Proximate mechanisms underlying the coevolution of diet quality and relative brain size in primates

Multiple primate species, including humans, have evolved brains that are surprisingly large relative to their body sizes. Studies of this variation have focused on either proximate (how) or ultimate (why) explanations by correlating species-average brain sizes with, e.g. the rate of genetic changes or certain socioecological variables, respectively. Here, we combined proximate and ultimate perspectives to identify genes that modulated the coevolutionary relationship between diet quality and relative brain size in primates. For n = 50 species, we estimated selection pressure (i.e. root-to-tip dN/dS) for approximately 8K genes and collected brain size, body size and diet quality data. We first used this novel dataset to build on previous studies and bolster findings that neurogenesis-related genes facilitate evolutionary changes in brain size. We then applied phylogenetic partial correlation analysis (to identify genes correlated with both brain size and diet quality) and phylogenetic path analysis (to compare different causal models). We found dozens of genes that may have facilitated the coevolution of diet quality and brain size in primates and show that these genes are involved in neurodevelopment and energy metabolism. This is likely to reflect that higher-quality diets provide more energy to grow and maintain metabolically expensive brains. Our novel approach provides new insight into the drivers of primate brain size evolution.This article is part of the Theo Murphy meeting issue 'Selection shapes diverse animal minds'.