Anatomical and Volumetric Description of the Baird’s Beaked Whale (Berardius bairdii) brain from Magnetic Resonance Imaging

Baird's beaked whale, a member of the family Ziphiidae, is one of the largest odontocetes, second only in body mass to the sperm whale. Baird's beaked whales are known for their ability to dive to exceptional depths; however, this behavior makes them difficult to observe in their natural habitat and has resulted in major gaps in our understanding of the species. To address part of this gap, this study provides a comprehensive description of the Baird's beaked whale brain using magnetic resonance imaging (MRI). We describe the external and internal neuroanatomy, including sulcal and gyral patterns, and provide quantitative measurements of the cerebral cortex, amygdala, hippocampus, ventricular system, superior and inferior colliculi, cerebellum, the mid-sagittal cross-sectional area of the corpus callosum, the gyrification index, and encephalization quotient. Baird’s beaked whale has a brain organization typical of odontocetes, including a large, exceptionally gyrencephalic neocortex. Both the encephalization quotient and the relative cerebellar volume are lower than in delphinids, consistent with findings in other deep-diving cetaceans. These differences may reflect energy allocation strategies related to diving behavior and body size. By contextualizing these traits within a broader mammalian neuroanatomical framework, this study contributes to our understanding of how ecological pressures may shape brain evolution, particularly in rarely-studied cetacean lineages like the Ziphiidae.