Gross Anatomy and Vascularization of the Brain of Pacarana (Dinomys branickii)

Background: The pacaranalives in South America and has herbivorous and nocturnal habits. It is a rare specieswith scarce data concerning its morphology and adding more data is important in establishing its vulnerability. The aim was to describe its macroscopic brain anatomy, as well as the brain vascularization. Materials, Methods & Results: Two specimens were available for this study, that were donated post-mortem. The animals were injected with latex and fixed with 10% formaldehyde. Upon exposure and removal of the brain its main features were described. The rhinal fissureis single and the lateral sulcusarises from its caudal part. There are two sagittal sulci, an extensive medial sulcusand a short lateral sulcus. The piriform lobe is vermiformand the rostral part is smaller. The caudal colliculus is larger than the rostral colliculus and they are separated by a sulcus. The cerebellum has oval shape and the flocculuslobe is not conspicuous. The cerebral arterial circlewas analyzed and described. The brain is supplied by the vertebrobasilar system only. The cerebral arterial circleis formed by the terminal branch of the basilar artery, the caudal communicating artery, the rostral cerebral arteryand the rostral communicating artery. The caudal and middle cerebellar arteriesare branches of the basilar artery. The terminal branch of the basilar arteryoriginates the rostral cerebellar arteryand the caudal cerebral artery. From the end of the caudal communicating artery and the beginning of the rostral cerebral arteryarises the middle cerebral artery. Discussion: The cerebral structures related to sensory inputs reflect the species usage of senses, or rather one is intrinsically correlated to the other. The caudal colliculus is larger than the rostral colliculus, as the former is related to hearing and the latter to the vision, this indicates that the visual sense is not so important. Indeed, the animals are nocturnal and have small eyes. The hearing on the other hand is used in social interactions, to indicate combat, threat and defensive situations, for example. The rhinencephalonhas the most conspicuous external sensory structures and the animals use olfactionfor social behaviors, even producing a gland exudate to mark territory. Most brain features are similar with other species in the infraorder Caviomorpha. The brain of the pacaranaresembles the brains of the Patagonian mara, capybara and guinea pig. The common porcupinepresents a different shape to the brain. The guinea pig and the common porcupineare lissencephalic. The animals that present sulci in the brain, follow this order from more to less girencephalic: capybara, maraand pacarana. The rhinal fissureis important because it delimits the rhinencephalonand it is present in all the animals mentioned above. In the mara, the colliculi are exposed in dorsal and lateral views, however, this does not happen for the pacarana. The cerebellum is similar in these species, but the flocculusis more pronounced in the guinea pig. The brain of Caviomorphaspecies is supplied only by the vertebrobasilar system in most species analyzed, as in the capybara, guinea pig, coypus, mara, chinchilla, degu and in pacarana, where the absence of the carotid artery was observed. In some species the rostral cerebral arteryanastomosis in a single branch that runs towards the corpus callosum (degu, capybara, chinchillaand coypus), but in the pacaranathe rostral cerebral arteryis present in both left and right sides, then branching towards the corpus callosum and the splenial sulcus. In summary, the pacaranapresents brain features similar to other Caviomorpha, with some specific species variation.