In a recent publication (see also our recent comment in BioEssays), we demonstrated that marsupials are not, as frequently thought, systematically smaller-brained than placentals. We also showed that partial correlations of gestation length, weaning age, litter size, basal metabolic rate (BMR) and brain size – all adjusted for body size – differ in marsupials and placentals. The difference between these two clades consists of the existence of a partial correlation between BMR and brain size in placentals, which we did not find in marsupials. We suggested that placentals differ from what could be called an ancestral mammalian pattern (we prefer the term ancestral rather than general for reasons of information content and accuracy) by having a placenta, through which increases in maternal BMR could benefit offspring brain sizes.
We agree with Isler’s title assessment in her recent review of our work that marsupials confirm an ancestral mammalian pattern – we hypothesise that it is the placentals that ‘added’ an additional avenue of energetically provisioning the growth of a large brain in their offspring. Also, as advocated by Isler, we took an energetic approach to our work (asking the question as to how the costs of increased brain size are met, by simultaneously testing multiple metabolic and reproductive variables). Isler’s suggestion that allo-maternal care correlates with increases in marsupial brain size also fits our predictions very well. However, we disagree with several other aspects of Isler’s characterisation of our study and her conclusions.