Fig. 6: Replication of allometric relationship between the cerebellum and cerebrum in the Stephan collection.

The Stephan et al.⁸² collection reported cerebellar and cerebral volumes for 34 species available in 10kTrees Arnold et al.¹⁴². Ancestral character estimations (ACEs) (a) and phylogenetic generalized squares regression (PGLS) (b) were repeated in this complementary dataset. Despite only partial overlap in species, ACEs for cerebellar-to-cerebral volume ratios (a) largely mirrored the main analysis (Fig. 2c). PGLS regression between cerebellar and cerebral volumes in the Stephan collection (b) illustrated generally similar scaling as the primary analysis (Fig. 3a), albeit reduced at approximately 0.92 (black line), and with its 95% confidence intervals (black dotted line), but not 99% confidence intervals (red dotted line), excluding isometry (blue line). All apes and lemurs fell on or above the regression line, approaching isometry and mirroring the main analysis (Fig. 3a). Altogether, the analyses again show how allometry causes diverse ratios between brain areas to arise across species. Legend: apes are colored as in Fig. 3a, with legend again shown here in (b). Lemuriformes are colored here as well, with solid grays representing species overlapping with the main analysis, and solid blacks indicating lemur species unique to the Stephan dataset (a, b). Lastly, other species overlapping between both datasets, that were not specifically considered, are colored a shaded gray (a, b). N = 34 species.