The terrestrial-aquatic transition impacts endocranial shape in caniform carnivorans

Mammals exhibit remarkable diversity in brain size and morphology, shaped by numerous ecological radiations throughout the Cenozoic. Although previous studies have demonstrated the influence of phylogeny, allometry, and various ecological variables on endocranial morphology in certain mammalian lineages, the extent to which locomotor behaviour influences brain shape evolution remains unclear. The suborder Caniformia, or 'dog-like' carnivorans, is an ecologically diverse clade, containing terrestrial, arboreal, fossorial, and semiaquatic species, each facing a unique set of sensory and motor challenges. Here, we evaluate the impact of phylogeny, allometry, and locomotor mode on brain shape evolution in caniforms. We examine endocranial morphology in 73 species using high-density 3D geometric morphometrics, principal component analysis and phylogenetic comparative methods. Our findings indicate that the terrestrial-to-aquatic transition significantly impacts endocranial shape across caniforms, which is associated with maximum dive duration. This effect is more pronounced in pinnipeds than in other semiaquatic fissipeds, such as mustelids and ursids, likely due to their more derived ecology and greater commitment to aquatic life. Aquatic caniforms tend to exhibit expanded neocortices alongside reduced paleocortices and olfactory bulbs. These morphological changes likely reflect selection pressures acting on both the brain and the surrounding cranial architecture in aquatic environments.