The size of a brain varies considerably within any species. As you might imagine, larger animals have larger brains. However, this does not mean that animals with larger brains are smarter than animals with smaller brains. For example, a larger brain is necessary to control larger muscles in larger animals and a larger brain is necessary to process more sensory information from the skin in larger animals - this has nothing to do with intelligence.

Figures for the average brain size of modern humans tend to vary according to the source of information, but a typical value is 1350 or 1400 cc (cubic centimeters); man's brain average weight is 1,500g. The brain of the sperm whale weighs 7,800g, the elephant's weighs 7,500g, the dolphin's 840g, and the brain of a mouse weighs 0,4g. As humans evolved, brain size increased dramatically.

A large brain requires an inordinate amount of care and feeding--a diet high in protein--and exquisite temperature control in order to function properly. Increases in brain size would therefore drive changes in early human diets, because of the need to increase the intake of protein. While human species remained omnivorous, a preference for meat eating did in fact occur over time. About 25% of our metabolism is devoted to brain function, which represents a huge investment of energy--and therefore a huge risk in terms of the overall chances for survival of the species.