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.
Abstract:
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.
PMID: 19018663;Abstract:
The ecological success of social insects is often attributed to an increase in efficiency achieved through division of labor between workers in a colony. Much research has therefore focused on the mechanism by which a division of labor is implemented, i.e., on how tasks are allocated to workers. However, the important assumption that specialists are indeed more efficient at their work than generalist individuals - the "Jack-of-all-trades is master of none" hypothesis - has rarely been tested. Here, I quantify worker efficiency, measured as work completed per time, in four different tasks in the ant Temnothorax albipennis: honey and protein foraging, collection of nest-building material, and brood transports in a colony emigration. I show that individual efficiency is not predicted by how specialized workers were on the respective task. Worker efficiency is also not consistently predicted by that worker's overall activity or delay to begin the task. Even when only the worker's rank relative to nestmates in the same colony was used, specialization did not predict efficiency in three out of the four tasks, and more specialized workers actually performed worse than others in the fourth task (collection of sand grains). I also show that the above relationships, as well as median individual efficiency, do not change with colony size. My results demonstrate that in an ant species without morphologically differentiated worker castes, workers may nevertheless differ in their ability to perform different tasks. Surprisingly, this variation is not utilized by the colony - worker allocation to tasks is unrelated to their ability to perform them. What, then, are the adaptive benefits of behavioral specialization, and why do workers choose tasks without regard for whether they can perform them well? We are still far from an understanding of the adaptive benefits of division of labor in social insects. © 2008 Anna Dornhaus.
PMID: 12548429;Abstract:
Foragers of Bombus terrestris are able to alert their nestmates to the presence of food sources. It has been supposed that this happens at least partially through the distribution of a pheromone inside the nest. We substantiate this claim using a behavioral test in which an alerting signal is transmitted from one colony to another by long distance air transport, so excluding all other modalities of information exchange. We then investigated the source of the pheromone and were able to show that a hexane extract from tergites V-VII of bumble bee workers elicits higher activity, like a successful forager does. Extracts from other glands, such as the mandibular, labial, hypopharyngeal, and Dufour's gland as well as extracts from other parts of the cuticle had no effect. This suggests that bumble bees possess a pheromone-producing gland, similar to the Nasanov gland in honey bees. Indeed, an extract from the honey bee Nasanov gland also proved to alert bumblebee workers, suggesting a possible homology of the glands.
Abstract:
The importance of the spatial information which is communicated in the Camolian Race of the Western Honeybee, Apis mellifera carnica (Pollmann 1879) waggle dance relative to other cues used by bees in finding food sources was investigated. The efficiency of recruitment with and without transmission of direction information in the waggle dance was quantified using artificial, plentiful unscented food sources and hives which were turned to a horizontal position to disrupt orientation of dancing bees and thereby eliminate the spatial information from dances. Transmission of location information seems to improve recruitment effect particularly at large distances. Recruitment declines more rapidly with distance if dances are disoriented, and for large distances it takes a few hours before a foraging group is established. However, this shows that even without dance information, foragers manage to recruit some bees to their food source. This process, however, is so slow that by the time a group of recruits has reached the food source, it may not be worth exploiting any more. Transmission of spatial information thus is especially important if distant food sources which often change in nectar availability are exploited.
