Ant-colony optimization is a search algorithm that is inspired by observing the behaviour individual ants as they search for food and the emergent behaviour of the colony. Ants perform random search, unless they either (1) find food - then they way as direct a way back to the nest as they remember, laying down a trail of pheromones as they walk, or (2) they smell a pheromone trail and then follow it towards the source of food. When the source of food is exhausted, the pheromones evaporate, so they can forget what they have learnt. The emergent behaviour is that they can collectively solve problems such as finding the shortest path to food, or solving the NP-complete travelling salesman problem efficiently.
Reference:
http://staff.washington.edu/paymana/swarm/stutzle99-eaecs.pd...