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Evolutionary strategies of odontocete brain development

The development of the brain was studied in several species of toothed whales (harbour porpoise, spotted dolphin, narwhal, and sperm whale). As embryos, odontocetes show the general mammalian bauplan. The olfactory bulb, lacking in adult toothed whales, forms in embryos but then disappears in early fetal development. In contrast, the terminal nerve persists and shows a great increase in neuron number. Some components of the limbic system are reduced while others grow to become relatively large. The pyramidal tract is inconspicuous. The auditory system and the extrapyramidal system show rapid size increase. In the sperm whale, growth of the telencephalic hemispheres relative to the size of the brain as a whole (telencephalization) and expansion of the cortical areas (neocorticalization) are greater than in other mammals. Increases in the diameters of the cranial nerves seem to be correlated with their presumed functional roles in the postnatal animal.