Cranial shape predicts arboreal activity of Sigmodontinae rodents

Brain enlargement is an adaptation in animals that use the space in three dimensions and need to integrate information of complex environments. As consequence, this adaptation can result in differences in skull shape among species with different types of locomotion and level of arboreality. We directly tested for the relation between the cranial shape and arboreality (based on field data) of 11 Sigmodontinae rodent species from a Neotropical savanna (Cerrado). For that, we used geometric morphometrics considering both dorsal and lateral views of the cranium. We also accounted for any possible allometric effects (i.e. the relationship between size and shape) and evolutionary constraints (i.e. phylogenetic signal) on the cranial shape. The dorsal view presented strong allometric effects but no phylogenetic signal. On the other hand, the lateral view showed a weak allometric effect and a significant phylogenetic signal. After controlling for size and evolutionary constraints, we found significant relationships between rodent arboreality and cranial shape for both lateral (r(2)=0.74) and dorsal cranium views (r(2)=0.40). Patterns for both views showed that arboreal rodents present more expanded crania, suggesting larger brains. The dorsal view showed a general cranial expansion in arboreal rodents, whereas the lateral view showed a more rounded and posteriorly expanded cranium. This later suggests a larger cerebellum, an important brain portion responsible for orientation and sense of balance. Our study provides evidence that the cranial shape can be used as a proxy to infer arboreal activity of sigmodontine rodents. Moreover, the observed differences in cranial shape between arboreal and terrestrial species are probably related to brain enlargement in the former driven by the need to cope with a three-dimensional environment.