C1139 Social Insects. Task Partitioning in Insect Societies. Lecture 16
Task Partitioning in Insect Societies
Aims
1. To provide information on the following topics in task partitioning: definition, relationship to division of labour, benefits and costs, taxonomic occurrence, patterns of material transfer, etc.
Objectives
1. Understand the relationship of task partitioning to division of labour in the organization of work in insect societies
2. Learn specific examples
Big Picture
Insect societies have sophisticated ways of organising their work. Two of the main principles used in work organisation are Division of Labour (DoL) and Task Partitioning (TP). DoL has long been studied whereas TP has been less studied. DoL and TP are not mutually exclusive. In fact, they often go together. Nectar collection in the honey bee involves both. A forager transfers her nectar to a receiver bee. This is TP as the task of collecting and storing nectar has been divided into two subtasks of collecting and storing. The receivers are a different, younger, group of workers than the foragers. This is DoL. Without TP, DoL between receivers and foragers would not be possible. TP in nectar collection is not essential. Forager bumble bees collect nectar and store it themselves. The following word equations summarise the difference between TP and DoL.
TP = task/workers (a task/two of more workers that do the subtasks)
DoL = workers/tasks (the workers in the colony/the tasks that need doing)
Basic Information
All examples of TP involve the handling of materials, usually materials moving into the nest (forage) but also materials moving through (leaves being processed in leafcutter ants) and out (e.g., garbage, excavated soil) of the nest. The study of TP raises many questions. For example, when does it occur, in what context does it occur, and in which species does it occur? How is material transferred (directly, indirectly, one transfer, two transfers etc.) What is the adaptive significance of TP—why do it? Why make things more complicated? How to organize things so that the sub-tasks are co-ordinated.
Possible benefits
TP occurs widely in insect societies. Presumably, it occurs because it enables the colony to function better. In Polybia wasps building is a partitioned task. Some forager workers collect wood pulp that they transfer directly to builders at the nest. A forager can collect a large load of wood pulp, sufficient for several builders. This reduces the number of foraging trips. Foraging is a risky activity so reducing the number of trips is beneficial. Honey bee foragers directly transfer their nectar at the nest to receiver bees. What is the advantage of doing this? No one knows for sure, but it may enable the forager to get back to the flowers more quickly. Flower patches do not last long, sometimes just a few days, so it may increase the total nectar collected by a forager over her life to work fewer patches more intensely. This is because when one flower patch finishes the forager must locate another, and this can take some time. For example, if the forager finds a new patch of flowers by following dances she will likely have to repeat this several times as there is only about a 25% chance of a dance follower locating the advertised patch. The lecture slides also show benefits in the collection of honeydew by ants and in the collection of leaves in Atta ants. By cutting leaves in trees and allowing them to fall to the ground, where they are cut up into small pieces, much walking is eliminated.
Possible benefits: hygiene in leafcutter ants
Atta colonies face the challenge of keeping their colony and fungus gardens healthy. The garden fungus is susceptible to the weed fungus Escovopsis. This can kill the fungus garden. This will then kill the colony or entail very costly remedies such as building a new nest. The ants have a whole series of defences against Escovopsis. They carefully lick and clean leaves brought into the nest. They weed the fungus garden and even groom individual strands of mycelium. They can direct more weeding to areas with Escovopsis. The worker ants also have an antibiotic bacterium growing on their bodies that kills Escovopsis. It gives the ants a whitish appearance. The workers also have a special gland, called the metapleural gland, which secretes disinfectant chemicals. Research has shown that Atta colonies also have hygienic adaptations in the disposal of garbage. These include several behavioural and organizational features.
Atta cephalotes can have massive colonies with many garden chambers in which their fungus crop is grown. These are connected to underground waste chambers that receive spent compost from the fungus garden and other waste. Workers working in the fungus garden take waste towards the garbage chambers. But few of them enter the garbage chamber. Instead, the waste is placed in a cache in the connecting tunnel. Workers living in the garbage chamber then take it the rest of the way. This is another example in which TP and DoL are combined. The use of separate chambers, TP, and DoL isolate the garbage chambers from the garden chambers. This is important as the waste may contain Escovopis and other pathogenic microorganisms. Ants working in the garbage chamber are not allowed back into the garden chamber. Ants contaminated with garbage odour are aggressed or killed if they try to enter the garden chamber. What A. cephalotes do is similar to the barrier washing machines found in many hospitals, in which the contaminated and clean materials are kept separate.
Atta colombica also has large colonies with many garden chambers in which their fungus crop is grown. But the waste is dumped externally above ground. There is division of labour between above ground workers. Foraging and garbage dumping are separate careers. Workers do not switch between these two activities. In addition, the garbage trail never leads in the same direction as the main foraging trail. The workers who work on the garbage heap itself are former garbage dumpers. The garbage dump is often below a convenient tree trunk or log to dump off. In this way the garbage dumping ants do not have to walk on the garbage dump itself. The garbage dump is always downhill to the nest entrance. Garbage is also dumped into a stream if one is nearby.
Costs
TP involves material transfer. Therefore, time is wasted in transferring material, looking for a transfer partner, queuing for partner, and sometimes in signalling to balance the system (e.g., tremble dance in honey bees). Delays occur even if the balance in the work capacities of the various groups of workers is perfect because of stochastic variation in arrival rates. There may be other costs, such as loss of material during transfer. Possibly, the increased inter-individual contact that occurs with TP may result in increased disease transmission.
Transfer delay costs might be reduced by caching because this eliminates the need to be served directly by another worker. Instead, material can be put down by one worker and later picked up by another worker. In this way, caches can even out the “supply” and “demand” problem of direct transfer. But indirect transfer via a cache does not eliminate all the difficulties associated with TP. Liquids are never cached even though it might be possible to have a wax nectar trough for honey bee nectar foragers to cache their nectar in. Cached materials might also degrade, be lost, or get stolen, and there is still the need to balance “cachers” and receivers.
Task partitioning results in a more complex arrangement of work and introduces the problem of organizing the system to balance the work capacities of the various sub-tasks. The fact that task partitioning occurs indicates that it must have substantial benefits, that are large enough to overcome these costs and more.
Occurence
TP occurs in ants, bees, wasps, and termites. It must have evolved many times independently. TP can vary within a species for different forage materials. For example, honeybees use TP in the collection of nectar but not in the collection of pollen. TP can vary between species for the same material. For example, nectar collection involves TP in honey bees but not in bumble bees.
TP occurs in bringing material into the nest. These include liquids such as nectar, honeydew, and water, and solids such as seeds, leaves, prey, wood pulp, and propolis. TP occurs in bringing various solids out of the nest, especially garbage and excavated soil. TP also probably occurs inside the nest but has been little studied. One example is in the processing of leaf fragments in leafcutter ants.
It is likely that many new examples of task partitioning will be found. While studying Atta leafcutter ants in Brazil, we discovered task partitioning in fruit cutting. Large ants cut the fruit but medium-sized workers transfer the cut pieces back to the nest.
Types and patterns of transfer
Direct transfer can be used for both solids and liquids but only solids are transferred indirectly via a cache. Transfer is always at the nest in bees and wasps, which have flying workers. But transfer may take place at the food collecting location or on the foraging trail in ants and termites, which have non-flying workers. Only non-flying social insects have foraging trails and much opportunity to meet each other outside the nest along a trail.
There are many different patterns of transfer. The most common is two interlocking cycles. When there are three interlocking cycles these can be in a line or in a clover leaf arrangement of three interlocking cycles. This occurs in Polybia occidentalis wasps (Vespidae: Polistinae: Epiponini) and probably in other epiponines. It also occurs in the British ant Lasius fuliginosus (which lives on the Sussex campus) which builds a nest out of material it has collected. There are cycles of honeydew foragers, building material foragers, and builders. There can be considerable diversity in the transfer methods used, with various combinations of direct transfer, indirect transfer, and no transfer.
Honey bee, Apis mellifera, foragers collect water, nectar, propolis (= plant resin), and pollen. Only pollen does not involve transfer. The other three are all transferred directly in the nest. Pollen foragers collect pollen in their pollen baskets. Pollen foragers are not unloaded by another bee but put the pollen directly into a cell by placing their legs into the cell and kicking the pollen off. Transfer of pollen would probably be inefficient as it would take two receivers to unload one forager. That is, one to take each of the two pollen loads. This would presumably be carried in the receiver’s mandibles after transfer. It is not possible to transfer a pollen load from one pollen basket to another. A pollen load is fragile and could easily break apart or be dropped during transfer. This is a second possible reason why pollen is not transferred.
Water is a liquid that is held in the forager’s crop and is transferred like nectar. A receiver extends her tongue and drinks from the forager’s mouth. Propolis is a solid and is held in the pollen basket. However, unlike pollen, it is subject to TP. A builder unloads a forager by taking the forager’s propolis in her mandibles. Why are pollen and propolis handled differently whereas water and nectar are not? It is suggested that propolis is directly transferred because a forager cannot unload herself because the propolis is sticky. Propolis foragers can sometimes wait hours to be unloaded if there is little demand for propolis.
Simulation modelling of 2-cycle TP
A computer simulation can be set up to investigate queuing delays. It is possible to vary the work capacities of the foragers and receivers, and the mean and variance in the durations of the foraging and receiving trips. The simulation generates the delays in being unloaded for both receivers and foragers for either “serve in random order” or “first come first served” protocols. (British people queue first come first served but in some other counties, they serve in random order, to the annoyance of any visiting British.)
Even when the work capacities are balanced there are still delays. This is because of stochastic variation in the arrival rates of foragers and receivers into the transfer area. The average stochastic delay in the larger colonies is smaller because large size lessens the effect. This suggests that TP is more likely in larger colonies because the delay cost will be lower.
The data support this prediction. Swarm-founding wasps and honey bees have TP in the handling of wood pulp and nectar, but Vespinae wasps and bumble bees do not. These species have colonies founded by a lone queen, so their colonies start small and often do not grow to large sizes. In Vespula wasps there are also data to show that nectar is handled via TP in larger population colonies. Research carried out in Yucatan, Mexico on stingless bees provided additional support for the hypothesis. Five species were studied and all had TP in nectar collection. Stingless bees have swarm-founded colonies with colony sizes typically in excess of 1000 workers.
References
Anderson, C., Ratnieks, F. L. W. 2000. Task partitioning in insect societies: novel situations. Insectes Sociaux 47: 198-199.
Anderson, C., Ratnieks, F. L. W. 1999. Task partitioning in insect societies. 1. Effect of colony size on queueing delay and colony ergonomic efficiency. American Naturalist 154: 521-535.
Anderson, C, Ratnieks, F. L. W. 1999. Worker allocation in insect societies: coordination of nectar foragers and nectar receivers in the honey bee. Behavioral Ecology and Sociobiology 46: 73-81.
Anderson, C., Ratnieks, F. L. W. 1999. Task partitioning in foraging: effect of colony size on queueing times and information reliability. Pages 31-50 in: Detrain C, Deneubourg J L, Pasteels J M (eds.). Information Processing in Social Insects. Birkhauser, Basel.