JOHNSON, J. I.*; MORRIS, J. A. ; FOBBS JR., A. J.; Michigan State Univ., East Lansing; Michigan State Univ., East Lansing; National Mus. Health Medicine, Armed Forces Inst. Pathol., Washington DC: Shared Characters of Cerebral Isocortex in Cetacean Bottlenosed Dolphins, Tursiops truncatus, and Artiodactyl Domestic Sheep, Ovis aries.

In many large brained mammals, sensory regions of isocortex are separated from one another by intervening regions of association cortex. In the large brains of cetacean bottlenosed dolphins, however, the main sensory and motor regions are grouped together on the superior surfaces of the hemispheres, with no other cortical regions intervening between them. The same characteristic of grouped sensory and motor regions is seen in the brains of domestic sheep, which are artiodactyls, now recognized as a sister group to the cetaceans. The extensive relative increase in the amount of isocortex in the dolphin brain occurs outside this grouping of sensory and motor regions, in a peripheral arc of extensive frontal, lateral, and posterior cortical regions. This contrasts with the pattern which evolved in expanded cortex of highly encephalized primate brains, and also in brains of advanced carnivores, where additional cortex was inserted between the main sensory and motor regions. Support: NSF grants IBN 0131267, 0131826, 0131028