The evolution of brain size and organization in vertebrates. A program for research.

In vertebrates, brain size variability relates to two main parameters: body size and ecological factors (in particular diet and foraging strategy). It has been considered by many authors that evolutionary brain growth is a unitary phenomenon whose main effect is to increase processing capacity. Alternatively, in this paper it is considered that brain growth is significantly associated with higher processing capacity only when it occurs associated with ecological circumstances (selection of behavioral or perceptual skills). This process is referred to as "active" growth. When the brain scales on body size, there is little change in processing capacity, and this will be referred to as "passive" growth. I propose that these two modes of phylogenetic brain growth relate to different developmental/evolutionary processes and are distinguishable at the level of adult and developing structure. Shortly, growth due to selection of behavioral capacities is associated with more differentiated brains in terms of number of areas, connectional rearrangements and cell types. Growth due to scaling of body mass produces little brain rearrangements, and many of those that occur relate to the maintenance of functions in a larger brain. In addition, active selection of brain size is triggered by plastic, ontogenic rearrangements of connectivity in the organisms, while passive growth produces the minor rearrangements that take place. Finally, I propose a research program oriented to test this model by separating the effects of body size and ecological variables in brain organization across species.