We found highly significant differences in the relative volume of all brain regions across populations (all regions p < 0.0001, figure 2 ; electronic supplementary material, figure S7 and table S9), and between sexes (all regions p < 0.05, figure 2 ; electronic supplementary material, table S9).
For the optic tectum and medulla, we found differences in the degree of sexual dimorphism in the relative volume of these regions across populations (interaction between sex and population: p < 0.05; figure 2 ; electronic supplementary material, table S9), although for the remaining regions this interaction remained borderline significant ( p < 0.1 figure 2 ; electronic supplementary material, table S9).