Primate encephalization

Encephalization is a concept that implies an increase in brain or neocortex size relative to body size, size of lower brain areas, and/or evolutionary time. Here, I review 26 large-scale comparative studies that provide robust evidence for five lifestyle correlates of encephalization (group living, a large home range, a high-quality diet, a strong reliance on vision, arboreal and forest dwelling), six cognitive correlates (better performance in captive tests, more tactical deception, innovation, tool use, social learning, all subsumed in part by general intelligence), one life history correlate (a longer lifespan), two evolutionary correlates (a high rate of change in microcephaly genes, an increase in brain size over macroevolutionary time), as well as three trade-offs (a slower juvenile development, a higher metabolic rate, sexually selected dimorphism). Of the 26 different encephalization measures used in these studies, corrected neocortex size, either with a ratio or a residual, is the most popular structural correlate of the functional variables, while residual brain size is the measure associated with the greatest number of them. Controversies remain on corrected or absolute measures of neural structure size, concerted versus mosaic evolution of brain parts and specialized versus domain-general brain structures and cognitive processes.