Fig. 5: Brain-body scaling in the primate sample.

Here, we plotted the number of standard deviations from the brain volume–body mass regression versus body mass to visualize how they covary among clades and major primate bifurcations (a). Gray shading indicates the 95% confidence interval for the linear regression. We also replicated Jerison’s encephalization quotient (EQ) in our data, describing brain-to-body scaling relative to expectation based on allometric scaling (b). We used the allometry described by Jerison¹⁰⁰ for mammals: \({EQ}=\frac{0.12}{{P}^{2/3}}\), with P = body mass. EQ varied greatly between clades, and somewhat less within them. Deviations from our primate brain-body regression show little within-clade consistency or relation to grouping. *Indicates suspected outliers; **indicates missing values. N = 34 species (63 observations).