Adaptation to the aquatic environment: from penguin heart rates to cetacean brain morphology

The evolutionary process of adaptation to the aquatic environment has dramatically modified the anatomy and physiology of secondarily-aquatic, air-breathing seabirds and marine mammals to address oxygen constraints and unique sensorimotor conditions. As taxa that have arguably undergone significant evolutionary transformations, deep-diving sphenisciforms (penguins) and obligatorily aquatic cetaceans (whales, dolphins, and porpoises) provide an excellent opportunity to study such physiological and anatomical adaptation. Investigation of heart rates of free-ranging emperor penguins (Aptenodytes forsteri) equipped with digital electrocardiogram recorders and time depth recorders revealed a phenomenal dive capacity extending to 431 m as well as extreme bradycardia, reaching heart rates as low as 10 beats min-1 during deep dives to promote oxygen conservation. The organization and potential function of the cetacean brain were examined with structural magnetic resonance imaging and diffusion tensor imaging of post-mortem killer whale (Orcinus orca) and bottlenose dolphin (Tursiops truncatus) brains. Structural images were acquired for an O. orca brain in situ and underwent manual segmentation to obtain volumetric measurements of neuroanatomy including gray and white matter, constituent neural regions (i.e., cerebrum, brainstem, and cerebellum), and subcortical and midbrain structures. This O. orca had one of the largest forebrains studied to date with cerebral volume comprising 81.51% of the total brain volume. Moreover, the cerebral white matter of O. orca and other delphinoids exhibited isometric scaling unlike other mammals suggesting that this divergent morphology may have evolved in response to the sensorimotor demands of the aquatic environment. Examination of T. truncatus cerebral white matter with diffusion tractography revealed widespread structural asymmetries potentially attributable to brain enlargement and isometrically-scaled white matter. Moreover, these structural asymmetries may underpin previously reported observations of functional and behavioral lateralization in cetaceans. These studies of cetacean anatomy and sphenisciform physiology provide insight into and promote our understanding of the evolution of arguably the most ocean-adapted seabirds and marine mammals