Fig. 3: Allometric and phylogenetic regressions highlight ansiform hyperscaling relative to the cerebello-cerebral system.

Phylogenetic generalized least squares (PGLS) regressions for cerebellar volume regressed on cerebral volume (a, b), and ansiform regressed on rest of cerebellar (c) and cerebral (d) volumes. Median volumetric data were taken from 34-species (a) and 13-species (b–d). Log₁₀-transformed neuroanatomical measures were plotted and overlaid with regression lines obtained in the respective PGLS models. 95% (black dotted line) and 99% (red dotted line) confidence intervals are provided alongside a fictive isometric scaling relationship with the same intercept (blue line). Exclusion of the isometric scaling relationship from the confidence intervals was taken to indicate significant allometry, as indicated by the asterisks. a, b Cerebellar volumes regressed on cerebral volumes for full data (a) and for the 13 species with complete data (b) both illustrate isometric scaling trending towards hypo-allometry. Lemuriformes and Hominidea were the two clades with the most impressive cerebellar-to-cerebral volume ratios (Fig. 2c) and were thus specifically colored here (Lemuriformes colored in a bold gray; (a–d) to show how ratios were confounded by the primate-general allometry between areas of the cerebello-cerebral system. Although species belonging to these infraorders displayed slightly higher cerebellum-to-cerebrum scaling than the primate sample as a whole (a), most of the differences in ratio resulted directly from allometry. Zooming in, (a) illustrates that while most Hominoidea, including Homo sapiens, Pongo pygmaeus, and Hylobates lar fall on the regression line of the sample, all but one of the Lemuriformes (Lemur catta; on the line) exceed the general trend. Additionally, in both clades, several species approach isometry, with one member of each falling on the isometric line (Gorilla gorilla gorilla and Mirza coquereli). (b) Illustrates reducing of the PGLS slope, connected to the smaller sample of species. c, d Ansiform volume regressed on the rest of cerebellar volume (c) and cerebral volume (d) both illustrate hyper-allometric scaling relationships. Because of strong positive allometry, species with larger cerebella and cerebra are expected to have larger relative ansiforms, directly accounting for the high ratios reported in (Fig. 2e). c, d Both illustrate that Hominoidea do not have uniquely large ansiforms. Although some Hominoidea lie above this steep regression line, so do several other—smaller—species (as colored in green; Mirza coquereli, Aotus trivirgatus, Cebus apella, and Papio hamadryas). NS = non-significant. N = 34 in (a), and N = 13 in (b–d).