Task allocation and partitioning in social insects

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Task allocation and partitioning is the way that tasks are chosen, assigned, subdivided, and coordinated within a colony of social insects. Task allocation and partitioning gives rise to the division of labor often observed in social insect colonies, whereby individuals specialize on different tasks within the colony (e.g., "foragers", "nurses"). Communication is closely related to the ability to allocate tasks among individuals within a group. This entry focuses exclusively on social insects. For information on human task allocation and partitioning, see division of labour, task analysis, and workflow.

Contents

Definitions

Introduction

Social living provides a multitude of advantages to its practitioners, including predation risk reduction, environmental buffering, food procurement, and possible mating advantages. The most advanced form of sociality is eusociality, characterized by overlapping generations, cooperative care of the young, and reproductive division of labor, which includes sterility or near-sterility of the overwhelming majority of colony members. With few exceptions, all the practitioners of eusociality are insects of the orders Hymenoptera (ants, bees, and wasps), Isoptera (termites), Thysanoptera (thrips), and Hemiptera (aphids). [5] [6] Social insects have been extraordinarily successful ecologically and evolutionarily. This success has at its most pronounced produced colonies 1) having a persistence many times the lifespan of most individuals of the colony, and 2) numbering thousands or even millions of individuals. Social insects can exhibit division of labor with respect to non-reproductive tasks, in addition to the aforementioned reproductive one. In some cases this takes the form of markedly different, alternative morphological development (polymorphism), as in the case of soldier castes in ants, termites, thrips, and aphids, while in other cases it is age-based (temporal polyethism), as with honey bee foragers, who are the oldest members of the colony (with the exception of the queen). Evolutionary biologists are still debating the fitness-advantage gained by social insects due to their advanced division of labor and task allocation, but hypotheses include: increased resilience against a fluctuating environment, reduced energy costs of continuously switching tasks, increased longevity of the colony as a whole, or reduced rate of pathogen transmission. [7] [8] Division of labor, large colony sizes, temporally-changing colony needs, and the value of adaptability and efficiency under Darwinian competition, all form a theoretical basis favoring the existence of evolved communication in social insects. [9] [10] [11] Beyond the rationale, there is well-documented empirical evidence of communication related to tasks; examples include the waggle dance of honey bee foragers, trail marking by ant foragers such as the red harvester ants, and the propagation via pheromones of an alarm state in Africanized honey bees.

Worker Polymorphism

One of the most well known mechanisms of task allocation is worker polymorphism, where workers within a colony have morphological differences. This difference in size is determined by the amount of food workers are fed as larvae, and is set once workers emerge from their pupae. Workers may vary just in size (monomorphism) or size and bodily proportions (allometry). An excellent example of the monomorphism is in bumblebees (Bombus spp.). Bumblebee workers display a large amount of body size variation which is normally distributed. The largest workers may be ten times the mass of the smallest workers. Worker size is correlated with several tasks: larger workers tend to forage, while smaller workers tend to perform brood care and nest thermoregulation. Size also affects task efficiency. Larger workers are better at learning, have better vision, carry more weight, and fly at a greater range of temperatures. However, smaller workers are more resistant to starvation. [12] In other eusocial insects as well, worker size can determine what polymorphic role they become. For instance, larger workers in Myrmecocystus mexicanus (a North America species of honeypot ant) tend to become repletes, or workers so engorged with food that they become immobile and act a living food storage for the rest of the colonies. [13]

In many ants and termites, on the other hand, workers vary in both size and bodily proportions, which have a bimodal distribution. This is present in approximately one in six ant genera. In most of these there are two developmentally distinct pathways, or castes, into which workers can develop. Typically members of the smaller caste are called minors and members of the larger caste are called majors or soldiers. There is often variation in size within each caste. The term soldiers may be apt, as in Cephalotes, but in many species members of the larger caste act primarily as foragers or food processors. In a few ant species, such as certain Pheidole species, there is a third caste, called supersoldiers.

Temporal polyethism

Temporal polyethism is a mechanism of task allocation, and is ubiquitous among eusocial insect colonies. Tasks in a colony are allocated among workers based on their age. Newly emerged workers perform tasks within the nest, such as brood care and nest maintenance, and progress to tasks outside the nest, such as foraging, nest defense, and corpse removal as they age. In honeybees, the youngest workers exclusively clean cells, which is then followed by tasks related to brood care and nest maintenance from about 2–11 days of age. From 11– 20 days, they transition to receiving and storing food from foragers, and at about 20 days workers begin to forage. [14] Similar temporal polyethism patterns can be seen in primitive species of wasps, such as Ropalidia marginata as well as the eusocial wasp Vespula germanica . Young workers feed larvae, and then transition to nest building tasks, followed by foraging. [15] Many species of ants also display this pattern. [16] This pattern is not rigid, though. Workers of certain ages have strong tendencies to perform certain tasks, but may perform other tasks if there is enough need. For instance, removing young workers from the nest will cause foragers, especially younger foragers, to revert to tasks such as caring for brood. [17] These changes in task preference are caused by epigenetic changes over the life of the individual. Honeybee workers of different ages show substantial differences in DNA methylation, which causes differences in gene expression. Reverting foragers to nurses by removing younger workers causes changes in DNA methylation similar to younger workers. [18] Temporal polyethism is not adaptive because of maximized efficiency; indeed older workers are actually more efficient at brood care than younger workers in some ant species. [17] Rather it allows workers with the lowest remaining life expectancy to perform the most dangerous tasks. Older workers tend to perform riskier tasks, such as foraging, which has high risks of predation and parasitism, while younger workers perform less dangerous tasks, such as brood care. If workers experience injuries, which shortens their life expectancies, they will start foraging sooner than healthy workers of the same age. [19]

Response-Threshold Model

A dominant theory of explaining the self-organized division of labor in social insect societies such as honey bee colonies is the Response-Threshold Model. It predicts that individual worker bees have inherent thresholds to stimuli associated with different tasks. Individuals with the lowest thresholds will preferentially perform that task. [7] Stimuli could include the “search time” that elapses while a foraging bee waits to unload her nectar and pollen to a receiver bee at the hive, the smell of diseased brood cells, or any other combination of environmental inputs that an individual worker bee encounters. [20] The Response-Threshold Model only provides for effective task allocation in the honey bee colony if thresholds are varied among individual workers. This variation originates from the considerable genetic diversity among worker daughters of a colony due to the queen’s multiple matings. [21]

Network representation of information flow and task allocation

To explain how colony-level complexity arises from the interactions of several autonomous individuals, a network-based approach has emerged as a promising area of social insect research. Social insect colonies can be viewed as a self-organized network, in which interacting elements (i.e. nodes) communicate with each other. As decentralized networks, colonies are capable of distributing information rapidly which facilitates robust responsiveness to their dynamic environments. [22] The efficiency of information flow is critical for colony-level flexibility because worker behavior is not controlled by a centralized leader but rather is based on local information.

Social insect networks are often non-randomly distributed, wherein a few individuals act as ‘hubs,’ having disproportionately more connections to other nestmates than other workers in the colony. [22] In harvester ants, the total interactions per ant during recruitment for outside work is right-skewed, meaning that some ants are more highly connected than others. [23] Computer simulations of this particular interaction network demonstrated that inter-individual variation in connectivity patterns expedites information flow among nestmates.

Task allocation within a social insect colony can be modeled using a network-based approach, in which workers are represented by nodes, which are connected by edges that signify inter-node interactions. Workers performing a common task form highly connected clusters, with weaker links across tasks. These weaker, cross-task connections are important for allowing task-switching to occur between clusters. [22] This approach is potentially problematic because connections between workers are not permanent, and some information is broadcast globally, e.g. through pheromones, and therefore does not rely on interaction networks. One alternative approach to avoid this pitfall is to treat tasks as nodes and workers as fluid connections.

To demonstrate how time and space constraints of individual-level interactions affect colony function, social insect network approaches can also incorporate spatiotemporal dynamics. These effects can impose upper bounds to information flow rate in the network. For example, the rate of information flow through Temnothorax rugatulus ant colonies is slower than would be predicted if time spent traveling and location within the nest were not considered. [24] In Formica fusca L. ant colonies, a network analysis of spatial effects on feeding and the regulation of food storage revealed that food is distributed heterogeneously within colony, wherein heavily loaded workers are located centrally within the nest and those storing less food were located at the periphery. [25]

Studies of inter-nest pheromone trail networks maintained by super-colonies of Argentine ants ( Linepithema humile ) have shown that different colonies establish networks with very similar topologies. [26] Insights from these analyses revealed that these networks – which are used to guide workers transporting brood, workers and food between nests – are formed through a pruning process, in which individual ants initially create a complex network of trails, which are then refined to eliminate extraneous edges, resulting in a shorter, more efficient inter-nest network.

Long-term stability of interaction networks has been demonstrated in Odontomachus hastatus ants, in which initially highly connected ants remain highly connected over an extended time period. [27] Conversely, Temnothorax rugatulus ant workers are not persistent in their interactive role, which might suggest that social organization is regulated differently among different eusocial species. [24]

A network is pictorially represented as a graph, but can equivalently be represented as an adjacency list or adjacency matrix. [28] Traditionally, workers are the nodes of the graph, but Fewell prefers to make the tasks the nodes, with workers as the links. [29] [30] O'Donnell has coined the term "worker connectivity" to stand for "communicative interactions that link a colony's workers in a social network and affect task performance". [30] He has pointed out that connectivity provides three adaptive advantages compared to individual direct perception of needs: [30]

  1. It increases both the physical and temporal reach of information. With connectivity, information can travel farther and faster, and additionally can persist longer, including both direct persistence (i.e. through pheromones), memory effects, and by initiating a sequence of events.
  2. It can help overcome task inertia and burnout, and push workers into performing hazardous tasks. For reasons of indirect fitness, this latter stimulus should not be necessary if all workers in the colony are highly related genetically, but that is not always the case.
  3. Key individuals may possess superior knowledge, or have catalytic roles. Examples, respectively, are a sentry who has detected an intruder, or the colony queen.

O'Donnell provides a comprehensive survey, with examples, of factors that have a large bearing on worker connectivity. [30] They include:

Task taxonomy and complexity

Anderson, Franks, and McShea have broken down insect tasks (and subtasks) into a hierarchical taxonomy; their focus is on task partitioning and its complexity implications. They classify tasks as individual, group, team, or partitioned; classification of a task depends on whether there are multiple vs. individual workers, whether there is division of labor, and whether subtasks are done concurrently or sequentially. Note that in their classification, in order for an action to be considered a task, it must contribute positively to inclusive fitness; if it must be combined with other actions to achieve that goal, it is considered to be a subtask. In their simple model, they award 1, 2, or 3 points to the different tasks and subtasks, depending on its above classification. Summing all tasks and subtasks point values down through all levels of nesting allows any task to be given a score that roughly ranks relative complexity of actions. [31] See also the review of task partitioning by Ratnieks and Anderson. [2]

Note: model-building

All models are simplified abstractions of the real-life situation. There exists a basic tradeoff between model precision and parameter precision. A fixed amount of information collected, will, if split amongst the many parameters of an overly precise model, result in at least some of the parameters being represented by inadequate sample sizes. [32] Because of the often limited quantities and limited precision of data from which to calculate parameters values in non-human behavior studies, such models should generally be kept simple. Therefore, we generally should not expect models for social insect task allocation or task partitioning to be as elaborate as human workflow ones, for example.

Metrics for division of labor

With increased data, more elaborate metrics for division of labor within the colony become possible. Gorelick and Bertram survey the applicability of metrics taken from a wide range of other fields. They argue that a single output statistic is desirable, to permit comparisons across different population sizes and different numbers of tasks. But they also argue that the input to the function should be a matrix representation (of time spent by each individual on each task), in order to provide the function with better data. They conclude that "... normalized matrix-input generalizations of Shannon's and Simpson's index ... should be the indices of choice when one wants to simultaneously examine division of labor amongst all individuals in a population". [33] Note that these indexes, used as metrics of biodiversity, now find a place measuring division of labor.

See also

Related Research Articles

<span class="mw-page-title-main">Colony (biology)</span> Living things grouping together, usually for common benefit

In biology, a colony is composed of two or more conspecific individuals living in close association with, or connected to, one another. This association is usually for mutual benefit such as stronger defense or the ability to attack bigger prey.

<span class="mw-page-title-main">Red harvester ant</span> Species of ant

Pogonomyrmex barbatus is a species of harvester ant from the genus Pogonomyrmex. Its common names include red ant and red harvester ant. These large ants prefer arid chaparral habitats and are native to the Southwestern United States. Nests are made underground in exposed areas. Their diets consist primarily of seeds, and they consequently participate in myrmecochory, an ant-plant interaction through which the ants gain nutrients and the plants benefit through seed dispersal. Red harvester ants are often mistaken for fire ants, but are not closely related to any fire ant species, native or introduced.

<span class="mw-page-title-main">Decentralised system</span> Systems without a single most important component or cluster

A decentralised system in systems theory is a system in which lower level components operate on local information to accomplish global goals. The global pattern of behaviour is an emergent property of dynamical mechanisms that act upon local components, such as indirect communication, rather than the result of a central ordering influence of a centralised system.

<i>Bombus terrestris</i> Species of bee

Bombus terrestris, the buff-tailed bumblebee or large earth bumblebee, is one of the most numerous bumblebee species in Europe. It is one of the main species used in greenhouse pollination, and so can be found in many countries and areas where it is not native, such as Tasmania. Moreover, it is a eusocial insect with an overlap of generations, a division of labour, and cooperative brood care. The queen is monandrous which means she mates with only one male. B. terrestris workers learn flower colours and forage efficiently.

<i>Apis florea</i> Species of bee

The dwarf honey bee, Apis florea, is one of two species of small, wild honey bees of southern and southeastern Asia. It has a much wider distribution than its sister species, Apis andreniformis. First identified in the late 18th century, Apis florea is unique for its morphology, foraging behavior and defensive mechanisms like making a piping noise. Apis florea have open nests and small colonies, which makes them more susceptible to predation than cavity nesters with large numbers of defensive workers. These honey bees are important pollinators and therefore commodified in countries like Cambodia.

<i>Lasioglossum malachurum</i> Species of bee

Lasioglossum malachurum, the sharp-collared furrow bee, is a small European halictid bee. This species is obligately eusocial, with queens and workers, though the differences between the castes are not nearly as extreme as in honey bees. Early taxonomists mistakenly assigned the worker females to a different species from the queens. They are small, shiny, mostly black bees with off-white hair bands at the bases of the abdominal segments. L. malachurum is one of the more extensively studied species in the genus Lasioglossum, also known as sweat bees. Researchers have discovered that the eusocial behavior in colonies of L. malachurum varies significantly dependent upon the region of Europe in which each colony is located.

<span class="mw-page-title-main">East African lowland honey bee</span> Subspecies of honey bee native to Africa

The East African lowland honey bee is a subspecies of the western honey bee. It is native to central, southern and eastern Africa, though at the southern extreme it is replaced by the Cape honey bee. This subspecies has been determined to constitute one part of the ancestry of the Africanized bees spreading through North and South America.

Spatial organization can be observed when components of an abiotic or biological group are arranged non-randomly in space. Abiotic patterns, such as the ripple formations in sand dunes or the oscillating wave patterns of the Belousov–Zhabotinsky reaction emerge after thousands of particles interact millions of times. On the other hand, individuals in biological groups may be arranged non-randomly due to selfish behavior, dominance interactions, or cooperative behavior. W. D. Hamilton (1971) proposed that in a non-related "herd" of animals, the spatial organization is likely a result of the selfish interests of individuals trying to acquire food or avoid predation. On the other hand, spatial arrangements have also been observed among highly related members of eusocial groups, suggesting that the arrangement of individuals may provide advantages for the group.

<span class="mw-page-title-main">Eusociality</span> Highest level of animal sociality a species can attain

Eusociality, the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care, overlapping generations within a colony of adults, and a division of labor into reproductive and non-reproductive groups. The division of labor creates specialized behavioral groups within an animal society which are sometimes referred to as 'castes'. Eusociality is distinguished from all other social systems because individuals of at least one caste usually lose the ability to perform at least one behavior characteristic of individuals in another caste. Eusocial colonies can be viewed as superorganisms.

<i>Ropalidia marginata</i> Species of insect

Ropalidia marginata is an Old World species of paper wasp. It is primitively eusocial, not showing the same bias in brood care seen in other social insects with greater asymmetry in relatedness. The species employs a variety of colony founding strategies, sometimes with single founders and sometimes in groups of variable number. The queen does not use physical dominance to control workers; there is evidence of pheromones being used to suppress other female workers from overtaking queenship.

<span class="mw-page-title-main">Bumblebee communication</span>

Bumblebees, like the honeybee collect nectar and pollen from flowers and store them for food. Many individuals must be recruited to forage for food to provide for the hive. Some bee species have highly developed ways of communicating with each other about the location and quality of food resources ranging from physical to chemical displays.

<span class="mw-page-title-main">Halictinae</span> Subfamily of bees

Within the insect order Hymenoptera, the Halictinae are the largest, most diverse, and most recently diverged of the four halictid subfamilies. They comprise over 2400 bee species belonging to the five taxonomic tribes Augochlorini, Thrinchostomini, Caenohalictini, Sphecodini, and Halictini, which some entomologists alternatively organize into the two tribes Augochlorini and Halictini.

<i>Formica truncorum</i> Species of ant

Formica truncorum is a species of wood ant from the genus Formica. It is distributed across a variety of locations worldwide, including central Europe and Japan. Workers can range from 3.5 to 9.0mm and are uniquely characterized by small hairs covering their entire bodies. Like all other ants, F. truncorum is eusocial and demonstrates many cooperative behaviors that are unique to its order. Colonies are either monogynous, with one queen, or polygynous, with many queens, and these two types of colonies differ in many characteristics.

<i>Halictus ligatus</i> Species of bee

Halictus ligatus is a species of sweat bee from the family Halictidae, among the species that mine or burrow into the ground to create their nests. H. ligatus, like Lasioglossum zephyrus, is a primitively eusocial bee species, in which aggression is one of the most influential behaviors for establishing hierarchy within the colony, and H. ligatus exhibits both reproductive division of labor and overlapping generations.

<i>Polybia occidentalis</i> Species of wasp

Polybia occidentalis, commonly known as camoati, is a swarm-founding advanced eusocial wasp. Swarm-founding means that a swarm of these wasps find a nesting site and build the nest together. This species can be found in Central and South America. P. occidentalis preys on nectar, insects, and carbohydrate sources, while birds and ants prey on and parasitize them. P. occidentalis workers bite each other to communicate the time to start working.

<i>Polistes japonicus</i> Species of wasp

Polistes japonicus is a eusocial paper wasp found in Japan. It was first described by Henri Louis Frédéric de Saussure in 1858. It is closely related to Polistes formosanus. This species lives in small colonies with few workers and a foundress queen. Nests of these wasps are sometimes used as a traditional medicine in Korea, China, and Japan.

<i>Megalopta genalis</i> Species of bee

Megalopta genalis is a species of the family Halictidae, otherwise known as the sweat bees. The bee is native to Central and South America. Its eyes have anatomical adaptations that make them 27 times more sensitive to light than diurnal bees, giving it the ability to be nocturnal. However, its eyes are not completely different from other diurnal bees, but are still apposition compound eyes. The difference therefore lies purely in adaptations to become nocturnal, increasing the success of foraging and minimizing the danger of doing so from predation. This species has served as a model organism in studies of social behavior and night vision in bees.

<i>Plebeia remota</i> Species of bee

Plebeia remota is a species of stingless bee that is in the family Apidae and tribe Meliponini. Bees of the species are normally found in a few states in southern Brazil and their nests can be found in tree cavities. Depending on the region, P. remota may have a different morphology and exhibit different behaviors. The bee's diet consists of nectar and pollen that are collected intensely from a few sources. Researchers have conducted a multitude of studies analyzing the changes that occur in the colony during reproductive diapause and what happens during the provisioning and oviposition process or POP.

<span class="mw-page-title-main">Social immunity</span> Antiparasite defence mounted for the benefit of individuals other than the actor

Social immunity is any antiparasite defence mounted for the benefit of individuals other than the actor. For parasites, the frequent contact, high population density and low genetic variability makes social groups of organisms a promising target for infection: this has driven the evolution of collective and cooperative anti-parasite mechanisms that both prevent the establishment of and reduce the damage of diseases among group members. Social immune mechanisms range from the prophylactic, such as burying beetles smearing their carcasses with antimicrobials or termites fumigating their nests with naphthalene, to the active defenses seen in the imprisoning of parasitic beetles by honeybees or by the miniature 'hitchhiking' leafcutter ants which travel on larger worker's leaves to fight off parasitoid flies. Whilst many specific social immune mechanisms had been studied in relative isolation, it was not until Sylvia Cremer et al.'s 2007 paper "Social Immunity" that the topic was seriously considered. Empirical and theoretical work in social immunity continues to reveal not only new mechanisms of protection but also implications for understanding of the evolution of group living and polyandry.

Polyethism is the term used for functional specialization of non-reproductive individuals in a colony of social organisms, particularly insects. Division of labour is considered a key aspect of eusociality and can be seen in a variety of forms. In some insects, there are distinct morphological differences among the individuals that decide their function in the colony, and this is termed as caste or morphological polyethism and is associated with polymorphism. Functions of individuals within the colony that are identical in morphology may however vary in the tasks taken up with the age of the individuals. In some species riskier activities are taken up by older individuals. This is termed as age polyethism. Time- and season-related specialization may also be termed more generically as temporal polyethism. The mechanisms involved in the control of polyethism has been an area of intense research in the field of sociobiology.

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