The superior neocortex ratios in primates and their distinctive relationship with sociality among terrestrial mammals are well documented. However, there has been an absence of research into relative neocortex size, its evolution and correlates in marine mammals, such as cetaceans (dolphins, porpoises and whales). This study uses the advanced radiological techniques of computed tomography and magnetic resonance imaging to establish neocortex ratios in dolphins and to re-assess these values for humans. It was found that freezing and defrosting did not significantly alter the neocortex ratios of dolphins and thus extra material as included in the analysis. Furthermore, equations for the estimation of neocortex ratios from eT and MRI have been applied to the cranial volumes calculated for 19 toothed whale species, in order to extend the range of analysis. Using these techniques, it appears that dolphin neocortex ratios are higher than those of other mammals, except for primates. A notable finding is that dolphin values lie between human and other primates and are closer to human ratios at 4.1, than to non-human ratios reaching 3.2~ (Dunbar, 1992). The highest delphinid neocortex ratio from MRI was 3.94 for common dolphins, while the highest estimated neocortex ratio was at 3.95 for killer whales. To establish the correlates of such high neocortex ratios in dolphins, their scores were related to variables representing foraging ecology, sound and sociality. Although delphinid neocortex ratios do not appear to be related to foraging variables, they are significantly related with sound and sociality variables. Of these relationships, the most substantial finding exists with respect to the relationship of delphinid neocortex ratios and their mean group size. The capacity to predict group size from relative neocortex size has not been noted in non-primate species, and has formed the basis for current theories of social intelligence and mental evolution. The findings of this study are therefore of considerable interest and may have substantial implications. These may impact on current theories of primate-human mental evolution and therefore it is strongly recommended that the mental capacities of other mammals, such as dolphins, be examined in greater detail to support or refute these claims. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1998.