This kind of scheme can help computer scientists find the best solution to many a thorny and time-consuming problem, Eric Bonabeau of the Santa Fe Institute in New Mexico, USA, and colleagues report in Nature 1 . Some problems can be solved only by trawling through all possible answers for the best one. One example is the classic Travelling Salesman Problem, the search for the shortest route connecting many cities. Such problems can be solved by simulating the journeys of a swarm of trail-laying ants, Bonabeau's team suggests. Each 'virtual ant' marks the journey with a 'pheromone' that characterizes how short its tour was.
Good routes become more attractive to other ants than poor routes.
Simulating pheromone evaporation at a steady rate ensures that the colony does not get trapped onto mediocre solutions that prevent them from looking for better ones. ACO is currently being explored for planning petrol truck routes in Switzerland. Ants generally move over open terrain. Engineers are finding it useful to place their virtual foraging insects inside networks. This can help to identify, for example, the best way of routing signals through a communications network.