From “No Need to Shrink Guts to Have a Larger Brain” (ScienceDaily, Nov. 9, 2011), we learn:
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 by researchers at the University of Zurich. They have 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.
That said, no one has recently questioned the ability of humans to store fat. 😉
The hitch is,
Nevertheless, the storage of fat plays a key role in brain size evolution. The researchers discovered another rather surprising correlation: the more fat an animal species can store, the smaller its brain. Although adipose tissue itself does not use much energy, fat animals need a lot of energy to carry extra weight, especially when climbing or running. This energy is then lacking for potential brain expansion. “It seems that large adipose deposits often come at the expense of mental flexibility,” says Karin Isler. “We humans are an exception, along with whales and seals — probably because, like swimming, our bipedalism doesn’t require much more energy even when we are a bit heavier.”
Note also the proposed role of cooking.