Second, at least in mammals, it also requires changes in female reproductive anatomy to accommodate larger craniums during childbirth. Third, brains are extremely energy-hungry and thus place intense demands on organisms to acquire enough calories to support them. For example, the human brain consumes about 20 percent of the body’s daily energy expenditures, more than any other single organ. The fact that so many long-persisting lineages, such as sharks, horseshoe crabs, and turtles, never bothered to evolve big brains underscores how costly and improbable it is.
