The so-called expensive-tissue hypothesis, which suggests a trade-off between the size of the brain and the size of the digestive tract, has been challenged. Researchers have now shown that brains in mammals have grown over the course of evolution without the digestive organs having to become smaller. The researchers have further demonstrated that the potential to store fat often goes hand in hand with relatively small brains — except in humans, who owe their increased energy intake and correspondingly large brain to communal child care, better diet and their ability to walk upright.