Studies of mammalian and primate brain evolution have traditionally focused on changes in encephalization, that is, changes in brain size statistically adjusted to compensate for changes in body size, rather than on changes in the internal organization of the brain. There are some very sound reasons for stressing size. Mammals do indeed vary dramatically in absolute and relative brain size: at a given body weight, brain weight can vary more than five-fold across species (Stephan et al., 1988). Moreover, brain size changes can have profound consequences for the developmental biology and ecology of mammalian taxa, because larger-brained taxa grow more slowly and live longer than do smaller-brained taxa of comparable body size (Sacher, 1982; Finlay & Darlington, 1995) and because brain tissue is energetically very demanding (Aiello & Wheeler, 1995). Conveniently, brain size is relatively tractable empirically, which is to say that one can measure it with reasonable precision in all sorts of living and extinct taxa, whereas the internal features of brain organization can be examined only with difficulty in extant taxa and not at all in extinct forms. Finally, there can be little doubt that variations in brain size are in some way related to variations in cognitive and behavioral abilities.
But in precisely what ways are brain size, cognition, and behavior related? Harry Jerison has ably articulated the view that encephalization serves as an index of general animal intelligence (see especially Jerison, 1961, 1973), and in doing so has provided the underpinning for modern brain allometry studies.