Dynamics of Information Flow and Task Allocation of Social Insect Colonies: Impacts of Spatial Interactions and Task Switching.

Models of social interaction dynamics have been powerful tools for understanding the efficiency of information spread and the robustness of task allocation in social insect colonies. How workers spatially distribute within the colony, or spatial heterogeneity degree (SHD), plays a vital role in contact dynamics, influencing information spread and task allocation. We used agent-based models to explore factors affecting spatial heterogeneity and information flow, including the number of task groups, variation in spatial arrangements, and levels of task switching, to study: (1) the impact of multiple task groups on SHD, contact dynamics, and information spread, and (2) the impact of task switching on SHD and contact dynamics. Both models show a strong linear relationship between the dynamics of SHD and contact dynamics, which exists for different initial conditions. The multiple-task-group model without task switching reveals the impacts of the number and spatial arrangements of task locations on information transmission. The task-switching model allows task-switching with a probability through contact between individuals. The model indicates that the task-switching mechanism enables a dynamical state of task-related spatial fidelity at the individual level. This spatial fidelity can assist the colony in redistributing their workforce, with consequent effects on the dynamics of spatial heterogeneity degree. The spatial fidelity of a task group is the proportion of workers who perform that task and have preferential walking styles toward their task location. Our analysis shows that the task switching rate between two tasks is an exponentially decreasing function of the spatial fidelity and contact rate. Higher spatial fidelity leads to more agents aggregating to task location, reducing contact between groups, thus making task switching more difficult. Our results provide important insights into the mechanisms that generate spatial heterogeneity and deepen our understanding of how spatial heterogeneity impacts task allocation, social interaction, and information spread.

Dynamics of task allocation in social insect colonies: scaling effects of colony size versus work activities.

The mechanisms through which work is organized are central to understanding how complex systems function. Previous studies suggest that task organization can emerge via nonlinear dynamical processes wherein individuals interact and modify their behavior through simple rules. However, there is very limited theory about how those processes are shaped by behavioral variation within social groups. In this work, we propose an adaptive modeling framework on task allocation by incorporating variation both in task performance and task-related metabolic rates. We study the scaling effects of colony size on the resting probability as well as task allocation. We also numerically explore the effects of stochastic noise on task allocation in social insect colonies. Our theoretical and numerical results show that: (a) changes in colony size can regulate the probability of colony resting and the allocation of tasks, and the direction of regulation depends on the nonlinear metabolic scaling effects of tasks; (b) increased response thresholds may cause colonies to rest in varied patterns such as periodicity. In this case, we observed an interesting bubble phenomenon in the task allocation of social insect colonies for the first time; (c) stochastic noise can cause work activities and task demand to fluctuate within a range, where the amplitude of the fluctuation is positively correlated with the intensity of noise.

Peripheral sensory organs vary among ant workers but variation does not predict division of labor.

The neural mechanisms underlying behavioral variation among individuals are not well understood. Differences among individuals in sensory sensitivity could limit the environmental stimuli to which an individual is capable of responding and have, indeed, been shown to relate to behavioral differences in different species. Here, we show that ant workers in Temnothorax rugatulus differ considerably in the number of antennal sensory structures, or sensilla (by 45% in density and over 100% in estimated total number). A larger quantity of sensilla may reflect a larger quantity of underlying sensory neurons. This would increase the probability that a given set of neurons in the antenna detects an environmental stimulus and becomes excited, thereby eliciting the expression of a behavior downstream at lower stimulus levels than an individual with comparatively fewer sensilla. Individual differences in antennal sensilla density, however, did not predict worker activity level or performance of any task, suggesting either that variation in sensilla density does not, in fact, reflect variation in sensory sensitivity or that individual sensory response thresholds to task-associated stimuli do not determine task allocation as is commonly assumed, at least in this social insect. More broadly, our finding that even closely related individuals can differ strongly in peripheral sensory organ elaboration suggests that such variation in sensory organs could underlie other cases of intraspecific behavioral variation.

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.

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.

Who Are the "Lazy" Ants? The Function of Inactivity in Social Insects and a Possible Role of Constraint: Inactive Ants Are Corpulent and May Be Young and/or Selfish.

Social insect colonies are commonly thought of as highly organized and efficient complex systems, yet high levels of worker inactivity are common. Although consistently inactive workers have been documented across many species, very little is known about the potential function or costs associated with this behavior. Here we ask what distinguishes these "lazy" individuals from their nestmates. We obtained a large set of behavioral and morphological data about individuals, and tested for consistency with the following evolutionary hypotheses: that inactivity results from constraint caused by worker (a) immaturity or (b) senescence; that (c) inactive workers are reproducing; that inactive workers perform a cryptic task such as (d) acting as communication hubs or (e) food stores; and that (f) inactive workers represent the "slow-paced" end of inter-worker variation in "pace-of-life." We show that inactive workers walk more slowly, have small spatial fidelity zones near the nest center, are more corpulent, are isolated in colony interaction networks, have the smallest behavioral repertoires, and are more likely to have oocytes than other workers. These results are consistent with the hypotheses that inactive workers are immature and/or storing food for the colony; they suggest that workers are not inactive as a consequence of senescence, and that they are not acting as communication hubs. The hypotheses listed above are not mutually exclusive, and likely form a "syndrome" of behaviors common to inactive social insect workers. Their simultaneous contribution to inactivity may explain the difficulty in finding a simple answer to this deceptively simple question.

Task switching is associated with temporal delays in Temnothorax rugatulus ants.

The major evolutionary transitions often result in reorganization of biological systems, and a component of such reorganization is that individuals within the system specialize on performing certain tasks, resulting in a division of labor. Although the traditional benefit of division of labor is thought to be a gain in work efficiency, one alternative benefit of specialization is avoiding temporal delays associated with switching tasks. While models have demonstrated that costs of task switching can drive the evolution of division of labor, little empirical support exists for this hypothesis. We tested whether there were task-switching costs in Temnothorax rugatulus. We recorded the behavior of every individual in 44 colonies and used this dataset to identify each instance where an individual performed a task, spent time in the interval (i.e., inactive, wandering inside, and self-grooming), and then performed a task again. We compared the interval time where an individual switched task type between that first and second bout of work to instances where an individual performed the same type of work in both bouts. In certain cases, we find that the interval time was significantly shorter if individuals repeated the same task. We find this time cost for switching to a new behavior in all active worker groups, that is, independently of worker specialization. These results suggest that task-switching costs may select for behavioral specialization.