The hypertrophic and much elongated epicranial (nasal) complex of sperm whales (Physeter macrocephalus) is a unique device to increase directionality and source levels of echolocation clicks in aquatic environments. The size and shape of the nasal fat bodies as well as the peculiar organization of the air sac system in the nasal sound generator of sperm whales are in favour of this proposed specialized acoustic function. The morphology of the sperm whale nose, including a ‘connecting acoustic window’ in the case and an anterior ‘terminal acoustic window’ at the rostroventral edge of the junk, supports the ‘bent horn hypothesis’ of sound emission. In contrast to the laryngeal mechanism described for dolphins and porpoises, sperm whales may drive the initial pulse generation process with air pressurized by nasal muscles associated with the right nasal passage (right nasal passage muscle, maxillonasolabialis muscle). This can be interpreted as an adaptation to deep-diving and high hydrostatic pressures constraining pneumatic phonation. Comparison of nasal structures in sperm whales and other toothed whales reveals that the existing air sac system as well as the fat bodies and the musculature have the same topographical relations and thus may be homologous in all toothed whales (Odontoceti). This implies that the nasal sound generating system evolved only once during toothed whale evolution and, more specifically, that the unique hypertrophied nasal complex was a main driving force in the evolution of the sperm whale taxon.

Dolphin brain size with respect to body size ranks between that of apes and humans. The hypertrophic auditory structures, the large cerebrum with extended gyrification and the highly cognitive capabilities of toothed whales seem to be in paradoxical contrast to their thin neocortex with a plesiomorphic or paedomorphic cytoarchitecture. The total number of neurons in the delphinid neocortex is comparable to that of the chimpanzee (Primates), but, in relation to body weight, in the magnitude of the hedgehog (Insectivora) neocortex since cetaceans may be able to obtain larger body sizes than terrestrial mammals due to reduced gravitational effects in water. During evolution, dolphins may have increased the computational performance of their cytoarchitectonically ‘simple’ neocortex by a multiplication of relevant structures (resulting in a hypertrophic surface area) instead of increasing its complexity. Based on this hypothesis, I suggest that the evolution of the large dolphin brain was possible due to a combination of different prerequisites based on adaptations to the aquatic environment including the sonar system. The latter facilitated a successful feeding strategy to support an increased metabolic turnover of the brain and led to a hypertrophic auditory system. Moreover, the rudimentary pelvic girdle did not limit brain size at birth. These adaptations favoured the evolutionary size increase of the cerebral cortex in dolphins facilitating highly cognitive capabilities as well as precise and rapid sound processing using a ‘simple’ kind of neocortical cytoarchitecture.