Significance of Evolutionary Lags in the Primate Brain Size/Body Size Relationship

INTRODUCTIONThe original brain lag hypothesis proposed that primate brain evolution depended on spare energy derivative of savings of scale enabled by increasing body size. Deaner & Nunn [1] concluded that, in fact, there was no evidence for a brain lag. However, their result may have been due to a number of possible confounds in their analysis. METHODSI revisit their analysis to test for potential confounds using updated datasets. I also ask how primates paid for the energy costs incurred by changes in brain and body mass, and whether the impetus for these changes was predation risk. Finally, I ask whether the observed patterns explain the brain/body size ratio trajectory observed in fossil hominins. RESULTSI show that using statistically more appropriate statistics and updated data yields a significant brain lag effect. However, contrary to the original brain lag hypothesis, the brain/body ratio does not converge back on the allometric regression line, but continues to evolve beyond it. Increases in brain size are correlated with exploiting large group size rather than body size as the principal defence against predation risk, with significant growth in brain size (but not body size) only being possible if species adopted a more frugivorous diet. Finally, I show that hominins followed a similar trajectory from an australopithecine baseline that fell on the relevant allometric regression. CONCLUSIONThe brain lag effect is much more complicated than the original hypothesis proposed, with a distinctive switch from body to brain over evolutionary time.