There have been very few studies of brain size and encephalization in cetaceans and essentially none that have made direct quantitative comparisons of cetaceans and another mammalian group using large normative samples. In the present study two different measures of encephalization were calculated and used to rank and compare 21 odontocete species and 60 anthropoid primate species. Comparisons were made both within and between the two groups. Results show that the encephalization level of Homo sapiens is still extraordinary relative to that of nonhuman species. Nevertheless, a subset of delphinid odontocetes are significantly more highly encephalized than the most highly encephalized anthropoid primates and narrow the gap in encephalization between humans and nonhumans substantially. These findings may have implications for comparative models of the relative importance of brain size versus brain organization for the evolution of intelligence.
Marino, L. (1998). A comparison of encephalization between odontocete cetaceans and anthropoid primates. Brain, Behavior and Evolution.