Figure 2. Brain mass is a better fundamental predictor of time to weaning than is female body mass.

The continuous independent variables ‘female body mass’ and ‘adult brain mass’ were evaluated as potential predictors of time to weaning. Time to weaning (in days post conception; PC) was plotted as a function of female body mass (Body Mass, in grams) and adult brain mass (Brain Mass, in grams), in left and right panels, respectively. Note the log-log scales. Sample is as in Fig. 1 and Table S1 (N = 67). Double circle: humans (this value represents the mean value for 46 natural fertility societies; see Fig. 5 [5]). Solid line: Model II linear regression (reduced major axis) on all species. R²- and p-values are given in the diagrams. R²-values indicate amount of variance accounted for by the respective model. Adult brain mass accounted for a larger amount of variance in time to weaning, demonstrating that this parameter is a better predictor of time to weaning than is female body mass.