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Dynamics of Information Flow and Task Allocation of Social Insect Colonies: Impacts of Spatial Interactions and Task Switching
Abstract
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.
... Additionally, the foraging patterns observed in ant colonies show notable similarities to other biological processes, such as the aggregation of immune cells and the propagation of chemical signals (Sumpter 2010). These interdisciplinary parallels provide deeper insights into the principles of self-organization and adaptation in complex systems (Bonabeau et al. 1999;Chen et al. 2024;Dorigo et al. 1996;Guo et al. 2020;Kang et al. 2015;Kang and Theraulaz 2016;Navas-Zuloaga et al. 2023;Pratt 2009;Wang et al. 2024). ...
Understanding the collective foraging strategies of ant colonies is essential for studying
self-organization and collective behavior in biological systems. This study introduces
a simplified foraging model that classifies worker roles into two functional groups-
available workers and active foragers-providing a concise yet effective framework for
analyzing foraging dynamics. Our model effectively reproduces the foraging dynamics
observed in more complex three-dimensional models, while taking a more tractable
form that is more conducive to mathematical analysis. We examine the effects of
stochasticity in mortality rates of foragers and workers on foraging state transitions,
with a particular emphasis on the critical noise threshold, transition probability, and
transition time. While the critical noise threshold is reduced by stochasticity in either
of the two mortality rates, that of active foragers has the greatest effect. We find that
slight increases in the arrival rate of available workers and the recruitment rate of active
foragers enhance the colony’s resilience to environmental stochasticity, suggesting that
colonies can self-regulate via a feedback loop of foraging and recruitment to maintain
their foraging while their environment changes around them. In contrast, varying the
mortality rate of available workers had little effect on this resilience, analogously
to experimental observations that older or unhealthy worker ants disproportionately
transition into foraging roles. This study not only advances our understanding of ant
foraging dynamics by simplifying complex models but also provides valuable insights
into the robustness of foraging activities under varying environmental conditions.
Many animal species divide space into a patchwork of home ranges, yet there is little consensus on the mechanisms individuals use to maintain fidelity to particular locations. Theory suggests that animal movement could be based upon simple behavioural rules that use local information such as olfactory deposits, or global strategies, such as long-range biases toward landmarks. However, empirical studies have rarely attempted to distinguish between these mechanisms. Here, we perform individual tracking experiments on four species of social insects, and find that colonies consist of different groups of workers that inhabit separate but partially-overlapping spatial zones. Our trajectory analysis and simulations suggest that worker movement is consistent with two local mechanisms: one in which workers increase movement diffusivity outside their primary zone, and another in which workers modulate turning behaviour when approaching zone boundaries. Parallels with other organisms suggest that local mechanisms might represent a universal method for spatial partitioning in animal populations.
Alarm signal propagation through ant colonies provides an empirically tractable context for analysing information flow through a natural system, with useful insights for network dynamics in other social animals. Here, we develop a methodological approach to track alarm spread within a group of harvester ants, Pogonomyrmex californicus. We initially alarmed three ants and tracked subsequent signal transmission through the colony. Because there was no actual standing threat, the false alarm allowed us to assess amplification and adaptive damping of the collective alarm response. We trained a random forest regression model to quantify alarm behaviour of individual workers from multiple movement features. Our approach translates subjective categorical alarm scores into a reliable, continuous variable. We combined these assessments with automatically tracked proximity data to construct an alarm propagation network. This method enables analyses of spatio-temporal patterns in alarm signal propagation in a group of ants and provides an opportunity to integrate individual and collective alarm response. Using this system, alarm propagation can be manipulated and assessed to ask and answer a wide range of questions related to information and misinformation flow in social networks.
Much like a single organism, colonies of social insects regulate their collective nutrition towards a specific intake of nutrients. Yet, the mechanisms behind this colony‐level regulation are not fully understood. Although foraging behaviour in social insects has been studied extensively, not much is known on its relation with feeding processes that occur within the nest. In the nest, food is commonly transferred between individuals in numerous oral feeding interactions (trophallaxis). Studies on the properties of these trophallactic networks have so far been limited by: (a) the difficulty to non‐intrusively measure food inside individual ants, let alone its nutritional composition, and (b) the meticulous manual labour involved in detecting trophallactic events.
Our dual‐fluorescence imaging set‐up is designed to track two food sources, each labelled with a different fluorophore, as they are disseminated throughout a freely behaving colony of individually tagged ants. Additionally, our image‐based deep learning algorithm for automatic detection of ant trophallaxis events efficiently yields a detailed record of all food‐transfer interactions.
Using a series of calibration experiments, we demonstrate the reliability of our measurements. We then exemplify the capabilities of our new method by tracking food dissemination in a colony of Camponotus sanctus ants supplied with two nutritionally distinct food sources. We follow the amount of each food type in the crop of each ant in the colony. Our data reveal the path in nutrient space that the colony took to reach its final nutrient intake, the contributions of the different forager ants to this path and the dynamic nutritional states of all ants in the colony. We further demonstrate the potential applicability of dual‐fluorescence food imaging to other insect species.
Our system provides access to data that were previously unavailable and is crucial for a complete description of nutrient regulation by social insects. More broadly, the ability to simultaneously and efficiently track two material flows in a behaving colony provides new opportunities for tackling many open and emerging questions on topics other than nutrition, such as disease spread and social regulation of colony development.
Reproductive division of labor (e.g., germ-soma specialization) is a hallmark of the evolution of multicellularity, signifying the emergence of a new type of individual and facilitating the evolution of increased organismal complexity. A large body of work from evolutionary biology, economics, and ecology has shown that specialization is beneficial when further division of labor produces an accelerating increase in absolute productivity (i.e., productivity is a convex function of specialization). Here we show that reproductive specialization is qualitatively different from classical models of resource sharing, and can evolve even when the benefits of specialization are saturating (i.e., productivity is a concave function of specialization). Through analytical theory and evolutionary individual-based simulations, we demonstrate that reproductive specialization is strongly favored in sparse networks of cellular interactions that reflect the morphology of early, simple multicellular organisms, highlighting the importance of restricted social interactions in the evolution of reproductive specialization.
Social insects allocate their workforce in a decentralised fashion, addressing multiple tasks and responding effectively to environmental changes. This process is fundamental to their ecological success, but the mechanisms behind it are not well understood. While most models focus on internal and individual factors, empirical evidence highlights the importance of ecology and social interactions. To address this gap, we propose a game theoretical model of task allocation. Our main findings are twofold: Firstly, the specialisation emerging from self-organised task allocation can be largely determined by the ecology. Weakly specialised colonies in which all individuals perform more than one task emerge when foraging is cheap; in contrast, harsher environments with high foraging costs lead to strong specialisation in which each individual fully engages in a single task. Secondly, social interactions lead to important differences in dynamic environments. Colonies whose individuals rely on their own experience are predicted to be more flexible when dealing with change than colonies relying on social information. We also find that, counter to intuition, strongly specialised colonies may perform suboptimally, whereas the group performance of weakly specialised colonies approaches optimality. Our simulation results fully agree with the predictions of the mathematical model for the regions where the latter is analytically tractable. Our results are useful in framing relevant and important empirical questions, where ecology and interactions are key elements of hypotheses and predictions.
The relationship between division of labor and individuals' spatial behavior in social insect colonies provides a useful context to study how social interactions influence the spreading of elements (which could be information, virus or food) across distributed agent systems. In social insect colonies, spatial heterogeneity associated with variations of individual task roles, affects social contacts, and thus the way in which agent moves through social contact networks. We used an Agent Based Model (ABM) to mimic three realistic scenarios of elements' transmission, such as information, food or pathogens, via physical contact in social insect colonies. Our model suggests that individuals within a specific task interact more with consequences that elements could potentially spread rapidly within that group, while elements spread slower between task groups. Our simulations show a strong linear relationship between the degree of spatial heterogeneity and social contact rates, and that the spreading dynamics of elements follow a modified nonlinear logistic growth model with varied transmission rates for different scenarios. Our work provides important insights on the dual-functionality of physical contacts. This dual-functionality is often driven via variations of individual spatial behavior, and can have both inhibiting and facilitating effects on elements' transmission rates depending on environment. The results from our proposed model not only provide important insights on mechanisms that generate spatial heterogeneity, but also deepen our understanding of how social insect colonies balance the benefit and cost of physical contacts on the elements' transmission under varied environmental conditions.
In the Florida harvester ant, Pogonomyrmex badius, foragers occur only in the top 15 cm of the nest, whereas brood and brood-care workers reside mostly in the deepest regions, yet the food and seeds foragers collect must be transported downward 30 to 80 cm to seed chambers and up to 2 m to brood chambers. Using mark-recapture techniques with fluorescent printer's ink, we identified a class of workers that ranges widely within the vertical structure of the nest, rapidly moving materials dropped by foragers in the upper regions downward, and excavated soil from deeper upward. Within the nest, only 5% of foragers were recovered below 20 cm depth, but about 30% of transfer workers and 82% of unmarked workers were found there. Below 70 cm depth, 90% of workers were unmarked, and were probably involved mostly in brood care. During the summer, the transfer workers comprise about a quarter of the nest population, while foragers make up about 40%. Workers marked as transfer workers later appear as foragers, while those marked as foragers die and disappear from the foraging population, suggesting that transfer workers are younger, and age into foraging. The importance of these findings for laboratory studies of division of labor are discussed. The efficient allocation of labor is a key component of superorganismal fitness.
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.
Metabolic rates of individual animals and social insect colonies generally scale hypometrically, with mass-specific metabolic rates decreasing with increasing size. Although this allometry has wide ranging effects on social behaviour, ecology and evolution, its causes remain controversial. Because it is difficult to experimentally manipulate body size of organisms, most studies of metabolic scaling depend on correlative data, limiting their ability to determine causation. To overcome this limitation, we experimentally reduced the size of harvester ant colonies (Pogonomyrmex californicus) and quantified the consequent increase in mass-specific metabolic rates. Our results clearly demonstrate a causal relationship between colony size and hypometric changes in metabolic rate that could not be explained by changes in physical density. These findings provide evidence against prominent models arguing that the hypometric scaling of metabolic rate is primarily driven by constraints on resource delivery or surface area/volume ratios, because colonies were provided with excess food and colony size does not affect individual oxygen or nutrient transport. We found that larger colonies had lower median walking speeds and relatively more stationary ants and including walking speed as a variable in the mass-scaling allometry greatly reduced the amount of residual variation in the model, reinforcing the role of behaviour in metabolic allometry. Following the experimental size reduction, however, the proportion of stationary ants increased, demonstrating that variation in locomotory activity cannot solely explain hypometric scaling of metabolic rates in these colonies. Based on prior studies of this species, the increase in metabolic rate in sizereduced colonies could be due to increased anabolic processes associated with brood care and colony growth. © 2017 The Author(s) Published by the Royal Society. All rights reserved.
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.
The concerted responses of eusocial insects to environmental stimuli are often referred to as collective cognition on the level of the colony.To achieve collective cognitiona group can draw on two different sources: individual cognitionand the connectivity between individuals.Computation in neural-networks, for example,is attributedmore tosophisticated communication schemes than to the complexity of individual neurons. The case of social insects, however, can be expected to differ. This is since individual insects are cognitively capable units that are often able to process information that is directly relevant at the level of the colony.Furthermore, involved communication patterns seem difficult to implement in a group of insects since these lack clear network structure.This review discusses links between the cognition of an individual insect and that of the colony. We provide examples for collective cognition whose sources span the full spectrum between amplification of individual insect cognition and emergent group-level processes.
Animal societies rely on interactions between group members to effectively communicate and coordinate their actions. To date, the transmission properties of interaction networks formed by direct physical contacts have been extensively studied for many animal societies and in all cases found to inhibit spreading. Such direct interactions do not, however, represent the only viable pathways. When spreading agents can persist in the environment, indirect transmission via 'same-place, different-time' spatial coincidences becomes possible. Previous studies have neglected these indirect pathways and their role in transmission. Here, we use rock ant colonies, a model social species whose flat nest geometry, coupled with individually tagged workers, allowed us to build temporally and spatially explicit interaction networks in which edges represent either direct physical contacts or indirect spatial coincidences. We show how the addition of indirect pathways allows the network to enhance or inhibit the spreading of different types of agent. This dual-functionality arises from an interplay between the interaction-strength distribution generated by the ants' movement and environmental decay characteristics of the spreading agent. These findings offer a general mechanism for understanding how interaction patterns might be tuned in animal societies to control the simultaneous transmission of harmful and beneficial agents.
High-density living is often associated with high disease risk due to density-dependent epidemic spread. Despite being paragons of high-density living, the social insects have largely decoupled the association with density-dependent epidemics. It is hypothesized that this is accomplished through prophylactic and inducible defenses termed 'collective immunity'. Here we characterise segregation of carpenter ants that would be most likely to encounter infectious agents (i.e. foragers) using integrated social, spatial, and temporal analyses. Importantly, we do this in the absence of disease to establish baseline colony organization. Behavioural and social network analyses show that active foragers engage in more trophallaxis interactions than their nest worker and queen counterparts and occupy greater area within the nest. When the temporal ordering of social interactions is taken into account, active foragers and inactive foragers are not observed to interact with the queen in ways that could lead to the meaningful transfer of disease. Furthermore, theoretical resource spread analyses show that such temporal segregation does not appear to impact the colony-wide flow of food. This study provides an understanding of a complex society's organization in the absence of disease that will serve as a null model for future studies in which disease is explicitly introduced.
We expect that human organizations and cooperative animal groups should be optimized for collective performance. This often involves the allocation of different individuals to different tasks. Social insect colonies are a prime example of cooperative animal groups that display sophisticated mechanisms of task allocation. Here we discuss which task allocation strategies may be adapted to which environmental and social conditions. Effective and robust task allocation is a hard problem, and in many biological and engineered complex systems is solved in a decentralized manner: human organizations may benefit from insights into what makes decentralized strategies of group organization effective. In addition, we often find considerable variation among individuals in how much work they appear to contribute, despite the fact that individual selfishness in social insects is low and optimization occurs largely at the group level. We review possible explanations for uneven workloads among workers, including limitations on individual information collection or constraints of task allocation efficiency, such as when there is a mismatch between the frequency of fluctuations in demand for work and the speed at which workers can be reallocated. These processes are likely to apply to any system in which worker agents are allocated to tasks with fluctuating demand, and should therefore be instructive to understanding optimal task allocation and inactive workers in any distributed system. Some of these processes imply that a certain proportion of inactive workers can be an adaptive strategy for collective organization.
Social insect colonies are often considered to be highly efficient collective systems, with division of labor at the root of their ecological success. However, in many species, a large proportion of a colony’s workers appear to spend their time completely inactive. The role of this inactivity for colony function remains unclear. Here, we investigate how inactivity is distributed among workers and over time in the ant Temnothorax rugatulus. We show that the level of inactivity is consistent for individual workers, but differs significantly among workers, that is, some workers effectively specialize on ‘inactivity’. We also show that workers have circadian rhythms, although intra-nest tasks tend to be performed uniformly across the whole day. Differences in circadian rhythms, or workers taking turns resting (i.e., working in shifts), cannot explain the observation that some workers are consistently inactive. Using extensive individual-level data to describe the overall structure of division of labor, we show that ‘inactive workers’ form a group distinct from other task groups. Hierarchical clustering suggests that inactivity is the primary variable in differentiating both workers and tasks. Our results underline the importance of inactivity as a behavioral state and the need for further studies on its evolution.
Social insect colonies are models for complex systems with sophisticated, efficient, and robust allocation of workers to necessary tasks. Despite this, it is commonly reported that many workers appear inactive. Could this be an artifact resulting from the simplified laboratory conditions in most studies? Here, we test whether the time allocated to different behavioral states differs between field and laboratory colonies of Temnothorax rugatulus ants. Our results show no difference in colony time budgets between laboratory and field observations for any of the observed behaviors, including ‘inactivity’. This suggests that, on the timescale of a few months, laboratory conditions do not impact task allocation at the colony level. We thus provide support for a previously untested assumption of laboratory studies on division of labor in ants. High levels of inactivity, common in social insects, thus appear to not be a laboratory artifact, but rather a naturally occurring trait.
How social insect colonies behave results from the actions of their workers. Individual variation among workers in their response to various tasks is necessary for the division of labor within colonies. A worker may be active in only a subset of tasks (specialist), perform all tasks (elite), or exhibit no particular pattern of task activity (idiosyncratic). Here we examine how worker activity is distributed among and within tasks in ants of the genus Temnothorax. We found that workers exhibited elitism within a situation, i.e., in particular sets of tasks, such as those associated with emigrations, nest building, or foraging. However, there was weak specialization for working in a particular situation. A few workers exhibited elitism across all situations, i.e., high performance in all tasks in all situations. Within any particular task, the distribution of activity among workers was skewed, with few ants performing most of the work and most ants performing very little of the work. We further found that workers persisted in their task preference over days, with the same individuals performing most of the work day after day. Interestingly, colonies were robust to the removal of these highly active workers; they were replaced by other individuals that were previously less active. This replacement was not short-lived; when the removed individuals were returned to the colony, not all of them resumed their prior high activity levels, and not all the workers that replaced them reduced their activity. Thus, even though some workers specialize in tasks within a particular situation and are persistent in performing them, task allocation in a colony is plastic and colonies can withstand removal of highly active individuals.
Size has profound consequences for the structure and function of biological systems, across levels of organization from cells to social groups. As tightly integrated units that vary greatly in size, eusocial insect colonies, in particular, are expected to exhibit social scaling relations. To address the question of how social organization scales with colony size, we quantified task performance in variably sized colonies of the harvester ant Pogonomyrmex californicus. We found a positive scaling relationship between colony size and division of labor in 2 different contexts. First, individual workers were more specialized in older, larger colonies. Second, division of labor increased with colony size, independently of colony age. Moreover, the proportional allocation of workers to tasks shifted during colony ontogeny--older, larger colonies performed relatively less brood care--but did not vary with colony size among same-aged colonies. There were no colony-size effects on per capita activity or the distribution of activity across workers. Size-related changes in task performance were correlated with changes in the rate of encounter between nest mates. These results highlight the importance of colony size for the organization of work in insect societies and raise broader questions about the role of size in sociobiology. Copyright 2011, Oxford University Press.
This paper extends the notion of spatial efficiency in the organization
of social insect colonies. We demonstrate for the first time that ants
(individually marked workers in three colonies of Leptothorax
unifasciatus (Latr.)) are not only faithful to particular positions
within the nest, but they also quickly readopt these positions, relative
to one another, when the colony emigrates to an entirely new nest site.
This phenomenon, which we term social resilience, has implications for
the role of learning in the maintenance of an efficient division of
labour to which, in part, the great ecological success of social insects
has been attributed. As we demonstrate with observations of another
three colonies over a period of six months, workers change their
positions asynchronously and different age cohorts are intermingled.
Thus the reconstruction of colony spatial order cannot be accounted for
by age-based task allocation (i.e. age polyethism), as at any one time
the colony meshwork represents a heterogeneous mixture of different
generations. These findings also show that ant colonies have a much more
precise spatial structure and greater cohesion than previously assumed,
and demonstrates the importance of detailed quantitative examination of
the sociogenesis or developmental biology of these societies.
Ants live in organized societies with a marked division of labor among workers, but little is known about how this division
of labor is generated. We used a tracking system to continuously monitor individually tagged workers in six colonies of the
ant Camponotus fellah over 41 days. Network analyses of more than 9 million interactions revealed three distinct groups that differ in behavioral
repertoires. Each group represents a functional behavioral unit with workers moving from one group to the next as they age.
The rate of interactions was much higher within groups than between groups. The precise information on spatial and temporal
distribution of all individuals allowed us to calculate the expected rates of within- and between-group interactions. These
values suggest that the network of interaction within colonies is primarily mediated by age-induced changes in the spatial
location of workers.
Social animals exchange information during social interaction. The rate of interaction and, hence, the rate of information exchange, typically changes with density and density may be affected by the size of the social group. We investigate models in which each individual may be engaged in one of several tasks. For example, the different tasks could represent alternative foraging locations exploited by an ant colony. An individual's decision about which task to pursue depends both on environmental stimuli and on interactions among individuals. We examine how group size affects the allocation of individuals among the various tasks. Analysis of the models shows the following. (1) Simple interactions among individuals with limited ability to process information can lead to group behaviour that closely approximates the predictions of evolutionary optimality models, (2) Because per capita rates of social interaction may increase with group size, larger groups may be more efficient than smaller ones at tracking a changing environment, (3) Group behaviour is determined both by each individual's interaction with environmental stimuli and by social exchange of information. To keep these processes in balance across a range of group sizes, organisms are predicted to regulate per capita rates of social interaction and (4) Stochastic models show, at least in some cases, that the results described here occur even in small groups of approximately ten individuals.
As conditions change, social insect colonies adjust the numbers of workers engaged in various tasks, such as foraging and
nest work. This process of task allocation operates without central control; individuals respond to simple, local cues. This
study investigates one such cue, the pattern of an ant's interactions with other workers. We examined how an ant's tendency
to perform midden work, carrying objects to and sorting the refuse pile of the colony, is related to the recent history of
the ant's brief antennal contacts, in laboratory colonies of the red harvester ant, Pogonomyrmex barbatus. The probability that an ant performed midden work was related to its recent interactions in two ways. First, the time an
ant spent performing midden work was positively correlated with the number of midden workers that ant had met while it was
away from the midden. Second, ants engaged in a task other than midden work were more likely to begin to do midden work when
their rate of encounter per minute with midden workers was high. Cues based on interaction rate may enable ants to respond
to changes in worker numbers even though ants cannot count or assess total numbers engaged in a task.
When its nest is damaged, a colony of the ant Leptothorax albipennis skillfully emigrates to the best available new site. We investigated how this ability emerges from the behaviors used by ants to recruit nestmates to potential homes. We found that, in a given emigration, only one-third of the colony's workers ever recruit. At first, they summon fellow recruiters via tandem runs, in which a single follower is physically led all the way to the new site. They later switch to recruiting the passive majority of the colony via transports, in which nestmates are simply carried to the site. After this switch, tandem runs continue sporadically but now run in the opposite direction, leading recruiters back to the old nest. Recruitment accelerates with the start of transport, which proceeds at a rate 3 times greater than that of tandem runs. The recruitment switch is triggered by population increase at the new site, such that ants lead tandem runs when the site is relatively empty, but change to transport once a quorum of nestmates is present. A model shows that the quorum requirement can help a colony choose the best available site, even when few ants have the opportunity to compare sites directly, because recruiters to a given site launch the rapid transport of the bulk of the colony only if enough active ants have been "convinced" of the worth of the site. This exemplifies how insect societies can achieve adaptive colony-level behaviors from the decentralized interactions of relatively poorly informed insects, each combining her own limited direct information with indirect cues about the experience of her nestmates.
Social insect colonies possess remarkable abilities to select the best among several courses of action. In populous societies with highly efficient recruitment behaviour, decision-making is distributed across many individuals, each acting on limited local information with appropriate decision rules. To investigate the degree to which small societies with less efficient recruitment can also employ distributed decision-making, we studied nest site selection in Leptothorax albipennis. Colonies were found to make sophisticated choices, taking into account not only the intrinsic qualities of each site, but also its value relative to the available options. In choices between two sites, individual ants were able to visit both sites, compare them and choose the better one. However, most ants encountered only one site in the course of an emigration. These poorly informed ants also contributed to the colony's decision, because their probability of initiating recruitment to a site depended on its quality. This led to shorter latencies between discovery and recruitment to a superior site, and so created greater amplification via positive feedback of the population at the better site. In short, these small colonies make use of a distributed mechanism of information processing, but also take advantage of direct decision-making by well-informed individuals. The latter feature may in part stem from the limitations of their social structure, but may also reflect the stringent demand for unanimous decisions by house-hunting colonies of any size.
Social insect colonies operate without central control, and colony organization results from the ways that individuals respond to local information. We investigated how temporal information, in particular the rate of interaction among workers, stimulates foraging activity in the red harvester ant (Pogonomyrmex barbatus). Patrollers scout the foraging area each morning. Previous work showed that the patrollers' safe return to the nest stimulates the foragers to leave the nest; if the patrollers do not return, the foragers do not emerge. Here, we tested whether contact with returning patrollers must occur at a particular rate to stimulate foraging. We varied the rates at which we introduced patroller mimics, glass beads coated with an extract of the cuticular hydrocarbons of patrollers. A return rate of 1 patroller mimic every 10 s stimulated the highest level of foraging activity. We found that the onset of foraging depends on the rate of patroller return. Adding beads coated with patroller hydrocarbons at a rate of 1 per 10 s caused otherwise undisturbed colonies to forage 17.9 ± 19.7 (standard deviation) min faster than colonies that received blank control beads. These results show that rate is a crucial source of information in the network of interactions among workers. Copyright 2007, Oxford University Press.
Division of labor is one of the most fascinating phenomena found in social insects and is probably responsible for their tremendous ecological success. We show how major features of this division of labor may represent self-organized properties of a complex system where individuals share an information data base (a stimulus environment), make independent decisions about how to respond to the current condition of that data base (stimulus environment), and alter the data base by their actions. We argue that division of labor can emerge from such systems even without a history of natural selection, that in fact such ordered behavior is an inescapable property of group living. We then show how natural selection can operate on self-organized complex systems (social organization) and result in adaptation of division of labor. © Inra/DIB/AGIB/Elsevier, Paris
Flexibility in task performance is essential for a robust system of division of labour. We investigated what factors determine which social insect workers respond to colony-level changes in task demand. We used radio-frequency identification technology to compare the roles of corpulence, age, spatial location and previous activity (intra-nest/extra-nest) in determining whether worker ants (Temnothorax albipennis) respond to an increase in demand for foraging or brood care. The less corpulent ants took on the extra foraging, irrespective of their age, previous activity or location in the nest, supporting a physiological threshold model. We found no relationship between ants that tended the extra brood and corpulence, age, spatial location or previous activity, but ants that transported the extra brood to the main brood pile were less corpulent and had high previous intra-nest activity. This supports spatial task-encounter and physiological threshold models for brood transport. Our data suggest a flexible task-allocation system allowing the colony to respond rapidly to changing needs, using a simple task-encounter system for generalized tasks, combined with physiologically based response thresholds for more specialized tasks. This could provide a social insect colony with a robust division of labour, flexibly allocating the workforce in response to current needs.
Complex living systems often exist in noisy environments and must have a way to respond to change. In social insects, the colony itself is a complex system composed of dozens to millions of essentially autonomous workers. Studying the behaviour of these workers in response to experimental disturbance provides insight into the mechanisms by which colonies, and complex systems in general, can achieve flexibility. Here, we explore dynamic task allocation within colonies of Temnothorax rugatulus ants by separately increasing the demand for three different types of work: nest maintenance, brood care and foraging. We investigate (1) whether colonies respond to dynamic task demand and the timeline of their responses, (2) whether the colony achieves this flexibility by recruiting new workers to these tasks or modulating individual worker effort and (3) the rules by which individual workers switch tasks. We found that T. rugatulus ants are responsive to colony perturbation, yet the means by which they achieve this flexibility are task dependent, as is the time it takes them to respond. Flexibility is achieved both by the increased effort of already active workers and the recruitment of new workers to the focal task. We suggest that newly recruited workers may come from task-specific reserve pools of unemployed workers: roaming ‘walkers’ appear to be a generalized reserve force for most tasks except for brood care, while previously inactive workers might act as a specialized reserve pool for brood care and be prompted to engage in this task when they locally encounter brood.
Protecting the colony
When we get a cold and then stay home from work, we are not only taking care of ourselves but also protecting others. Such changes in behavior after infection are predicted in social animals but are difficult to quantify. Stroeymeyt et al. looked for such changes in the black garden ant and found that infected workers did alter their behavior—and healthy workers altered their behavior toward the sick. The changed behavior was especially valuable for protecting the most important and vulnerable members of the colony.
Science , this issue p. 941
Patchy distributions matter, from the point of view of the animals involved, because the individuals tend to find more others of their own kind right around them than would be the case in a random distribution. The animals are more 'crowded', in this sense, than their mean density would lead one to believe. For data from randomly placed quadrats, the proposed parameter 'mean crowding' () attempts to measure this effect by defining the mean number per individual of other individuals in the same quadrat. 'Mean crowding' is algebraically identical with mean density, augmented by the amount the ratio of variance to mean exceeds unity, i.e. . It can be viewed as equal to that increased mean density which a patchily distributed population could have, and be no more 'crowded' on the average than it is now, if it had a random distribution. The contention that populations with differing mean densities but the same 'mean crowding' would suffer the same density-dependent effects on mortality, natality, and dispersal implies the untested assumption that the effect of crowding on each individual varies linearly with the number of others around it, which are responsible for the crowding. Also implied is that each local situation (quadrat) is equally good habitat for the animals, has an equal rate of supply of expendable resources, and provides an equal amount of 'room', or cover, which the animals can use to avoid each other. If this assumption is perfectly fulfilled, however, then the patchy distribution itself becomes an ecological enigma: if the effects of crowding are more severe in the locally densest patches, then the local density should decrease more rapidly and increase more slowly there than elsewhere. The overall patchiness should decrease, in fact, until the distribution eventually comes to resemble a random one. Undoubtedly, a great deal of the patchines that one finds in continuous habitats can be explained in terms of local differences in habitat suitability. On the other hand, there is often a large component that appears to be utterly capricious. Since all of the capricious component should long since have been eroded away by density-dependent effects of crowding-if these are important-I must conclude that they are not important in such species, or, if they are, that other population processes (e.g. interactions with other species) regenerate patchiness as rapidly as it is destroyed. This fits in well with a general conviction that patchiness has a great deal more to do with undercrowding than with overcrowding, since 'mean crowding' often appears to be as great in rare species as it is in common ones (of comparable size). A suitable measure of patchiness is the ratio of 'mean crowding' to mean density. Where the distribution can be adequately approximated by the negative binomial, with parameters m and k, we have . The sample estimate for 'mean crowding' is . Standard errors are given for the cases where k̂ is estimated by maximum likelihood, by moments, by the number of empty quadrats, and also for the truncated negative binomial. The idea is briefly extended to apply to 'mean crowding' between species and to 'subjective' species diversity, without attempting in these cases to develop standard errors. Most importantly, I have tried to specify the types of habitats and distributions to which the parameter 'mean crowding' is not intended to apply.
The social organization of insect colonies has fascinated biologists and natural historians for centuries. Aristotle wrote in History of Animals about a division of labor among workers within the hive that is based on age. He observed that the field bees foraging for nectar and pollen have less “hair” on their bodies than the hive bees that care for young larvae and tend the nest. He concluded that the more pubescent hive bees must be older. We now know that, in fact, the field bees are older and have less hair because the hairs break off as the bees age. The phenomenon of age related changes in behavior, age-polyethism, is now well documented for many social insects (Oster and Wilson 1978).
Evidence of the ecological success of social insects is inescapable. Virtually everywhere you look you see them or the results of their activities.
In social insect colonies, workers perform a variety of tasks, such as foraging, brood care and nest construction. As the needs of the colony change, and as resources become available, colonies adjust the numbers of workers engaged in each task. Task allocation is the process that results in specific workers being engaged in specific tasks, in numbers appropriate to the current situation.
BIOLOGICAL SYSTEMS, RANGING from the molecular to the cellular to the organism level, are distributed and in most cases operate without central control. The key for successful studies at the intersection of distributed computing and biology is to identify problems in which similar constraints and goals may apply to both systems. Networks provide one of many popular abstractions that have been immensely useful in understanding large, distributed systems. Information processing in biology is also often based on message passing. Cells secrete proteins to interact with other cells in order to activate various signaling networks. The previous models assume nodes communicate by exchanging messages. Another popular distributed communication method is the use of shared memory.
This paper investigates how the movement zones of all the ants in a colony are organized inside the nest. The workers in nine colonies of the antLeptothorax unifasciatus(Latr.) were marked individually and their positions in the nest were recorded over 33 periods of observation spread throughout the year. Results from randomization tests demonstrated that the individual workers inL.unifasciatuscolonies had movement zones of limited area. These are termed spatial fidelity zones (SFZs). SFZs were specific to individuals. They occurred with partial overlap, in a sequence from the colony centre to the colony periphery. The size of SFZs increased from the centre of the colony towards the periphery. The median size of SFZ in a colony varied with the time of year; they expanded gradually after hibernation with a peak in May and then contracted gradually until the following hibernation. The frequency of a worker's brood care behaviour was related to the amount of overlap between her SFZ and the spatial distribution of the brood. Individuals on the periphery of the colony were most likely to leave the nest. No clear segregation on the basis of age was observed. The division of labour inL.unifasciatuswas flexibly organized along the continuum of SFZs where each worker performed the tasks within her spatial fidelity zone.
Harvester ants, Pogonomyrmex barbatus, were marked when engaged in one of four activities outside the nest: foraging, patrolling, nest maintenance and upkeep of the colony refuse pile. In undisturbed older colonies, each activity was done by a distinct group of workers. In undisturbed younger colonies, nest maintenance workers were likely to forage. Further experiments examined the conditions under which workers of each task group engaged in other activities. Marked workers were observed in the course of perturbations that increased the numbers engaged in one activity. In response to perturbations, most workers would switch tasks to forage, and nest maintenance workers were likely to switch to other tasks. Previous work showed that changes in the numbers of ants engaged in one activity alter the numbers engaged in other activities. The experiments with marked individuals described here show that task switching among exterior workers does not account for the observed responses to perturbations. This means that within a time scale of several hours, one worker group receives information about events affecting other worker groups.
All of the principal and some of the minor volatile products of Acanthomyops claviger were identified chemically, their glandular source determined, and their functions analysed. Undecane was shown to be an efficient spreading agent for formic acid. All of the more abundant substances in the C10–C13 range, including undecane, were shown to be alarm pheromones, with behavioral threshold concentrations varying in order of magnitude from 1010 to 1012 and yielding potential signal distances in the centimetre range. Among the substances tested behaviorally, olfactory efficiency varies with molecular weight rather than structure. The C10–C13 substances are optimal for alarm signalling because, apparently on the basis of molecular weight alone, they combine moderate olfactory efficiency with sufficiently high vapour pressure to broadcast in the centimetre range when present in microgram quantities or less.
1. The higher formicine ants base their chemical alarm-defense systems primarily on a limited array of acyclic terpenes discharged from the mandibular glands, and alkanes and ketones discharged from Dufour's gland. All of these substances appear to be utilized in defense, and most, especially those at the lower end of the range of molecular weight (C9-C13), also function as alarm pheromones. The active space of the pheromones reaches over a distance of centimeters and is relatively short-lived. The alarm response of the various species can be classified roughly as either "panic" or "aggressive" in nature. 2. Two major adaptive alterations in the basic alarm-defense system have occurred within the higher Formicinae. In the genus Acanthomyops, the mandibular gland has been enlarged and made the site of storage of unusually large quantities of citronellal and two isomers of citral. We suggest that the changes are causally linked to the development of a strongly aggressive form of alarm communication. The res...
In studying the relationship between insect dispersion and population density, we need an index which allows us to separate statistical artifacts from biologically significant effects. I used a simulation model to generate patterns of egg dispersion and tested several dispersion indices as predictors of these patterns. Green's coefficient and standardized Morisita's coefficient were not influenced by population density and are good measures of dispersion. Variance/mean was only weakly correlated with density and has the advantage of being easy to compute and readily understandable. Dispersion indices related to k of the negative binomial are not appropriate for data either more or less clumped than the negative binomial and should only be used with caution.
In social insect colonies, workers perform a variety of tasks, such as foraging, brood care and nest construction. As the needs of the colony change, and as resources become available, colonies adjust the numbers of workers engaged in each task. Task allocation is the process that results in specific workers being engaged in specific tasks, in numbers appropriate to the current situation.
Abstract 1. Data were compiled from the literature and our own studies on 24 ant species to characterise the effects of body size and temperature on forager running speed.
2. Running speed increases with temperature in a manner consistent with the effects of temperature on metabolic rate and the kinetic properties of muscles.
3. The exponent of the body mass-running speed allometry ranged from 0.14 to 0.34 with a central tendency of approximately 0.25. This body mass scaling is consistent with both the model of elastic similarity, and a model combining dynamic similarity with available metabolic power.
4. Even after controlling for body size or temperature, a substantial amount of inter-specific variation in running speed remains. Species with certain lifestyles [e.g. nomadic group predators, species which forage at extreme (>60 °C) temperatures] may have been selected for faster running speeds.
5. Although ants have a similar scaling exponent to mammals for the running speed allometry, they run slower than predicted compared with a hypothetical mammal of similar size. This may in part reflect physiological differences between invertebrates and vertebrates.
When deprived of minor workers under expermental conditions, major workers of the ant Pheidble pubiventris dramatically increase their repertory and rate of activity, and the change is due in good part to the greater attention they pay the brood. When minor workers are reinstated in appropriate numbers, the majors reduce their attention to the immature stages to the ordinary, low levels. Their response consists of the active avoidance of minors while in the vicinity of the immature stages. However, majors do not turn from other majors near the brood as much as they do from the minors, and they do not avoid minors at all while in other parts of the nest. In addition, minors do not avoid either minors or majors anywhere in the nest. The result is a striking division of labor with reference to brood care.
Within-group communication is a fundamental feature of animal societies. In order for animal groups to function as adaptive
units, the members must share information such that group mates respond appropriately to each others’ behavior. One important
function of social communication is to affect the allocation of tasks among group members. Theoretical and empirical findings
on a diverse array of social insect taxa show that interactions among workers often play important roles in structuring division
of labor. We review worker interactions that regulate division of labor in insect societies, which we refer to as worker connectivity. We present a framework for synthesizing and analyzing the study of worker connectivity. The widespread reliance on worker
connectivity among eusocial insect taxa and the diversity of communicative mechanisms used to recruit workers suggest that
the nature of worker interactions has evolved by natural selection. We suggest that colony-level selection acting on variation
in task allocation has been an important force in the evolution of mechanisms for worker connectivity. We also propose that
there are important links between individual worker cognition and task allocation at the colony level. Evolutionary changes
in the cognitive aspects of worker responses may affect task allocation as much as changes in the communicative signals themselves.
Previous studies suggested that juvenile hormone (JH) is involved in the regulation of physiological processes that are associated with division of labor in honey bees but the effects of JH on behavior were not clear. The hypothesis that JH affects worker age polyethism was tested by observing individually marked bees topically treated with different doses of the JH analog methoprene. Methoprene caused dose-dependent changes in the timing and frequency of occurrence of four important age-dependent tasks: brood and queen care, food storage, nest maintenance, and foraging. Weak or no effects were observed for social interactions, self-grooming, and other non-task behaviors that were not performed in an age-dependent manner. These results support the hypothesis that JH is involved in the control of age polyethism. A model is presented that explains the role of JH in regulating division of labor. JH may regulate the colony's allocation of labor by altering the probabilities of response to tasks. According to this model, hormone titers increase with age according to a genetically determined pattern of development, but this rise may be modulated by environmental and colony factors such as food availability and population structure. Extrinsic regulation of JH may be a mechanism underlying the ability of workers to respond to changing colony needs.
Infectious processes in a social group are driven by a network of contacts that is generally structured by the organization
arising from behavioral and spatial heterogeneities within the group. Although theoretical models of transmission dynamics
have placed an overwhelming emphasis on the importance of understanding the network structure in a social group, empirical
data regarding such contact structures are rare. In this paper, I analyze the network structure and the correlated transmission
dynamics within a honeybee colony as determined by food transfer interactions and the changes produced in it by an experimental
manipulation. The study demonstrates that widespread transmission in the colony is correlated to a lower clustering coefficient
and higher robustness of the social network. I also show that the social network in the colony is determined by the spatial
distribution of various age classes, and the resulting organizational structure provides some amount of immunity to the young
individuals. The results of this study demonstrates how, using the honeybee colony as a model system, concepts in network
theory can be combined with those in behavioral ecology to gain a better understanding of social transmission processes, especially
those related to disease dynamics.
Social interactions are critical to the organization of worker activities in insect colonies and their consequent ecological
success. The structure of this interaction network is therefore crucial to our understanding of colony organization and functioning.
In this paper, I study the properties of the interaction network in the colonies of the social wasp Ropalidia marginata. I find that the network is characterized by a uniform connectivity among individuals with increasing heterogeneity as colonies
become larger. Important network parameters are found to be correlated with colony size and I investigate how this is reflected
in the organization of work in colonies of different sizes. Finally, I test the resilience of these interaction networks by
experimental removal of individuals from the colony and discuss the structural properties of the network that are related
to resilience in a social network.
Social insect colonies are characterized by extensive interactions among individuals, exchanges that can also potentially
transmit pathogens. The large majority of these social interactions in a honeybee colony result from food transfer among individuals.
Since colony hunger is likely to have a significant influence on these interactions, we investigated its effect on the distribution
of food within the colony. By pulsing two colonies having different amounts of stored food with a radioactive label, we found
that a starved colony sent out a larger number of foragers, brought in more food, and stored more of it than the satiated
colony. We also found that the food brought into a starved colony was distributed more uniformly within each age class than
that in the satiated colony. The queen and the young individuals received the lowest exposure to the label even though the
label entered different regions of the colony at the same rate. The satiation level of the colony did not influence the relative
exposures of different age groups to the label but a higher amount of it was stored in the hungry colony. We discuss the significance
of these results in terms of the role played by the organizational structure of the honeybee colony on the transmission dynamics
of an infectious disease.
KeywordsFood distribution-Trophallaxis-Colony hunger-Radiolabel-Honeybees
Resource distribution is fundamental to social organization, but it poses a dilemma. How to facilitate the spread of useful resources but restrict harmful substances? This dilemma reaches a zenith in famine relief. Survival depends on distributing food fast but that could increase vulnerability to poisons. We tested how Temnothorax albipennis ants solve this dilemma in the distribution of honey solution after 48 h of starvation in four colonies with individually marked workers. We constructed the complete network of liquid food transmission (trophallaxis) between individuals. Within the first 30 min of famine relief, 95% of the workers received food and the distribution rate was an order of magnitude faster compared to the controls. We tested the assumptions of a simple analytical model that best fitted our data. Good mixing during famine relief was facilitated by the movement of internal workers away from the brood pile and the movement of foragers with food away from the nest entrance. This is intriguing because T. albipennis workers have spatial fidelity zones and in the controls internal and external workers were segregated. We discovered that colony vulnerability to poisons during famine relief might be mitigated by: (1) the dilution of food from the same source through mixing, (2) the concentration of food in workers positioned midway between the colony centre and its periphery and (3) the existence of living ‘silos’. The latter are expendable foragers, who stay inside the nest and store food during famine relief, thus acting as potential disposable testers for food toxicity.
Members of an ant colony perform a variety of tasks outside the nest, such as foraging and nest maintenance work. The number of ants actively performing each task changes, because workers switch from one task to another and because workers are sometimes active, sometimes inactive. In field experiments with harvester ants (
[2] and [3]), a perturbation that directly affects only the number of workers engaged in one task, causes changes in the numbers engaged in other activities. These dynamics must be the outcome of interactions among individuals; an ant cannot be expected to assess and respond to colony-level changes of behaviour. Here we present a parallel distributed model of the processes regulating changes in numbers of workers engaged in various tasks. The model is based on a Hopfield net, but differs from conventional Hopfield models in that when a unit or ant changes state, it changes its interaction patterns. Simulation results resemble experimental results; perturbations of one activity propagate to others. Depending on the pattern of interactions among worker groups, the distribution of active workers in different tasks either settles into a single, global attractor, or shows the dynamics associated with a landscape containing multiple attractors.
Despite more than a decade of experimental work in multi-robot systems, important theoretical aspects of multi-robot coordination mechanisms have, to date, been largely untreated. To address this issue, we focus on the problem of multi-robot task allocation (MRTA). Most work on MRTA has been ad hoc and empirical, with many coordination architectures having been proposed and val- idated in a proof-of-concept fashion, but infrequently an- alyzed. With the goal of bringing objective grounding to this important area of research, we present a formal study of MRTA problems. A domain-independent taxon- omy of MRTA problems is given, and it is shown how many such problems can be viewed as instances of other, well-studied, optimization problems. We demonstrate how relevant theory from operations research and combinato- rial optimization can be used for analysis and greater un- derstanding of existing approaches to task allocation, and show how the same theory can be used in the synthesis of new approaches.