The evolution of brain size and structure in primates

Abstract

The pressures and constraints influencing the wide variation in primate brain size and composition are hotly contested. Comparative biologists have proposed many alternative hypotheses with no consensus yet emerging. This thesis uses phylogenetic comparative techniques and new data to explore the core issues in primate brain evolution; examining how behavioural ecology is associated with brain size and structure variation and what life history correlates reveal about possible developmental mechanisms producing this variation.
The thesis raises a number of important issues for the field. Firstly, evidence of selection at the level of individual structures independently of overall brain size further challenges the utility of whole brain size as a meaningful measure in comparative enquiry. Secondly, by analysing multiple datasets, I demonstrate that fluctuations in data quality are a major cause of inconsistency in results. Finally, the pursuit of explanatory frameworks based on single niche dimensions appears to yield unclear results; contributing to the lack of consensus in the literature. The concept of adaptive syndromes of correlates, while more difficult to operationalise, is likely more meaningful in terms of selection on function.
The findings demonstrate different patterns of covariation of structures across orders and varying correlates of individual structures within primates. This suggests that primate brain evolution has been characterised by the mosaic evolution of individual structures in response to ecological, social and developmental factors, and that selection on function is the primary cause of the observed phenotypic variation. Life history traits were also associated with structure size in a manner predicted from their developmental trajectories, suggesting that selection induces variation in brain composition by modifying the duration of specific life history phases to adjust the relative growth of individual structures.