The influence of beacon-aiming on the routes of wood ants.

Many insects have an innate propensity to approach conspicuous objects. We explore how such beacon aiming determines the shape of a wood ant's habitual route. We find that a single large black cylinder within an arena biases the route taken by ants as they run from a start position at one end of the arena to reach a feeder at the other. Ants learn a stable route with the first segment of their trajectory aimed at the cylinder, which becomes an intermediate goal on the way to the feeder. When in occasional tests the cylinder is removed or displaced, ants head for the usual site of the cylinder. They also aim for the same site when the cylinder is removed and the ant's normal start position is changed. This behaviour suggests that visual features of the arena are learnt from the vantage point of the cylinder and that this stored snapshot guides the ant to that site. Ants thus reinforce their ability to reach the cylinder by learning other visual features in their surroundings that can also steer them to its location. The use of beacon aiming in fixing routes has several benefits. Because the same path will be traversed on every trial, beacon aiming facilitates the acquisition of routes. Beacon aiming also increases the robustness of learnt routes: ants straying from the route will be attracted to the closest beacon and so regain their habitual paths.