How quickly do brains catch up with bodies? A comparative method for detecting evolutionary lag

A trait may be at odds with theoretical expectation because it is still in the process of responding to a recent selective force. Such a situation can be termed evolutionary lag. Although many cases of evolutionary lag have b een suggested, almost all of the arguments have focused on trait fitness. A n alternative approach is to examine the prediction that trait expression i s a function of the time over which the trait could evolve. Here we present a phylogenetic comparative method for using this 'time' approach and we ap ply the method to a long-standing lag hypothesis: evolutionary changes in b rain size lag behind evolutionary changes in body size. We tested the predi ction in primates that brain mass contrast residuals, calculated from a reg ression of pairwise brain mass contrasts on positive pairwise body mass con trasts, are correlated with the time since the paired species diverged. Con trary to the brain size lag hypothesis, time since divergence was not signi ficantly correlated with brain mass contrast residuals. We found the same r esult when we accounted for socioecology, used alternative body mass estima tes and used male rather than female values. These tests do not support the brain size lag hypothesis. Therefore, body mass need not be viewed as a su spect variable in comparative neuroanatomical studies and relative brain si ze should not be used to infer recent evolutionary changes in body size.