Think Big, Evolutionary Allometry as a Major Factor in Rates, Trajectories and Scaling of Morphological Evolution of the Primate Brain Shape

Primates constitute one of the most successful and diverse mammalian clades. One key factor in their diversification is the evolution of their peculiar brain morphology. However, the evolutionary and developmental processes determining the relevant shape changes in the primate brain remain largely unknown. In this study, we used 3D-geometric morphometrics, phylogenetic comparative methods and Bookstein’s novel concept of scaling in shape variation to understand the factors influencing rates, trajectories and scaling of brain shape in a sample of 146 species including members from each major primate clade (excluding Homo). We found that only Hominoidea and Cercopithecinae showed a significant evolutionary allometry after controlling for phylogeny, whereas Strepsirrhini, Colobinae and Platyrrhini did not. However, Hominoidea and Cercopithecinae both showed markedly high rates of morphological evolution, whereas Strepsirrhini and Platyrrhini display a significant slowdown. As a consequence, Hominoidea and Cercopithecinae have different trajectories and magnitudes of shape changes when compared to the remaining clades. Apes, lesser apes and cercopiths tend to have an overall globular brain shape with more developed frontal lobes. Furthermore, there is a large-scale effect (global pattern of variation) of size on brain S212 shape in Hominoidea and Cercopithecinae, whereas size better describes smaller scales of variation (local pattern) in the slowly evolving clades. In conclusion, our results suggest that the evolution of allometry may have favored shape changes at larger scales and promoted the rapid evolution exhibited by Hominoidea and Cercopithecinae. On the other hand, the smaller scale effect of size on brain shape of Strepsirrhini and Platyrrhini might have had a key role in their reduced evolutionary rates.