CORPUS-CALLOSUM SIZE IN DELPHINID CETACEANS

The midsagittal surface area of the corpus callosum was determined by computer-assisted morphometry in juvenile and adult members of 13 spec ies of the cetacean family Delphinidae. In 57 brains, absolute callosa l areas ranged from 104 to 829 mm(2). When compared to other mammal gr oups possessing a corpus callosum, callosal area in dolphins was small er in relation to brain mass with a ratio range (mm(2)/g) of 0.08-0.31 . The corpus callosum was decreased relative to brain mass in the larg er-brained odontocetes, suggesting that increases in brain size were n ot necessarily allied with needs for equivalent increases in callosal linkage. One delphinid species, Tursiops truncatus, for which the larg est single-species sample was available, was examined for sex differen ces in callosal size relative to brain mass. Among 10 males and 5 fema les the averaged ratio was not distinguishable between sexes.