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.

This study successfully utilised the non-invasive neuroimaging techniques of Computerised Tomography (CT) and Magnetic Resonance Imaging (MRI) to establish that dolphins have high relative brain size values, transcending the primate range for neocortex volume and neocortex ratio. Bottlenose dolphins superseded human values of the neocortex ratio and common dolphins marked the upper limit of the range for the dolphin species under investigation. In addition this study was the first to find a correlation between sociality and neocortex ratio in dolphins (R.I.M. Dunbar, pers.comm), which supports the hypothesis of neocortical development in relation to sociality/group size (Sawaguchi & Kudo 1990; Dunbar 1992) and social/Machiavellian intelligence (Byrne & Whiten 1988; Byrne 1995). The study devised new measures of relative brain size, including the grey-white matter and higher cortical ratios and these require further research before verification of their efficacy. Equations were calculated to allow estimation of: (1) MRI values of total brain volumes from CT values, (2) total brain volume from cranial volume using CT, (3) cerebral cortex volume from cranial or total brain volume (CT) and (4) cerebral cortex and cerebellar cortex volume from total brain volume (MRI). The effects of freezing and defrosting on volume and density of CT and MRI values were investigated. Additionally, the relationship between relative brain size (EQ) and sociality was investigated for other dolphin research, using previously published figures, but no significant correlations were found. Finally, dolphin values were compared to primate values for neocortex volume and neocortex ratio with the finding that the only primate within the dolphin range of neocortex was the human, positioned higher than the solitary humpback dolphin, but below all of the other, more socially complex, dolphin species. / Thesis (M.A.)-University of Natal, Durban, 1996.

Populations of the humpback dolphin in Natal, South Africa, are subject to increasing pressures including capture in the shark nets and habitat degradation, and concern has been raised about the status of the population. A minimum of 95 humpback dolphins were caught in the shark nets during the period from 1980 to 1992. Capture and sighting records of the Natal Sharks Board revealed a relatively high occurrence of humpback dolphins at Richards Bay. Elsewhere, in southern Natal, the infrequent sightings and captures were attributed to a seasonal occurrence of dolphins, possibly due to temporary movements away from resident areas. Sighting rates reported by the Natal Sharks Board has decreased by 55%from 1984-86 to 1990-92 and may reflect a decrease in the population. In a photo-identification study, searches took place in ten search areas in Natal. The sighting rates in the different areas revealed a relatively high density of humpback dolphins occurring in north central Natal, from the Tugela River to the St. Lucia estuary (including Richards Bay). This distribution correlated significantly with the turbidity of the water and the width of the inshore continental shelf, and was inversely related to the density of bottlenose dolphins. Within the northern Tugela Bank region, higher densities of dolphins were found surrounding the five river mouths and estuaries. The Natal population was estimated to be between 161 to 166 animals (95% confidence limits 134 to 229). The annual mortality due to shark net captures approximates 4,5%of the population. Various evidence, including a high mortality rate and a decrease in the annual sighting per unit effort reported by the Natal Sharks Board suggest that the humpback dolphin population in Natal is vulnerable and may be decreasing in size. A proposal is made to reduce the capture rate by relocating shark nets away from the Richards Bay harbour. / Thesis (M.Sc.)-University of Natal, 1994.