You don't have to be particularly smart to hunt. You do have to be particularly smart to eat fruit. The Economist says that eating fruit is a mentally-taxing activity:

If this is a man | Economist.com : Many primates, monkeys in particular, are fruit-eaters. Eating fruit is mentally taxing in two ways. The first is that fruiting trees are patchily distributed in both space and time (though in the tropics, where almost all monkeys live, there are always trees in fruit somewhere). An individual tree will provide a bonanza, but you have to find it at the right moment. Animals with a good memory for which trees are where, and when they last came into fruit, are likely to do better than those who rely on chance. Also, fruit (which are a rare example of something that actually wants to be eaten, so that the seeds inside will be scattered) signal to their consumers when they are ready to munch by changing colour. It is probably no coincidence, therefore, that primates have better colour vision than most other mammals. But that, too, is heavy on the brain. The size of the visual cortex in a monkey brain helps to explain why monkeys have larger brains than their weight seems to warrant.

The intelligence rocket's second stage was almost certainly a way of dealing with the groups that fruit-eating brought into existence. Because trees in the tropics come into fruit at random, an animal needs a lot of fruit trees in its range if it is to avoid starving. Such a large range is difficult for a lone animal to defend. On the other hand, a tree in fruit can feed a whole troop. For both these reasons, fruit-eating primates tend to live in groups.

But if you have to live in a group, you might as well make the most of it. That means avoiding conflict with your rivals and collaborating with your friends%u2014which, in turn, means keeping track of your fellow critters to know who is your enemy and who your ally. That, in turn, demands a lot of brain power.

One of the leading proponents of this sort of explanation for intelligent minds is Robin Dunbar, of Liverpool University in England. A few years ago, he showed that the size of a primate's brain, adjusted for the size of its body, is directly related to the size of group it lives in. (Subsequent work has shown that the same relationship holds true for other social mammals, such as wolves and their kin.) Humans, with the biggest brain/body ratio of all, tend to live in groups of about 150. That is the size of a clan of hunter-gathers. Although the members of such a clan meet only from time to time, since individual families forage separately, they all agree on who they are. Indeed, as Dr Dunbar and several other researchers have noticed, many organisations in the modern world, such as villages and infantry companies, are about this size.

Living in collaborative groups certainly brings advantages, and those may well offset the expense of growing and maintaining a large brain. But even more advantage can be gained if an animal can manipulate the behaviour of others, a phenomenon dubbed Machiavellian intelligence