Ants are notoriously good at finding the most direct route to food. Laying down volatile pheromones to guide their colleagues, ant colonies eventually home in on the most direct path as pheromone is lost from longer paths and added to more heavily used shorter routes. Finding the optimal paths through networks is also a challenge for human delivery agents, telephone routers and systems analysts, so software engineers have turned to trail-blazing ants for inspiration to solve these problems. Chris Reid and Madeleine Beekman from Sydney University, Australia, and David Sumpter from Uppsala University, Sweden, explain that one network-solving algorithm uses virtual ants to lay down virtual pheromone trails to identify the most direct route through, but it doesn't do well when networks continually change, as they do in real life. However, ants have faced constantly changing landscapes for millions of years, so Reid and his co-workers decided to find out how Argentine ants identify the optimal route through a changing network when they presented the insects with the Towers of Hanoi puzzle (p. 50).

Of course the team didn't ask the ants to start moving puzzle discs between pegs. Instead, they converted the task into a graph of possible moves and converted that in turn into a maze of hexagons. Having initially allowed a colony of ants to explore the maze and lay down pheromones (short-lived foraging pheromone and long-lasting exploration pheromone), the team then placed food at the end of the maze and filmed the ants as they tried to identify the optimal route through the maze to the food while the team blocked off routes and opened up alternatives.

The ants had little problem coping with change. Initially, the insects found the shortest route, scurrying along the outer edge of the maze to the food source. However, when the team blocked this route and opened up another through the middle of the maze, the ants changed course, zigzagging across the maze, perpendicular to their original route until they found the alternative. The team suspects that both pheromones played critical roles, with short-lived foraging pheromone allowing the ants to respond quickly when the route changed and long-lived exploration pheromone rapidly stabilising the optimal route. Also, when an ant encountered an obstacle, Reid and his colleagues noticed that she began searching for a route around the blockage, rather than retracing her steps. The team suspects that the ants don't just rely on geometry to identify paths and negotiate obstacles; they use an internal compass to help them reach their goal. Finally, the trio hopes that their new discovery that ants use multiple pheromones and possibly compasses will inspire software engineers to develop alternative ant-based algorithms to solve complex dynamic network problems.

C. R.
D. J. T.
Optimisation in a natural system: Argentine ants solve the Towers of Hanoi
J. Exp. Biol.