The use of neuroimaging in the assessment of brain size and social structure in odontocetes.

Abstract

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 Bottlenose dolphins superseded human values of the neocortex ratio and common dolphins marked the upper limit of the range for the dolphin species under investigation.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.