Abstract
Social insect colonies are highly successful, self-organized complex systems. Surprisingly however, most social insect colonies contain large numbers of highly inactive workers. Although this may seem inefficient, it may be that inactive workers actually contribute to colony function. Indeed, the most commonly proposed explanation for inactive workers is that they form a ‘reserve’ labor force that becomes active when needed, thus helping mitigate the effects of colony workload fluctuations or worker loss. Thus, it may be that inactive workers facilitate colony flexibility and resilience. However, this idea has not been empirically confirmed. Here we test whether colonies of Temnothorax rugatulus ants replace highly active (spending large proportions of time on specific tasks) or highly inactive (spending large proportions of time completely immobile) workers when they are experimentally removed. We show that colonies maintained pre-removal activity levels even after active workers were removed, and that previously inactive workers became active subsequent to the removal of active workers. Conversely, when inactive workers were removed, inactivity levels decreased and remained lower post-removal. Thus, colonies seem to have mechanisms for maintaining a certain number of active workers, but not a set number of inactive workers. The rapid replacement (within 1 week) of active workers suggests that the tasks they perform, mainly foraging and brood care, are necessary for colony function on short timescales. Conversely, the lack of replacement of inactive workers even 2 weeks after their removal suggests that any potential functions they have, including being a ‘reserve’, are less important, or auxiliary, and do not need immediate recovery. Thus, inactive workers act as a reserve labor force and may still play a role as food stores for the colony, but a role in facilitating colony-wide communication is unlikely. Our results are consistent with the often cited, but never yet empirically supported hypothesis that inactive workers act as a pool of ‘reserve’ labor that may allow colonies to quickly take advantage of novel resources and to mitigate worker loss.
Highlights
Complex systems are a broad class of systems in which behavior emerges from the actions and interactions of a number of independent units
To ensure that worker removals should have a significant impact on colony activity and inactivity levels, we compared the activity and inactivity levels of whole colonies pre-removal to the calculated mean colony activity level of colonies without the workers that would later be removed
We show that colonies maintained pre-removal activity levels after active workers were removed, despite calculations showing that activity should have been significantly reduced had the colony not compensated for the loss of active workers. We show that it is the previously inactive workers who became active subsequent to the removal of ‘active’ workers. This constitutes evidence for the hypothesis that inactive workers function as a reserve against worker loss and is relevant to the general question of task organization and how the colony responds to changes in task demands, including by task switches and switches to being active [55,77]
Summary
Complex systems are a broad class of systems in which behavior emerges from the actions and interactions of a number of independent units These can range from human-made systems such as computer networks [1,2], robot swarms [3,4], transportation networks [5,6], human organizations [7], and economic systems [8], as well as biological systems such as embryogenesis and organogenesis [9], disease transmission networks [10], genes expression networks [11,12,13], the organization of multicellular systems [14], and social insect colonies [15,16,17]. Social insect colonies are highly successful, evolved, self-organized collectives which are often used as models for the organization of complex systems They are thought to employ sophisticated individual and group-level strategies for the allocation of workers to tasks. Task allocation in insect colonies is expected to be flexible to changes in demand for different types of work [27], and robust to individual failure [28,29]
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