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.Significance statementPheromones 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.Electronic supplementary materialThe online version of this article (doi:10.1007/s00265-016-2187-y) contains supplementary material, which is available to authorized users.
Highlights
Several methods of communication are used by social insects to recruit nestmates to profitable resources
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
There were strong correlations between the number of ants feeding at the feeders and the ant traffic counts each of the food quality treatments, suggesting that there was no overcrowding on either the food source or on the branches leading to the food source
Summary
Several methods of communication are used by social insects to recruit nestmates to profitable resources. The pheromones are deposited along foraging trails, providing positive and negative feedback to make decisions on colony foraging (Hölldobler and Wilson 1990; Traniello and Robson 1995; Camazine et al 2001; Ratnieks 2008; Czaczkes et al 2015). These decisions will affect fitness due to their influence on resource acquisition, leading to evolutionary pressure to make more accurate decisions (Marshall et al 2009; Chittka et al 2009; Stroeymeyt et al 2010). Social insects are required to make these decentralised decisions during processes such as choosing a new nest site (Mallon et al 2001; Evison et al 2012b), avoiding predation (Lamon and Topoff 1981), and organisation while foraging for food (Beckers et al 1992; Sumpter and Beekman 2003; Devigne and Detrain 2006)
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