The expensivness of the brain.
The brain, from a metabolic point of view, is a very costly organ. In the case of the human body, the brain accounts for about 2
percent in terms of mass of an adult, but uses about 20 percent of the energy that is available. To illustrate this, for every five spoonfuls of food, one goes directly to the brain.
It also turns out that intensive mental effort increases the energy requirements of the brain. Glucose is the primary food source of the brain, basically it's fuel, and this may be the reason that brainboxes often feel the craving for chocolate. Studies have shown that the harder the task that the brain is required to perform, the faster the decrease in glucose levels in the blood vessels that are connected with the
active areas of the brain. However, mental efforts make up a relatively small portion of brain activity - hence we should not expect to loose any weight after doing arithmetic sums. Energy is used
predominantly in the reception (the collecting of information), and the sending out of nerve signals.
The brain consists of roughly 1012 cells, and every signal is a big energy
usage on a cellular level. On top of this, the brain needs to process information for the functioning of the organism itself. No one knows how much energy
expenditure is needed for this type of processing, but as it is concerned with the creation of electrical potentials, it cannot be small. The human nervous system, in a second, collects 10 billion bits of
information from all parts of the body, from this only 100 bits per second are used to steer the organism. This is a huge selection of information (1 : 1 000 000 000), and the mechanism by which this is done, is yet unknown.
Furhtermore, we must consider the energy that is required to keep the cells alive, regardless of whether they are carrying out any
data processing activity or not. Complicated chemical compounds such as
neurotransmitters and proteins constantly need to be synthesized, the regulation of
potassium and sodium levels, the sustaining of the intake of chemical compounds from the bloodstream,
are also highly energy requiring processes.
Its no wonder that in cases where a brain is not needed, evolution has mechanisms for decreasing its mass. This can be seen on a daily basis. People with small brains have more energy for activities that do not require brain power, for example (no offence) for sport or politics.