Endocranial shape variation and allometry in Euarchontoglires.

While brain size in primates and their relatives within Euarchontoglires is well-studied, less research has examined brain shape, or the allometric trajectories that underlie the relationship between size and shape. Defining these patterns is key to understanding evolutionary trends. 3D geometric morphometric analyses of endocranial shape were performed on 140 species of extant euarchontoglirans using digital cranial endocasts. Principal component analyses on Procrustes shape variables show a clear phylogenetic pattern in endocranial shape, supported by an ANOVA which identified significant differences in shape among several groups (e.g., Platyrrhini, Strepsirrhini, Scandentia, Rodentia, and Lagomorpha). ANOVAs of shape and size also indicate that allometry has a small but significant impact on endocranial shape across Euarchontoglires, with homogeneity of slopes tests finding significant differences in the scaling relationship between shape and size among these same groups. While most of these clades possess a distinct endocranial morphotype, the highly derived platyrrhines display the strongest relationship between size and shape. Rodents show the most diversity in endocranial shape, potentially attributed to their comparatively weak relationship between shape and size. These results suggest fundamental differences in how shape and size covary among Euarchontoglires, which may have facilitated the adaptive radiations that characterize members of this group.

Endocranial asymmetry in New World monkeys: a comparative phylogenetic analysis of morphometric data.

Brain lateralization is a widespread phenomenon although its expression across primates is still controversial due to the reduced number of species analyzed and the disparity of methods used. To gain insight into the diversification of neuroanatomical asymmetries in non-human primates we analyze the endocasts, as a proxy of external brain morphology, of a large sample of New World monkeys and test the effect of brain size, home range and group sizes in the pattern and magnitude of shape asymmetry. Digital endocasts from 26 species were obtained from MicroCT scans and a set of 3D coordinates was digitized on endocast surfaces. Results indicate that Ateles, Brachyteles, Callicebus and Cacajao tend to have a rightward frontal and a leftward occipital lobe asymmetry, whereas Aotus, Callitrichinae and Cebinae have either the opposite pattern or no directional asymmetry. Such differences in the pattern of asymmetry were associated with group and home range sizes. Conversely, its magnitude was significantly associated with brain size, with larger-brained species showing higher inter-hemispheric differences. These findings support the hypothesis that reduction in inter-hemispheric connectivity in larger brains favors the lateralization and increases the structural asymmetries, whereas the patterns of shape asymmetry might be driven by socio-ecological differences among species.