Ant colonies, for example, send out ‘scouts’ in search of a potential nest – say, a narrow crack in a rock. Such a scout has to estimate whether the area is big enough to house the colony. Not having a tape measure and a slide rule, it does this by using a heuristic. It runs around randomly, leaving pheromone trails of a certain length, then leaves. After a period, it returns, this time moving on a different random path. By noting the frequency with which it encounters the old trail, it can estimate the size of the cavity. The heuristic is remarkably accurate:
