When it comes to estimating the intelligence of various animal species, it may be as simple measuring overall brain size. In fact, making corrections for a species' body size may be a mistake."It's long been known that species with larger body sizes generally have larger brains," said Robert Deaner. "Scientists have generally assumed that this pattern occurs because larger animals require larger nervous systems to coordinate their larger bodies. But our results suggest a simpler reason: larger species are typically smarter."
The new results showed that some primate species consistently outperform others across a broad range of cognitive tasks. It compared how well eight different brain size measures predicted the domain-general cognition variable generated in the earlier study. To the researchers' surprise, overall brain size and overall neocortex size proved to be good predictors, but the various measures that controlled for body size did not. The results did not change even when various statistical assumptions were altered.
Another unexpected finding was that the overall size of the whole brain proved to be just as good a predictor of intelligence as was the overall size of the neocortex. Scientists making cross-species comparisons have often assumed that the neocortex would be more closely linked to intelligence, since it is considered the "thinking part" of the brain. link
Overall Brain Size, and Not Encephalization Quotient, Best Predicts Cognitive Ability across Non-Human Primates. 2007. R. O. Deaner, et al. Brain, Behavior, and Evolution 70: 115-124.