Figure 2.

Cube-root slopes are averaged across available studies to produce robust measures of relative organ growth in 10 eutherian species (electronic supplementary material, table S1). (a) Eutherian variation in average whole body and organ cube-root slopes; brain slopes exhibit the lowest degree of variation among the organs studied. (b) Brain cube-root slopes in 10 eutherians, two marsupials, and two birds. Species are ordered by increasing adult brain size, which is unrelated to brain acceleration. Relative basal metabolic rate in ml O₂ g⁻¹ h⁻¹ (BMR), placental invasiveness (green, haemochorial; red, endotheliochorial) and precociality/altriciality (P/A) are unrelated to brain acceleration. Eutherian ranges indicate variation in estimates from individual studies (electronic supplementary material, figures S6–S17). (c) OLS regression models predicting organ slopes from whole-body slope in the eutherian sample (solid lines; n = 10). Body acceleration significantly predicts acceleration in every organ except the brain. In phylogenetic models (electronic supplementary material, tables S6), brain acceleration is predicted by body acceleration (p = 0.043), but much more weakly than other organs are (all p < 0.001).