cerebellar morphology that is of particular interest is the degree of folding, or foliation, of the cerebellum and its functional significance. Here, we present the first comprehensive analysis of variation in cerebellar foliation in birds with the aim of determining the effects that allometry, phylogeny and development have on species differences in the degree of cerebellar foliation. Using both conventional and phylogenetically based statistics, we assess the effects of these variables on cerebellar foliation among 91 species of birds. Overall, our results indicate that allometry exerts the strongest effect and accounts for more than half of the interspecific variation in cerebellar foliation. In addition, we detected a significant phylogenetic effect. A comparison among orders revealed that several groups, corvids, parrots and seabirds, have significantly more foliated cerebella than other groups, after accounting for allometric effects. Lastly, developmental mode was weakly correlated with relative cerebellar foliation, but incubation period and fledging age were not. From our analyses, we conclude that allometric and phylogenetic effects exert the strongest effects and developmental mode a weak effect on avian cerebellar foliation. The phylogenetic distribution of highly foliated cerebella also suggests that cognitive and/or behavioral differences play a role in the evolution of the cerebellum.