Numerical Analysis of the Cerebral Cortex in Diprotodontids (Marsupialia; Australidelphia) and Comparison with Eutherian Brains.

Diprotodontids are a diverse group of Australian metatherians, which occupy a range of ecological niches from nectar and pollen-feeders to grazers and folivores. The group encompasses small-brained nectar-feeding species (Tarsipes) and large-brained grazing and browsing species (macropods). This group of Australian metatherians therefore represents an opportunity to examine how the cerebral cortex has expanded in an adaptive radiation quite independent of that occurring among eutherians. We have used the Nelson Brain Collection and online resources to perform a quantitative analysis of the isocortex, hippocampal formation and olfactory structures in diprotodontids. We found that the scaling relationship between iso- and periallocortical grey matter and brain size, and between subcortical white matter and iso- and periallocortex grey matter, are both almost identical among diprotodontids and eutherians. By contrast, the relationship between gyrification and brain size is strikingly different between diprotodontids and eutherians, with gyrification being much lower for a given brain size among the diprotodontids, although gyrification is much more varied among macropods than other diprotodontids. The scaling of iso- and periallocortical volume with dorsal striatal and dorsal thalamic volume is almost identical among the diprotodontids and eutherians, but the claustrum is smaller, and amygdala larger, for a given brain size among diprotodontids than eutherians. The hippocampal formation and central olfactory areas (anterior olfactory region and piriform cortex) both scale more steeply with brain size among diprotodontids compared to eutherians. Our findings suggest that, although white matter scaling is identical among all therians, there are significant differences between diprotodontids and eutherians in the way that cortical folding and expansion of allocortical structures occurs with brain enlargement.