Pheromone trails are used by many ants to guide foragers between nest and food1,2,3,4. But how does a forager that has become displaced from a trail know which way to go on rejoining the trail? A laden forager, for example, should walk towards the nest. Polarized trails would enable ants to choose the appropriate direction, thereby saving time and reducing predation risk. However, previous research has found no evidence that ants can detect polarity from the pheromone trail alone3,5,6,7. Pharaoh's ants (Monomorium pharaonis) produce elaborate trail networks throughout their foraging environment8. Here we show that by using information from the geometry of trail bifurcations within this network, foragers joining a trail can adaptively reorientate themselves if they initially walk in the wrong direction. The frequency of correct reorientations is maximized when the trail bifurcation angle is approximately 60 degrees, as found in natural networks. These are the first data to demonstrate how ant trails can themselves provide polarity information. They also demonstrate previously unsuspected sophistication in the organization and information content of networks in insect societies.
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We thank K. Boomsma, A. Bourke, V. Fourcassié, A. Hart, L. Keller, S. Martin and G. Theraulaz for discussions and comments on the manuscript. We also thank P. Mitchell for statistical advice. This work was funded by the EPSRC and British Telecom.
The authors declare that they have no competing financial interests.
This table shows reorientation results for fed and unfed foragers when placed at the ends of a Y-shaped pheromone trail (angle 30-120 degrees) representing a bifurcating foraging trail leading from the nest or a straight trail. (DOC 90 kb)
Additional information to accompany the above Supplementary Table. (DOC 20 kb)
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Jackson, D., Holcombe, M. & Ratnieks, F. Trail geometry gives polarity to ant foraging networks. Nature 432, 907–909 (2004) doi:10.1038/nature03105
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