Comparative tests of primate cognition: Different scaling methods produce different results

Although early comparative studies supported hypotheses that ecological dem ands selected for primate cognition, later work indicated that social deman ds were more important. One difference between earlier and later studies is that earlier studies scaled brain structures by (A) taking residuals from an interspecific regression of the brain structure in question on body mass , whereas later studies scaled them by (B) taking residuals from an intersp ecific regression of the brain structure in question on another brain struc ture or by (C) taking ratios of the brain structure in question to another brain structure. We conducted a series of comparative tests to explore the possibility that the different methods are responsible for the discrepancy between earlier and later studies. Specifically, we tested the ability of a social variable - group size - and an ecological variable - home range siz e to explain variation in the non-V1 isocortex (isocortex minus primary vis ual cortex) when this structure was scaled with the three different methods . In multiple regression analysis, group size was a better predictor of the non-V1 isocortex with method (B). With methods (A) and (C), however, resul ts were ambiguous: either home range size or group size explained more of t he variation, depending on the inclusion of outliers, the use of independen t contrasts, and whether home range size was scaled relative to body mass. We examine the three scaling methods and find no reasonable basis for prefe rring any of them. Hence, our results do not allow a distinction between so cial and ecological hypotheses. The general implications of our study are t hat (1) previous comparative studies are inconclusive and (2) further resea rch is needed to develop a scaling method where relative measures of brain structure size are demonstrated to correspond with behavioral performance. Copyright (R) 2000 S. Karger AG, Basel.