Stop and go: exploring alternative mechanisms for task allocation in social insects - response and satisfaction thresholds trade off cost, accuracy, and speed differently

Abstract

Division of labor, a key feature of many complex systems, requires a mechanism that allows individuals to choose tasks. The popular response threshold hypothesis posits that some workers start engaging in particular tasks at a lower level of need than others. However, individuals may only have access to information about need after they actually engage in a task. We therefore introduce two novel interpretations of this task-allocation mechanism. While the response threshold mechanism determines when individuals start working, the satisfaction threshold mechanism drives when individuals stop working. We also model a composite threshold mechanism where workers consider task need both to start and end working. Second, we model the possibility that the stimulus perceived by workers is a completion cue instead of a demand cue. While these may seem like subtle variations, we show here that they can yield dramatically different collective dynamics. In simulations with biologically relevant parameter ranges, response thresholds produced the quickest reaction to increases in task demand, satisfaction thresholds yielded the lowest task-switching rate, and composite thresholds most closely matched the number of workers allocated to the number needed. Different threshold types thus differentially trade off speed, cost, and accuracy. We did not model benefits of specialization; purely in terms of allocating workers to tasks, we also found that response thresholds usually perform worse than a null random choice model in terms of cost and efficiency, and variation among workers does not improve task allocation. Colonies utilizing task demand cues also tend to perform better than those using task completion cues. Our results ultimately suggest that different threshold mechanisms may be suited for different situations or types of tasks. Author SummaryDivision of labor is a phenomenon where workers in a community consistently differ in the tasks they work on. Many scientists believe division of labor arises in social insects (i.e. ants and bees) as a result of difference in workers responsiveness to cues that correspond to the demand for work in a task. For example, some ants in a colony start feeding brood much sooner than others, possibly because of a higher sensitivity, or lower response threshold, to brood pheromone. We show that instead of using such a cue to decide when to start on a task, theoretically workers may instead use it only to decide when to stop working; similarly, workers may use a cue that tells them how much work is needed in a task, or they may use one that corresponds to how much work has already been done. These seemingly subtle differences affect how much a colony invests in work and how quickly stability is reached when the balance of work needed in different tasks changes. Therefore, these different mechanisms may evolve to solve different problems.