Task allocation and partitioning of social insects
Encyclopedia
Task allocation and partitioning refers to the way that tasks are chosen, assigned, subdivided, and coordinated (here, within a single colony of social insects). Closely associated are issues of communication
that enable these actions to occur.
This entry focuses exclusively on social insects. For information on human task allocation and partitioning, see division of labour
, task analysis
, and workflow
.
, 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 (ant
s, bee
s, and wasp
s), Isoptera (termite
s), Thysanoptera (thrips
), and Hemiptera (aphid
s).
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).
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. 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, and the propagation via pheromones of an alarm state in Africanized honey bees
.
approach to model communication in animals, including that related to task performance. A network is pictorially represented as a graph
, but can equivalently be represented as an adjacency list
or adjacency matrix
. Traditionally, workers are the nodes of the graph, but Fewell prefers to make the tasks the nodes, with workers as the links. 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". He has pointed out that connectivity provides three adaptive advantages compared to individual direct perception of needs:
O'Donnell provides a comprehensive survey, with examples, of factors that have a large bearing on worker connectivity. They include:
; 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. See also the review of task partitioning by Ratnieks and Anderson.
ones, for example.
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".
Note that these indexes, used as metrics of biodiversity
, now find a place measuring division of labor.
Communication
Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...
that enable these actions to occur.
This entry focuses exclusively on social insects. For information on human task allocation and partitioning, see division of labour
Division of labour
Division of labour is the specialisation of cooperative labour in specific, circumscribed tasks and likeroles. Historically an increasingly complex division of labour is closely associated with the growth of total output and trade, the rise of capitalism, and of the complexity of industrialisation...
, task analysis
Task analysis
Task analysis is the analysis of how a task is accomplished, including a detailed description of both manual and mental activities, task and element durations, task frequency, task allocation, task complexity, environmental conditions, necessary clothing and equipment, and any other unique factors...
, and workflow
Workflow
A workflow consists of a sequence of connected steps. It is a depiction of a sequence of operations, declared as work of a person, a group of persons, an organization of staff, or one or more simple or complex mechanisms. Workflow may be seen as any abstraction of real work...
.
Definitions
- Task allocation "... is the process that results in specific workers being engaged in specific tasks, in numbers appropriate to the current situation. [It] operates without any central or hierarchical control..." The concept of task allocation is individual-centric. It focuses on decisions by individuals about what task to perform. However, different biomathematical models give different weights to inter-individual interactions vs. environmental stimuli.
- Task partitioning refers to the division of one task into sequential actions done by more than one individual. The focus here is on the task, and its division, rather than on the individuals performing it.
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 eusocialityEusociality
Eusociality is a term used for the highest level of social organization in a hierarchical classification....
, 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 (ant
Ant
Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...
s, bee
Bee
Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila...
s, and wasp
Wasp
The term wasp is typically defined as any insect of the order Hymenoptera and suborder Apocrita that is neither a bee nor an ant. Almost every pest insect species has at least one wasp species that preys upon it or parasitizes it, making wasps critically important in natural control of their...
s), Isoptera (termite
Termite
Termites are a group of eusocial insects that, until recently, were classified at the taxonomic rank of order Isoptera , but are now accepted as the epifamily Termitoidae, of the cockroach order Blattodea...
s), Thysanoptera (thrips
Thrips
Thrips are tiny, slender insects with fringed wings . Other common names for thrips include thunderflies, thunderbugs, storm flies, thunderblights, and corn lice...
), and Hemiptera (aphid
Aphid
Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies or whiteflies, are small sap sucking insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions...
s).
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
Polymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
), 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
Honey bee
Honey bees are a subset of bees in the genus Apis, primarily distinguished by the production and storage of honey and the construction of perennial, colonial nests out of wax. Honey bees are the only extant members of the tribe Apini, all in the genus Apis...
foragers, who are the oldest members of the colony (with the exception of the queen).
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. Beyond the rationale, there is well-documented empirical evidence of communication related to tasks; examples include the waggle dance
Waggle dance
Waggle dance is a term used in beekeeping and ethology for a particular figure-eight dance of the honey bee. By performing this dance, successful foragers can share with their hive mates information about the direction and distance to patches of flowers yielding nectar and pollen, to water...
of honey bee foragers, trail marking by ant foragers, and the propagation via pheromones of an alarm state in Africanized honey bees
Africanized bee
Africanized honey bees, known colloquially as "killer bees", are a hybrid variety of the European honeybee , generated by a man-made breeding of the African honey bee, A. m. scutellata, with various European honey bees such as the Italian bee A. m. ligustica and A. m. iberiensis. These bees are far...
.
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. Similar temporal polyethism patterns can be seen in even primitive species of wasps, such as Ropalidia marginata. Young workers feed larvae, and then transition to nest building tasks, followed by foraging. Many species of ants also display this pattern.Network representation of tasks and communication
Numerous scientists have used a social networkSocial network
A social network is a social structure made up of individuals called "nodes", which are tied by one or more specific types of interdependency, such as friendship, kinship, common interest, financial exchange, dislike, sexual relationships, or relationships of beliefs, knowledge or prestige.Social...
approach to model communication in animals, including that related to task performance. A network is pictorially represented as a graph
Graph (mathematics)
In mathematics, a graph is an abstract representation of a set of objects where some pairs of the objects are connected by links. The interconnected objects are represented by mathematical abstractions called vertices, and the links that connect some pairs of vertices are called edges...
, but can equivalently be represented as an adjacency list
Adjacency list
In graph theory, an adjacency list is the representation of all edges or arcs in a graph as a list.If the graph is undirected, every entry is a set of two nodes containing the two ends of the corresponding edge; if it is directed, every entry is a tuple of two nodes, one denoting the source node...
or adjacency matrix
Adjacency matrix
In mathematics and computer science, an adjacency matrix is a means of representing which vertices of a graph are adjacent to which other vertices...
. Traditionally, workers are the nodes of the graph, but Fewell prefers to make the tasks the nodes, with workers as the links. 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". He has pointed out that connectivity provides three adaptive advantages compared to individual direct perception of needs:
- 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.
- 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.
- 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. They include:
- graph degreeDegree (graph theory)In graph theory, the degree of a vertex of a graph is the number of edges incident to the vertex, with loops counted twice. The degree of a vertex v is denoted \deg. The maximum degree of a graph G, denoted by Δ, and the minimum degree of a graph, denoted by δ, are the maximum and minimum degree...
- size of the interacting group, especially if the network has a modular structure
- sender distribution (i.e. a small number of controllers vs. numerous senders)
- strength of the interaction effect, which includes strength of the signal sent, recipient sensitivity, and signal persistence (i.e. pheromone signal vs. sound waves)
- recipient memory, and its decay function
- socially-transmitted inhibitory signals, as not all interactions provide positive stimulus
- specificity of both the signal and recipient response
- signal and sensory modalities, and activity and interaction rates
Task taxonomy and complexity
Anderson, Franks, and McShea have broken down insect tasks (and subtasks) into a hierarchical taxonomyTaxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
; 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. See also the review of task partitioning by Ratnieks and Anderson.
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. 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 workflowWorkflow
A workflow consists of a sequence of connected steps. It is a depiction of a sequence of operations, declared as work of a person, a group of persons, an organization of staff, or one or more simple or complex mechanisms. Workflow may be seen as any abstraction of real work...
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 matrixMatrix (mathematics)
In mathematics, a matrix is a rectangular array of numbers, symbols, or expressions. The individual items in a matrix are called its elements or entries. An example of a matrix with six elements isMatrices of the same size can be added or subtracted element by element...
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
Shannon index
The Shannon index, sometimes referred to as the Shannon-Wiener Index or the Shannon-Weaver Index, is one of several diversity indices used to measure diversity in categorical data. It is simply the Information entropy of the distribution, treating species as symbols and their relative population...
and Simpson's index
Simpson index
Simpson's diversity index is one of a number of diversity indices, used to measure diversity. In ecology, it is often used to quantify the biodiversity of a habitat. It takes into account the number of species present, as well as the relative abundance of each species...
... should be the indices of choice when one wants to simultaneously examine division of labor amongst all individuals in a population".
Note that these indexes, used as metrics of biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
, now find a place measuring division of labor.