Sensorimotor Complexity and Cognition as Predictors of Primate Brain Size

OBJECTIVES: As animal brain size increases, cognitive performance generally increases. However, other key brain functions, such as the regulation of somatic processes, processing of sensory input, and planning and initiation of motor actions, are also closely tied to brain size. Thus, the observed correlations between cognitive performance and brain size may be spurious, a suggestion boosted by recent work showing fishes perform as well as primates on various cognitive tests. MATERIALS AND METHODS: Here, we applied multi-response Bayesian phylogenetic mixed models, in which the four main functions of brains were dependent variables predicted by brain size as the independent variable, as well as a phylogenetic path analysis. RESULTS: Among varying sets of primate species, brain size strongly predicted somatic functions (as indexed by body size), with similar, somewhat weaker effect sizes for the other three functional traits. The path analysis suggested that body size also independently affected brain size, which in turn predicted the three non-somatic functions with nearly equal strength. DISCUSSION: This result suggests that the correlation between body and brain size is the sum of two distinct processes: correlated evolution between body and brain size and a brain size effect on somatic functions, although estimating the strength of the two components is currently impossible. We reject the challenge posed by cognition data on fishes, but find that primate cognition tightly coevolved with sensory and motor functions.