Analysis of the brain sizes of 197 marsupial and 457 placental mammals has found that marsupial mammals (e.g. kangaroos, possums), had relative brain sizes that are at least as big as placental mammals. Previous belief that marsupials have relatively smaller brains appears to be produced by the inclusion of the one outlier group — primates. In both placental and marsupial groups, big brains were correlated to length of maternal care (i.e. lactation). Basal metabolic rate (the energy an animal expends at rest), although correlated with brain size in placental mammals, did not correlate with marsupial brain size. Because brain tissue uses so much energy, it has been assumed that a high metabolism was a prerequisite for a big brain.
The new findings indicate that maternal investment is a more critical factor than metabolic rate. It may also be that primates have been especially advantaged by combining both methods of increasing brain size: fast growth in the womb with the help of the mother’s high metabolic rate (placental method), and slower but lengthy growth after birth with the help of extended lactation (marsupial method).
(2010). Brain size, life history, and metabolism at the marsupial/placental dichotomy.
Proceedings of the National Academy of Sciences. 107(37), 16216 - 16221.