Perhaps the most important limitation of brain size is the fact that brain tissue is metabolically very intensive and it, therefore, consumes a lot of energy. The heart, the liver and the kidneys need a similar amount of energy per gram of tissue. However, their size is much more determined by body weight than brain size i.e. it varies much less between species. Other organs, bones, muscles, skin etc. consume per gramme a fraction of the energy the brain consumes (…)
That means that it is more difficult to achieve a certain increase in brain size through natural selection than it is, for example, to achieve an equivalent increase in muscle mass or general body size.
Another difficulty is that an organism cannot temporarily shut down its brain to save energy because the brain needs exactly the same amount of energy at rest.
(Carel van Schaik, Karin Isler 2010: Gehirne, Lebensläufe und die Evolution des Menschen, in: Fischer/ Wiegandt: Evolution, pp. 142-169, pp. 153s)
The brain is a very expensive organ for a living organism because it consumes a lot of energy. Nevertheless, in mammals, primates, monkeys, great apes and finally hominids all the way up to humans, a steady increase in relative brain size can be observed.
beginning, two statistics related to energy consumption should make the costs
of the brain clear: 1. The brain is responsible for approximately 20% of the
total energy consumption of the body. 2. More than 50% of the energy that the
human foetus absorbs is used to build up its brain.
three authors who have weighed in on the issue of the costs of the brain in the
context of human evolution will have their say. Robin Dunbar, from whom I used
the first percentage, wrote:
Brain tissue is unusually expensive to grow and maintain. It needs about ten times more energy than one would expect, based on its weight, and it is the most expensive tissue after that of the heart and liver.