APS 323, Social Insects, Lecture 6 Division of labour among workers Aims 1. To provide information on the tasks performed by workers and how these may be divided among the workers in a colony according to age and size. The primary focus will be on the <a href="/keyword/honey-bee/" >honey bee</a> Apis mellifera, in which workers of different ages do different talks (age polyethism), and Atta ants, in which workers of different sizes do different tasks. 2. To become aware of possible advantages of division of labour, the kinds of tasks that need to be performed, and how this varies among species; control of age polyethism. 3. To show how diversity in the sizes and shapes of ant workers may arise from some simple underlying relationships concerning the overall range of sizes of workers, and allometric differences between workers of different body size. Objectives 1. Learn the main features of division of labour in the <a href="/keyword/honey-bee/" >honey bee</a> and Atta. 2. Be aware of some hypotheses for the adaptive significance of division of labour at both colony and individual levels. 3. Understand the basics of allometry/isometry in workers ants. The Big Picture Reproductive division of labour is the key feature of eusociality, with one or more individuals per colony specializing in reproduction and the others specializing in work. But in insect societies there is also division of labour among the working individuals. Over a period of time, from days to their whole life, individual workers specialize on a sub-set of the tasks within the colony. This is division of labour among workers. That is, the work that needs doing is divided among the workers. In most species, individual workers typically change their specialization as they age (&quot;age polyethism&quot;). The almost universal pattern is for workers to work within the nest initially, with foraging, the most dangerous task, last. Where morphological differences occur, as in some ants such as Atta, this generally has a great effect on the tasks performed by different workers. The size and shape of different workers allow them to perform particular tasks more efficiently and to fill particular colony needs. Hypothesized advantages of DoL The fact that DoL occurs indicates that it is advantageous. But for whom? Individual or colony? Most hypotheses for the advantage of DoL are based on increased colony performance. That is, DoL among workers increases colony survival and reproduction. In other words, it allows workers to work more efficiently. One clear advantage of leaving foraging to last is that this will increase the work output per worker lifetime, because foraging is dangerous whereas working inside the nest is not. To see the logic, imagine that a worker will work in the nest for one week and then forage. If it works in the nest first it will almost certainly survive to forage, so will have a full in-nest career followed by a full foraging career. If it forages for one week first it will only be able to work in the nest if it survives the week of foraging. There are many risks in foraging: getting lost, getting killed by heavy rain or sudden cold weather, being eaten by a predator, being parasitized by one of the various flies or wasps that lay eggs on host insects. Specialization may also increase the efficiency of individual task performance. Workers of different sizes may be better suited to different tasks. In Atta ants, the large workers would be unable to tend the fungus garden because this involves delicate operations, such as grooming individual strands of fungal hyphae and removing weed fungi. Equally, the very small workers are unsuited to defending the nest against predators. The large jaws of the largest Atta workers can cut human skin. There is also an economic aspect to consider. Even if the big workers were as good as the small workers at tending the fungus garden, it would still be advantageous for the colony to rear small workers to do this task as the small workers need less food to rear and sustain. Specialization may also increase efficiency by reducing time wasted moving between spatially separated tasks. This has been studied in the <a href="/keyword/honey-bee/" >honey bee</a> Apis mellifera and in Pheidole dentata ants. <a href="/keyword/honey-bee/" >honey bee</a> workers of a particular age tend to do a range of tasks that are located together spatially. You may like to consider how similar principles apply to human organizations such as a factory or a hospital. There is usually specialization of tasks to workers with particular skills and tools. Different parts of a factory do different tasks and have workers who are located there. Thus, a metal working factory may have APS 323, Social Insects, Lecture 6 an office area, foundry, machine shop, paint shop etc. In a hospital, it would not be efficient for surgeons, who are costly to rear (long expensive training) and sustain (high salary), to clean the floors. The above hypotheses for DoL are based on increased colony performance. The workers are doing what is best for the colony as a whole. An alternative explanation for the fact that foraging is the final task is based on direct advantages to individual workers. Thus, in species without morphologically-distinct queens and workers, a worker can, in principle, replace the queen if she dies. To increase this possibility individuals may delay foraging or forage less. What work needs doing? The most basic distinction is between tasks inside and outside of the nest. Typical in-nest tasks include nest cleaning, tending and feeding brood (&quot;nursing&quot;) and feeding the queen and other nestmates; food handling and storage within the nest; nest building and maintenance. The main outside-nest task is foraging for food and building materials. In some ants this can also involve clearing trails and laying pheromone along trails. Many termites even build a roof over their foraging trails. There is often a lot of communication involved in foraging, and these behaviours may be made both inside (e.g., waggle dance of <a href="/keyword/honey-bee/" >honey bee</a> s) and outside the nest (e.g., pheromone trail laying of ants, termites and some stingless bees). Every species will be a little bit different. Many differences arise from specific ways of life. In Atta ants, a major task is tending the fungus garden. The entire economy of a leafcutter ant colony is based around collecting and processing leaves, tending the fungus garden, and dumping large quantities of waste from the fungus garden and soil excavated to make the underground garden chambers. In Vespinae and Polistinae wasps nests are built of plant fibres, so some foragers collect wood pulp, whereas in <a href="/keyword/honey-bee/" >honey bee</a> s the internal structure of the nest is largely made of wax which is secreted by the bees. But <a href="/keyword/honey-bee/" >honey bee</a> s also use some propolis (tree resin) in building which they forage for. Some stingless bees (Meliponinae) also collect soil and even feces for nest building. <a href="/keyword/honey-bee/" >honey bee</a> foragers collect nectar, pollen, water, and propolis, and about 10% of the foragers are scouts who find new patches of flowers. So even within foraging there are different tasks Worker ants are often considered to be hard working, and any time you see a worker bee, wasp or ant away from the nest it is usually busy collecting food. However, when you look inside a social insect colony most of the workers are standing still and not obviously working. In addition, workers spend a lot of time walking about the nest &quot;patrolling&quot; and grooming. What might the adaptive significance of these activities be? For a worker to be efficient at helping the colony should it always be active, or are there times when a worker can best help the colony by being inactive and so use up less energy? Methods of study Research on DoL often uses observation nests and marked individuals (numbered discs glued on thorax; paint marks; wire rings etc.) Often, a group (&quot;cohort&quot;) of workers are marked on emergence from their pupae and the activities they perform noted as they age. In focal sampling, individuals are followed to determine what they do. In scan sampling, locations in the nest are scanned and the activities of marked individuals in specific locations are noted. It is also possible to experimentally manipulate the nest by changing the need for a particular task, or by removing workers doing certain tasks. For example, if many of the workers in a leafcutter ant colony of the size that normally cut leaves are removed, those that remain work harder. In the <a href="/keyword/honey-bee/" >honey bee</a> , if there are not enough receiver bees (workers who unload nectar foragers inside the nest and store the nectar in cells), the foragers will cause more in-nest workers to become receivers by making a special dance, the tremble dance, that recruits workers to this task. T. D. Seeley studied this by marking workers who were seen receiving nectar with paint dots. At the end of the day he removed them to investigate how the colony coped in the nest day's foraging. He then put them back into the colony. Age polyethism in the <a href="/keyword/honey-bee/" >honey bee</a> , Apis mellifera The immature period lasts 21 days (egg 3 days, larva 6-7 days, pupa 11-12 days). An adult worker takes a few minutes to chew her way through the wax capping of her cell. She then begins to work. The first job is cleaning cells from which workers have emerged to make them ready for another egg to be laid by the queen. The adult lifespan of a worker is approximately 30 days, but during winter they live much longer because there is little foraging or brood rearing. They pass through four age castes (Seeley 1985) each performing a discretized range of tasks that tend to co-occur spatially, in a particular part of the nest. 1. 0-2 days. Nest centre. Cleaning cells from which pupae have emerged so they can be reused. 2. 2-11 days. Nest centre and throughout nest. Feeding brood, queen; capping brood (wax cap on cell with fully fed larva), comb building, ventilating. APS 323, Social Insects, Lecture 6 3. 11-20 days. Nest periphery. Receiving and storing nectar; pollen packing 4. 20+ days. Outside nest. Foraging Physiology Although a <a href="/keyword/honey-bee/" >honey bee</a> worker cannot change her morphology her physiology does change in an adaptive way in relation to her age and the tasks performed at that age. The clearest example of this is the high activity of the mandibular glands in the heads of nurse bees. These secrete brood food. (The nurse bees eat pollen stored in the hive and convert it into these secretions.) Wax glands are fully developed in somewhat older bees and then degenerate in forager bees. The venom reservoir is not full in young bees and their stores of alarm pheromone are also low, but when they are older and most likely to be guarding the nest entrance their venom reservoir is full and alarm pheromone is present in quantity. (In contrast, queen <a href="/keyword/honey-bee/" >honey bee</a> s do not guard but instead use their sting to kill rival sister queens following swarming. The queen venom reservoir and venom gland are both larger than in the worker, and production of venom is more rapid following emergence from the pupa.) As a bee ages there are also changes in brain function, such as the switching on of the &quot;forager&quot; gene. In some way this prepares the worker's brain for foraging duties. Specialist or idiosyncratic tasks Some tasks are performed at a relatively low rate in the <a href="/keyword/honey-bee/" >honey bee</a> nest. For example undertaking (removing corpses of bees that die in the nest), guarding the entrance, scouting, and propolis collection. Not all bees will perform these tasks. However, all bees perform the main tasks, such as cleaning, nursing, foraging. Genetic variation in tasks In general, DoL among workers or between queen and workers is not due to genetic differences. However, there are heritable differences in the tendencies for workers to do different tasks. It is possible, for example, to breed bees that are more hygienic. That is, which have a greater tendency to remove diseased or dead larvae from cells. W. Rothenbuhler found that there is genetic variation in the tendency both to uncap cells contain a dead larva, in the tendency to remove the dead larva when the cell is uncapped. By selective breeding Rothenbuhler obtained colonies in which the workers performed zero, one, or both of these tasks. Robinson and Page (1988, Nature 333:356-358; 1989, Behav Ecol &amp; Sociobiol 24: 317-323): showed heritable differences in the tendency for workers to perform idiosyncratic tasks such as scouting and undertaking. The adaptive significance of this genetic variation is not well understood. Regulation of division of labour The age schedule given above is not absolute. Worker <a href="/keyword/honey-bee/" >honey bee</a> s of any age can perform any task. This is necessary for <a href="/keyword/honey-bee/" >honey bee</a> colonies to function properly because there are often breaks in brood rearing, leading to long periods during which no young bees are emerging from cells. For example, when a swarm founds a new nest there is a 21-day break before the first adult workers reared in the newly-constructed combs emerge. Division of labour is greatly influenced by juvenile hormone. The juvenile hormone titre of workers increases as the bee ages. Worker bees to which artificial juvenile hormone has been applied begin to forage at a younger age. If an experimental nest is made up of workers all just a few days old, some bees start foraging at a young age. Those that forage have higher juvenile hormone titres than their equal-aged nestmates who do not foarge (Robinson et al. 1989, Science 246: 109-112). In some way, colony conditions and needs are detected by the workers and affect the endocrine system, which in turn affects behaviour. The regulation of DoL is extremely complex in <a href="/keyword/honey-bee/" >honey bee</a> s (and presumably in other species as well), and involves worker bees receiving stimuli concerning colony conditions and needs from a variety of sources (pheromones and other communication signals from nestmates; monitoring the colony conditions both directly and indirectly), and complex processes inside each worker involving the nervous and hormonal systems to control her passage through the general age polyethism schedule and also the performance of idiosyncratic tasks. Morphological worker castes 15% of ant genera have at least one species with two or more morphological worker castes. (Many termites also have morphologically distinct working individuals.) That is, workers of very different size and sometimes shape. In Pheidole dentata there are two morphological castes of workers (...