Neural tissue is nearly an order of magnitude more energetically taxing than other tissue types, so evolutionarily, larger brains would only evolve if they endowed a selective advantage large enough to compensate for the high energy cost. On the molecular level, increases in brain size could be accounted for by various gene duplications and divergences. Here are two examples of specific genes that impact brain size:
1) ASPM codes for a protein that is necessary for the function of the mitotic spindle in neural progenitor cells. Ali and Meier sequenced ASPM exons of 28 primate types and correlated codon changes in each primate lineage with regional brain volumes. They found a positive selection effect of 16 amino acids for cerebral cortex volume but not cerebellar nor whole brain volume. It makes sense that increases in brain density due to ASPM were localized to the cortex because that’s where ASPM is primarily expressed. So, polymorphisms here could account for some regional differences in brain size.
2) DAB1 codes for a protein that is phosphorylated at a tyrosine residue following the binding of reelin and regulates cell positioning in the developing brain. As evidence for this, knock-out studies have shown that without DAB1 mice have misplaced neurons (ectopias) in various brain regions. Pramatarov et al used a mice model that expressed 15% of wildtype DAB1 and found that they had reduced cerebellar volumes. So, polymorphisms at this locus could also account for regional differences in brain size.
Across species, animals with larger bodies tend to have larger brains on an absolute scale. However, as size increases brains become relatively smaller.
For example, Roth and Dicke point out that among larger mammals, humans have relatively the largest brains at 2% of body mass. Whereas shrews, the smallest mammals, have brain sizes of ~ 10% of their body mass. They then chart the log brain mass vs log body weight for 20 mammals:
Most of the increase in brain size in larger animals is probably due to more need for motor / sensory integration, not intelligence. Instead, interspecies intelligence differences are mainly due to increases in the size of the neocortex, as indicated by the correlation between the neocortex to overall brain size ratio and degree of socialization among primates:
Given the huge metabolic cost of neural tissue, another covariate of brain size is metabolic capacity. In mammals, Isler et al show that 2.6% of the variance in brain mass across mammals can be explained by increases in metabolic turnover (indexed by BMR):
Mammals can meet the metabolic requirements of more brain tissue by intaking more energy content or by reducing allocation to other functions like reproduction, locomotion, etc. One hypothesis for the solution of this “energy crisis” is a reduction in gut size and an increase in food nutritional value and digestibility. Exactly how this trade-off works remains an open question as far as I know.
Inspired by CalTech’s Question #15 for cognitive scientists: “Why do some animals have larger brains than others? Why do animals with larger bodies have larger brains? How does brain size relate to metabolism or to longevity?”
Pramatarova A, et al. 2008 A genetic interaction between the APP and Dab1 genes influences brain development. doi:10.1016/j.mcn.2007.09.008
Ali F, Meier R. 2008 Positive Selection in ASPM Is Correlated with Cerebral Cortex Evolution across Primates but Not with Whole-Brain Size. doi:10.1093/molbev/msn184
Isler K, et al. 2006 Metabolic costs of brain size evolution. Biology Letters doi: 10.1098/rsbl.2006.0538
Dunbar RM. 1998 The social brain hypothesis. Evolutionary Anthropology, pdf here.
Roth G and Dicke U. 2005 Evolution of the brain and intelligence. Trends in Cognitive Sciences doi:10.1016/j.tics.2005.03.005