Many studies have shown that heavily domesticated birds and mammals tend to have reduced brain:body ratios, compared to their wild ancestors. But does this mean that domestication reduced brain size (if you get fed no matter what, and have no choice of mates, why waste energy on a big brain?), or does it mean that domestication increased body size (more meat for us humans)? A little bit of both. However, in some species, domestication also brought a significant change in brain region proportions, which is not explicable in terms of changing body size. For example, a previous study by Boire & Baron (1994) reported that the telencephalon is significantly smaller, relative to the remaining brain, in domestic chickens than in the chicken's ancestor, Red Jungle Fowl (see Figure below; red circles represent the parrots; blue triangles represent galliform birds).
