The limbic lobe of the dolphin brain: a quantitative cytoarchitectonic study

In these cytoarchitectonic studies of the cortical limbic formations of the bottlenose dolphin and other whale brains we have carried out quantitative analyses of the entire limbic lobe, including all of its sectors: supracallosal, retrosplenial and temporal. The limbic lobe proper has been examined as well as transitional areas between the limbic lobe and the archicortical and paleocortical formations and the extralimbic neocortices, including the entorhinal area and presubiculum. Analyses include total cortical thickness, thickness of individual cortical layers, overall cortical cell densities and glia/neuron ratios, individual laminar counts and glia/neuron ratios and neuron size. Comparisons have been made between these parameters in the brains of the dolphin (Tursiops truncatus), beluga whale (Delphinapterus leucas) and humpback whale (Megaptera novaeangliae). Cortical neuron density values (cells per mm3) in these three species of whales and in the human brain have been compared with similar data given in the literature for elephant, fin whale (Balaenoptera physalus) and human brains. Our values reflect the inverse relationship between brain size and neuron density. Thus, the dolphin shows approximately 13,000 neurons/mm3 in its limbic cortex, compared to 12,000 in the beluga whale and 8,000 in the humpback whale. Further, the data provide the first quantitative accounts on a layer by layer basis of the limbic cortices in the whale brain. In the dolphin, the anterior limbic cortices have a much lower cell density than the posterior limbic area. However, in the humpback whale these two cortices have similar neuron densities. In the temporal region, the entorhinal area is well differentiated into many architectonic subdivisions in the dolphin though not to the extent described in the primate brains. Our findings in the three whale species are discussed in terms of their possible significance and provide quantitative data for future comparative studies with other mammalian species.