The brain endocast of the Canary Islands giant rats (Canariomys, Muridae, Rodentia): paleobiological and evolutionary implications

Insular mammal faunas have been the focus of numerous studies in evolutionary biology, specifically regarding the patterns of dwarfism and gigantism. Previous work has shown either increase or decrease in relative brain size in various clades, including elephants, hippos, lagomorphs, bovids, and multituberculates. Decrease in specific senses such as audition and vision were also observed. In this study, we describe the virtual brain endocasts of the giant rats of the Canary Islands, Canariomys bravoi and Canariomys tamarani, and make morphological and quantitative comparisons with 10 extant mainland and insular Murinae. We measured endocranial volumes and the relative sizes of brain regions, including the olfactory bulb and petrosal lobule volumes and the neocortical and paleocortex surface areas. Our results show that intraspecific variation was higher in Ca. bravoi compared to its closest extant relative Arvicanthis niloticus, which supports the idea that phenotypic variability is more prevalent on islands than on the continent. Canariomys tamarani could represent an intermediate form between Arvicanthis and Ca. bravoi, as previously hypothesized. The midbrain exposure in Ca. bravoi is likely a derived feature due to the reduction in the neocortex. The relatively small size of the olfactory bulbs in Canariomys might be related to decreased predation risk. The relatively smaller petrosal lobules in Ca. tamarani and insular extant rodents could have resulted from a shift to slower locomotion compared to their mainland relatives. Overall, we show that, as in the insular lagomorph Nuralagus rex, decrease in various senses has also occurred in rodents living on islands.