Who needs 'lazy' workers? Inactive workers act as a 'reserve' labor force replacing active workers, but inactive workers are not replaced when they are removed

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.

Fig 1

Mean worker time spent active and inactive pre-removal, 1 week after removal, and 2 weeks after removal of (a) active workers (20% most inactive in each colony) (b) inactive workers (20% most inactive in each colony), and (c) randomly selected workers (20% random workers from each colony). Boxplots show median (bar), quartiles (box), and extremes (whiskers) for best illustration (all figures). *Model: LMM, fixed: Worker inactivity/activity ~ Trial, random: Colony–Contrast significance determined using Tukey post hoc tests.

Fig 2

a) (left) Mean time spent active by the most active workers (top 20th percentile) pre-removal, 1 week after removal, and 2 weeks after removal of active workers. (b) (right) Mean time spent inactive by the most inactive workers (top 20^th percentile) pre-removal, 1 week after removal, and 2 weeks after removal of inactive workers. c) (left) Mean time spent active by the most active and (right) the most inactive workers (top 20^th percentile) pre-removal, 1 week after removal, and 2 weeks after removal of randomly selected workers. *Model: LMM, fixed: Worker inactivity/activity ~ Trial, random: Colony–Contrast significance determined using Tukey post hoc tests.

Fig 3. Mean time spent on specific tasks by the most active workers (top 20^th percentile) pre-removal and by the most active workers (of those remaining) 2 weeks after removal of active workers.

*Model: LMM, fixed: Worker time on task ~ Trial, random: Colony.

Fig 4

a) Time spent inactive pre-removal predicts time spent inactive post-removal when ‘inactive’ and randomly selected workers are removed (b and c), but not when ‘active’ workers are removed (a). Time spent active pre-removal did not predict time spent active post-removal for any treatment (a, b and c). Colored points highlight different colonies. Marginal (variance explained by fixed effects, i.e. by time spent active/inactive pre and post removal) and Conditional (total variance explained by fixed effects and random effects, i.e. including between-colony differences) R²: a) Active Removal—Activity: Marginal R² = 0.007, Conditional R² = 0.120; Inactivity: Marginal R² = 0.024, Conditional R² = 0.231; b) Inactive Removal—Activity: Marginal R² = 0.005, Conditional R² = 0.130; Inactivity: Marginal R² = 0.081, Conditional R² = 0.337; c) Random Removal—Activity: Marginal R² = 0.018, Conditional R² = 0.658; Inactivity: Marginal R² = 0.048, Conditional R² = 0.495. *Model: LMM, fixed: Worker activity/inactivity 2 weeks post ~ Worker activity/inactivity pre, random: Colony.

Fig 5

a) (left) Prior to ‘active’ worker removal, the 20% most active workers were mainly from the Nurse, Walker, and Forager task groups, but 2 weeks after removals the most active workers were mainly from the inactive and Walker task groups. b) (right) Prior to ‘inactive’ worker removal, the 20% most inactive workers were solely from the inactive task group, while 2 weeks after removals the most inactive workers were still mainly from the inactive task group, but there were also Nurses and Walkers (Fisher’s exact test, p<0.0001). c) After removal of random workers, colonies had (left) more inactive workers and less foragers in the top 20% most active, and (right) less inactive workers and more of each other task group in the 20% most inactive workers. * Fisher’s exact tests comparing frequencies of task groups pre- and 2 weeks post-removal.