Fig. 2. Histograms showing distributions of (A) gestation periods and adult body mass for mammals (first author's dataset) and (B) neonatal mass and adult body mass for primates (data from Smith and Leigh, 1998). In both cases, it can be seen that neither variable meets the criterion of normality of distribution, as indicated by the superimposed curves. (See text for tests of significance.) Body mass (M_b; g); gestation (days).