Mother’s care is key to a big brain

The evolution of big-brained mammals may be due to maternal investment, rather than metabolism, according to a new study by scientists at UCL (University College London) and the University of Cambridge. Published today in Proceedings of the National Academy of Sciences (PNAS) the study analysed data sets of the brain sizes of 197 marsupial and 457 placental mammals to test the influences of metabolism versus maternal investment on brain size evolution. Contrary to popular hypotheses, researchers found that marsupial mammals, for example kangaroos and possums, had relative brain sizes that are just as big as placental mammals (dogs, horses etc), and even tend to be bigger-brained in some cases. Big brains in both groups were correlated to length of maternal care, for example a longer period of lactation. However, basal metabolic rate, or the energy an animal expends at rest, did not correlate with marsupial brain size, whereas they did correlate in placental mammals. High metabolism was previously thought to be a requirement for big brains as brain tissue is costly to run. Marsupial brains grow slowly and mainly after birth in the mother's pouch, whereas placental mammal brains grow rapidly during gestation, where they benefit from the high metabolic rate of the mother.