Comparing Vertebrate Brains

Abstract

In the preceding chapters, we have tried to figure out how vertebrates differ in intelligence including mental functions, and we arrived at a certain ranking order. In this chapter we will ask to what degree these differences in intelligence can be correlated with brain traits. In the past, there have been many such attempts. The first trait that comes to our mind is absolute brain size (grams/kilograms or cubic centimeters), because many experts including Rensch were convinced that “bigger is better,” i.e., bigger brains mean higher intelligence. Another much-discussed trait is relative size, i.e., percent of body size, of the entire brain, or of alleged “seats” of intelligence like the mesonidopallium in birds or cerebral cortex in mammals. Since it becomes clear that much of brain size is determined by body size, experts have tried to determine the degree of encephalization, i.e., brain size beyond the mass related to body size, e.g., Jerison’s encephalization quotient or corrected relative brain size. One could also look for neurobiologically more meaningful traits like the number of neurons in the entire brain or in the “intelligence centers,” the degree of connectivity, etc., relevant for “information processing capacity.” Finally, one could look for “unique” properties that could best explain the observed differences in intelligence. Let us first study the significance of absolute brain size.