Stereology of the neocortex in Odontocetes: qualitative, quantitative, and functional implications

We investigated the quantitative morphology of the neocortex (gray matter) in 2 toothed whale (odontocete) species (harbor porpoise, Phocoena phocoena; bottlenose dolphin, Tursiops truncatus) with stereological methods. The 4 primary projection areas (motor, somatosensory, auditory, and visual fields) are analyzed for their cell densities in layers III and V with standard design-based stereology methods. Along cortical areas M1, S1, A1, and V1 in Tursiops, neuron density is always higher in layer III than in layer V, whereas the data in Phocoena are variable. Moreover, neuron density in layer III is generally around 1.5 times higher in Tursiops than in Phocoena. Maximal density values are seen in layer III of A1 and V1 in Tursiops and the ratio of layer III/layer V density is maximal in A1 of this species. Thus, layer III could have a higher capacity in the bottlenose dolphin with regard to intrinsic connectivity. Extant knowledge on toothed whale neurobiology and behavior suggests that quantitative/stereological differences between the 2 odontocete species regarding the neuron density of standard cortical units may be correlated with specific adaptations to their respective habitats. In contrast to layers V and VI which mainly serve as an executive system, layer III could represent an intermediate level in sensory and premotor processing which works more tangentially in the cortices via horizontal connections with other cortical areas, respectively. The generally higher density of cortical layer III in Tursiops suggests a higher connectivity of this layer in view of the more agile and complicated behavior of these gregarious animals including versatile phonation by complex sound and ultrasound signals.