title: Symmetry breaking in mass-recruiting ants: extent of foraging biases depends on resource quality.
abstract: The communication involved in the foraging behaviour of social insects is integral to their success. Many ant species use trail pheromones to make decisions about where to forage. The strong positive feedback caused by the trail pheromone is thought to create a decision between two or more options. When the two options are of identical quality, this is known as symmetry breaking, and is important because it helps colonies to monopolise food sources in a competitive environment. Symmetry breaking is thought to increase with the quantity of pheromone deposited by ants, but empirical studies exploring the factors affecting symmetry breaking are limited. Here, we tested if (i) greater disparity between two food sources increased the degree to which a higher quality food source is favoured and (ii) if the quality of identical food sources would affect the degree of symmetry breaking that occurs. Using the mass-recruiting Pharaoh ant,Monomorium pharaonis, we carried out binary choice tests to investigate how food quality affects the choice and distribution of colony foraging decisions. We found that colonies could coordinate foraging to exploit food sources of greater quality, and a greater contrast in quality between the food sources created a stronger collective decision. Contrary to prediction, we found that symmetry breaking decreased as the quality of two identical food sources increased. We discuss how stochastic effects might lead to relatively strong differences in the amount of pheromone on alternative routes when food source quality is low.
  
  
  
  Pheromones used by social insects should guide a colony via positive feedback to distribute colony members at resources in the most adaptive way given the current environment. This study shows that when food resources are of equal quality, Pharaoh ant foragers distribute themselves more evenly if the two food sources are both of high quality compared to if both are of low quality. The results highlight the way in which individual ants can modulate their response to pheromone trails which may lead colonies to exploiting resources more evenly when in a resource rich environment.
authors: School of Biology, University of Leeds, Leeds, UK ; Department of Ecology and Evolution, Biophore, University of Lausanne, 1015 Lausanne, Switzerland.
authors: R I'Anson
authors: RI
authors: Price
authors: Institute of Zoology, Johannes Gutenberg University Mainz, Johannes von Müller Weg 6, 55099 Mainz, Germany.
authors: C
authors: C
authors: Grüter
authors: School of Life Sciences, University of Sussex, Brighton, BN1 9QG UK.
authors: W O H
authors: WO
authors: Hughes
authors: Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN UK.
authors: S E F
authors: SE
authors: Evison
keywords: Colony organisation; Foraging; Monomorium pharaonis; Symmetry breaking; Trail pheromones