Fig. 6: Curvature in the average brain allometry observed within species explains the macroevolutionary metaphenomenon of curvature in the BBM relationship.

a The predicted relationship between brain and body size from an alternative hierarchical model separated into within-species effects (orange) and across-species effects (purple). In this model, we do not incorporate a random effect for species-level slopes (as we did for our main hierarchical model depicted in Fig. 5). The overall curvature in the average within-species slopes is owing to the pattern of diminishing slope value with increasing body size. In (b, c), we plot the posterior distributions of estimated parameter estimates from our alternative hierarchical model. We also include the posterior distribution of parameter estimates from the across species global curve model (from Fig. 4b) for comparison (grey). In this alternative hierarchical model, there is no significant curvature across species.