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Let's look at some research about the neocortex...
Scientists Heinz, Beron, and Frahm decided to compare the brain size with sizes of particular brain structures, such as the medulla, cerebellum, hippocampus, and neocortex (among others). They compared structures in the brains of the orders Insectivora and Scadentia and primate suborders such as prosimians, simians, and man. They found that the neocortex was the "most progressive structure". This means that when comparing across the different orders and suborders, the neocortex of Insectivora is 2.7 larger than Tenrecinae; in prosimians, 21 times larger; in nonhuman simians, 51 times larger; and in man, 200 times larger.
Researchers Stephan and Andy wanted to compare the size of a structure in a given mammal with the size that would be expected in a basal insectivore of comparable body weight. They found that man does indeed "possess much more neocortex than would be expected in a basal insectivore of comparable body weight, yet man does not possess much more neocortex than would be expected in a basal insectivore of comparable brain weight" (Macphail, 251). Basal insectivores (for example, hedgehogs) are assumed to be relatively un-specialized in terms of their behaviors and adaptations. A progression index was calculated (see footnote below for how this was done). According to the figure, we see that the neocortex shows a dramatic progression in size as we go from the insectivores to the 'lower' primates and on to the 'higher' primates. The importance of this figure is that while neocortex maintains a relatively constant percentage of brain volume as it expands (Fig. 7.5), other forebrain structures gradually decrease in percentage of brain volume as the volume increases.
Thus, we can say with relative accuracy that size of neocortex is related to size of the brain. Man has just the amount of neocortex expected in an average primate of comparable brain size (but not body size). In other words, if man has more neocortex than, say, another primate, it would most likely be due to the possession of more brain for a given body weight.
So, both studies we have seen conclude that the main statistic that makes substantial distinctions between mammals is relative brain size rather than relative amounts of neocortex...
Where do you want to go now?
Footnote: the expected size of a structure in a basal insectivore for a given body weight is calculated for each structure by applying the simple allometry formula to data gathered from basal insectivores of various weights, and extrapolating in the usual way to other body weights; the ratio between the actual size of a structure and the expected size in a basal insectivore is the "progression index"-- see Fig. 7.6 for progression indices for a number of brain structures in primates, Macphail 250
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