Encephalization of Australian marsupials was analyzed using the endocranial volume (ECV) of 52 species of Dasyuromorphia and Notoryctemorphia, 14 species of Peramelemorphia and 116 species of Diprotodontia from Australia and New Guinea and compared with 16 species of Ameridelphian marsupials and 3 species of native and recently introduced Australian eutherian carnivores (dingo, feral cat and feral fox). Linear regression analysis of the relationship between ECV and body weight for marsupials revealed that allometric parameters for these groups are different from those previously derived for samples of (mainly eutherian) mammals, with higher slopes for Dasyuromorphia and Diprotodontia and lower slopes for Ameridelphians and Peramelemorphia. Absolute ECV for small Australian and New Guinea marsupial carnivores (Antechinus and Sminthopsis) were found to be comparable to eutherians of similar body weight, but large marsupial carnivores such as the Tasmanian devil and thylacine had substantially smaller ECVs than eutherian carnivores of similar body weight. Similarly, members of some superfamilies within Diprotodontia (Burramyoidea, Petauroidea, Tarsipedoidea) had ECVs comparable to prosimians, whereas bandicoots, bilbies and many macropods were found to be poorly encephalized. When both encephalization quotient (EQ) and residuals from regression analysis were used to compare relative ECV of extinct/threatened species with common species there were no significant differences for any of the orders of Australian marsupials, suggesting that encephalization is not a major factor in the current extinction crisis for Australian marsupials. Similarly there were no consistent differences in relative ECV between marsupials from New Guinea and associated islands compared to Australia or between arid and non-arid Australian regions for any of the marsupial orders. The results indicate that marsupials are not uniformly poorly encephalized and that small marsupial carnivores and some members of Diprotodontia are of comparable encephalization to eutherians of similar body weight. In particular, honey possums and some gliders show an encephalization level comparable to prosimians, perhaps reflecting convergence in adaptation to similar arboreal niches.
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