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      Do Ants Need to Estimate the Geometrical Properties of Trail Bifurcations to Find an Efficient Route? A Swarm Robotics Test Bed

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          Interactions between individuals and the structure of their environment play a crucial role in shaping self-organized collective behaviors. Recent studies have shown that ants crossing asymmetrical bifurcations in a network of galleries tend to follow the branch that deviates the least from their incoming direction. At the collective level, the combination of this tendency and the pheromone-based recruitment results in a greater likelihood of selecting the shortest path between the colony's nest and a food source in a network containing asymmetrical bifurcations. It was not clear however what the origin of this behavioral bias is. Here we propose that it results from a simple interaction between the behavior of the ants and the geometry of the network, and that it does not require the ability to measure the angle of the bifurcation. We tested this hypothesis using groups of ant-like robots whose perceptual and cognitive abilities can be fully specified. We programmed them only to lay down and follow light trails, avoid obstacles and move according to a correlated random walk, but not to use more sophisticated orientation methods. We recorded the behavior of the robots in networks of galleries presenting either only symmetrical bifurcations or a combination of symmetrical and asymmetrical bifurcations. Individual robots displayed the same pattern of branch choice as individual ants when crossing a bifurcation, suggesting that ants do not actually measure the geometry of the bifurcations when travelling along a pheromone trail. Finally at the collective level, the group of robots was more likely to select one of the possible shorter paths between two designated areas when moving in an asymmetrical network, as observed in ants. This study reveals the importance of the shape of trail networks for foraging in ants and emphasizes the underestimated role of the geometrical properties of transportation networks in general.

          Author Summary

          Most ant species form transportation networks, be they foraging trails linking food sources to the main colony or underground galleries connecting the different parts of the nest. As for human transportation networks (roads, airlines, etc.), the design and the placement of the connecting points (or nodes) dramatically affects the movement of individuals and hence the exchanges of material and information. In a previous study, we have shown that the geometrical configuration of these nodes (i.e., the angles between the different exiting branches) can affect the route followed by an ant in a network of galleries and, as a consequence, the efficiency of the pheromone-based recruitment toward a food source. Here we show that we can reproduce these results using ant-like robots with minimal perceptual and cognitive capabilities. We demonstrate that the simple interaction between the displacement of an ant and the geometrical configuration of the gallery network can greatly affect the foraging performances of the colony. This result increases our understanding of how workers move through structures built by ant colonies and more generally points toward possible improvements for the design of man-made transportation networks.

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          Most cited references 13

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          Self-organization, embodiment, and biologically inspired robotics.

          Robotics researchers increasingly agree that ideas from biology and self-organization can strongly benefit the design of autonomous robots. Biological organisms have evolved to perform and survive in a world characterized by rapid changes, high uncertainty, indefinite richness, and limited availability of information. Industrial robots, in contrast, operate in highly controlled environments with no or very little uncertainty. Although many challenges remain, concepts from biologically inspired (bio-inspired) robotics will eventually enable researchers to engineer machines for the real world that possess at least some of the desirable properties of biological organisms, such as adaptivity, robustness, versatility, and agility.
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            The ant odometer: stepping on stilts and stumps.

            Desert ants, Cataglyphis, navigate in their vast desert habitat by path integration. They continuously integrate directions steered (as determined by their celestial compass) and distances traveled, gauged by as-yet-unknown mechanisms. Here we test the hypothesis that navigating ants measure distances traveled by using some kind of step integrator, or "step counter." We manipulated the lengths of the legs and, hence, the stride lengths, in freely walking ants. Animals with elongated ("stilts") or shortened legs ("stumps") take larger or shorter strides, respectively, and concomitantly misgauge travel distance. Travel distance is overestimated by experimental animals walking on stilts and underestimated by animals walking on stumps.
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              Is Open Access

              Optimal traffic organisation in ants under crowded conditions

              Efficient transportation, a hot topic in nonlinear science, is essential for modern societies and the survival of biological species. Biological evolution has generated a rich variety of successful solutions, which have inspired engineers to design optimized artificial systems. Foraging ants, for example, form attractive trails that support the exploitation of initially unknown food sources in almost the minimum possible time. However, can this strategy cope with bottleneck situations, when interactions cause delays that reduce the overall flow? Here, we present an experimental study of ants confronted with two alternative routes. We find that pheromone-based attraction generates one trail at low densities, whereas at a high level of crowding, another trail is established before traffic volume is affected, which guarantees that an optimal rate of food return is maintained. This bifurcation phenomenon is explained by a nonlinear modelling approach. Surprisingly, the underlying mechanism is based on inhibitory interactions. It implies capacity reserves, a limitation of the density-induced speed reduction, and a sufficient pheromone concentration for reliable trail perception. The balancing mechanism between cohesive and dispersive forces appears to be generic in natural, urban and transportation systems.

                Author and article information

                Role: Editor
                PLoS Comput Biol
                PLoS Comput. Biol
                PLoS Computational Biology
                Public Library of Science (San Francisco, USA )
                March 2013
                March 2013
                28 March 2013
                : 9
                : 3
                [1 ]Centre de Recherche sur la Cognition Animale, UMR-CNRS 5169, Université Paul Sabatier, Bât 4R3, Toulouse, France
                [2 ]CNRS, Centre de Recherches sur la Cognition Animale, Toulouse, France
                [3 ]Department of Biological Sciences, New Jersey Institute of Technology, Newark, New Jersey, United States of America
                Ecole Polytechnique Fe´de´rale de Lausanne EPFL, Switzerland
                Author notes

                The authors have declared that no competing interests exist.

                Participated in the revision of the manuscript: CJ GT. Conceived and designed the experiments: SG CJ GT. Performed the experiments: SG. Analyzed the data: SG. Contributed reagents/materials/analysis tools: MC. Wrote the paper: SG.


                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                Page count
                Pages: 12
                This work was partly supported by the Programme Cognitique from the French Ministry of Scientific Research. Simon Garnier was supported by a research grant from the French Ministry of Education, Research and Technology, and by an ATUPS grant from the University Paul Sabatier. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Research Article
                Evolutionary Biology
                Animal Behavior
                Animal Behavior

                Quantitative & Systems biology


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