Anna R Dornhaus
Social insect colonies are complex systems in which the interactions of many individuals lead to colony-level collective behaviors such as foraging. However, the emergent properties of collective behaviors may not necessarily be adaptive. Here, we examine symmetry breaking, an emergent pattern exhibited by some social insects that can lead colonies to focus their foraging effort on only one of several available food patches. Symmetry breaking has been reported to occur in several ant species. However, it is not clear whether it arises as an unavoidable epiphenomenon of pheromone recruitment, or whether it is an adaptive behavior that can be controlled through modification of the individual behavior of workers. In this paper, we used a simulation model to test how symmetry breaking is affected by the degree of non-linearity of recruitment, the specific mechanism used by individuals to choose between patches, patch size, and forager number. The model shows that foraging intensity on different trails becomes increasingly asymmetric as the recruitment response of individuals varies from linear to highly non-linear, supporting the predictions of previous work. Surprisingly, we also found that the direction of the relationship between forager number (i.e., colony size) and asymmetry varied depending on the specific details of the decision rule used by individuals. Limiting the size of the resource produced a damping effect on asymmetry, but only at high forager numbers. Variation in the rule used by individual ants to choose trails is a likely mechanism that could cause variation among the foraging behaviors of species, and is a behavior upon which selection could act.
Many ant species are polydomous, forming multiple spatially segregated nests that exchange workers and brood. However, why polydomy occurs is still uncertain. We investigated whether colonies of Crematogaster torosa form new polydomous nests to better exploit temporally stable food resources. Specifically, we tested the effect of food presence or absence and distance on the likelihood that colonies would form a new nest. Because this species also forms little-known structures that house only workers without brood (outstations), we also compared the function of this structure with true nests. Laboratory-reared colonies were connected to a new foraging arena containing potential nest sites with or without food for 4 months. When food was present, most colonies formed polydomous nests nearby and the remainder formed outstations. When food was absent, the behavior of colonies differed significantly, frequently forming outstations but never polydomous nests. Distance had no effect on the type of structure formed, but when food was present, a larger proportion of the workforce moved shorter distances. Workers often fortified the entrances to both structures and used them for storage of dried insect tissue ("jerky"). In an investigation of spatial fidelity, we found that workers on the between-nest trail were associated with the original nest, whereas workers collecting food were more likely to be associated with the new nest or outstation. C. torosa appears to have a flexible colony structure, forming both outstations and polydomous nests. Polydomous nests in this species were associated with foraging and were only formed near food resources. © 2010 Springer-Verlag.
Specialized defence traits and strategies are crucial in surviving enemy attacks and in resource acquisition. In numerous social insect lineages, soldiers function as specialized defence traits of the colony, but associated defence strategies are poorly known. The turtle ant Cephalotes rohweri is an obligate cavity-nesting ant with highly specialized soldiers. To maximize growth and reproduction, colonies must use their limited availability of soldiers to defend multiple cavities. Using laboratory experiments informed by field data, we addressed how soldier 'deployment' across cavities adjusts to changes in cavity availability and quality. From initial field-like conditions, soldier deployment to newly available cavities was rapid, stabilized quickly, and at least doubled the number of cavities defended by each colony. New cavities were defended by fewer soldiers than original cavities still in use. Nevertheless, when new cavities differed in size, an important quality metric, large cavities were used more often and defended by more soldiers than small cavities. Despite these dynamic responses, total soldier deployment to new cavities was limited to an approximately constant proportion (0.4) of overall soldier availability across colonies and resource contexts. Moreover, there was a significant positive relationship between total soldier deployment to new cavities (greater for larger colonies) and both the number of newly defended cavities and their average level of defence. These results demonstrate that colony-wide soldier deployment is dynamic, predictable and context sensitive but ultimately constrained by the availability of soldiers in the colony. Furthermore, the consistently lower number of soldiers in new cavities, which always limits the potential losses to enemies, is concordant with a 'conservative bet-hedging' life history strategy. Broadly, our findings show that a specialized soldier caste can be associated with a far more sophisticated defence strategy than previously recognized. This provides a more complete perspective on the evolution of soldier-based defences in insect societies. © 2012 The Association for the Study of Animal Behaviour.