The relationships between brain regions and forelimb dexterity in marsupials (Marsupialia): a comparative test of the principle of proper mass

A behavioural index of forelimb dexterity and comparative statistics were u sed to analyse the relationships between proximal (shoulder, upper and lowe r forelimb) and distal (wrist, forepaw, digits) forelimb dexterity and four aspects of brain morphology (overall brain, cortex, cerebellum and telence phalon sizes) in 18 species of marsupials. On the basis of the principle of proper mass, it was expected that an increase in forelimb dexterity (eithe r proximal or distal) would be positively correlated with the size of the b rain and the three brain components. Using independent contrast analysis to remove the effects of phylogeny revealed three significant correlations be tween: cortex size and distal dexterity, cerebellum size and proximal dexte rity, and telencephalon size and distal dexterity. The relationship between cortex size and distal dexterity was subsequently corroborated by Spearman rank correlations. These results suggest that the execution of finely coor dinated forelimb movements may not be dependent upon overall brain size, bu t may be dependent upon the size of brain components, thus supporting the p rinciple of proper mass.