Ant foraging on complex trails: route learning and the role of trail pheromones in Lasius niger

Abstract

Summary Ants are central place foragers and use multiple information sources to navigate between the nest and feeding sites. Individual ants rapidly learn a route, and often prioritize memory over pheromone trails when tested on a simple trail with a single bifurcation. However, in nature ants often forage at locations which are reached via more complex routes with multiple trail bifurcations. Such routes may be more difficult to learn so that ants benefit from additional information. We hypothesized that trail pheromones play a more significant role in ant foraging on complex routes, either by assisting in navigation or route learning or both. We studied Lasius niger workers foraging on a doubly-bifurcating trail with 4 endpoints. Route learning was slower and errors greater on alternating (e.g. left-right) versus repeating routes (e.g. left-left) - error rates 32% and 3%, respectively. However, errors on alternating routes decreased by 30% when trail pheromone was present. Trail pheromones also aid route learning, leading to reduced errors in subsequent journeys without pheromone. If an experienced forager makes an error when returning to a food source, it reacts by increasing pheromone deposition on the return journey. In addition, high levels of trail pheromone suppress further pheromone deposition. This negative feedback mechanism may act to conserve pheromone or to regulate recruitment. Taken together, these results demonstrate further complexity and sophistication in the foraging system of ant colonies, especially in the role of trail pheromones and their relationship with learning and private information in relation to the challenges of foraging in a complex environment.