Plasticity In Colony Division Of Labor

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Colony division of labor, though highly structured, also shows great plasticity. Colonies respond to changing needs by adjusting the ratios of individual workers engaged in different tasks. This is a consequence of the flexibility of the individual workers. For example, there is plasticity in age-related division of labor, with workers able to respond to changes in colony age demography with accelerated, retarded, or reversed behavioral development. Workers also can shift to emphasizing a different task that is part of their age-specific repertoire or they can simply work harder. Morphologically specialized workers can be induced to shift their behavior; majors, normally specialized in foraging or defense, can care for the brood in the absence of minor workers. This plasticity in division of labor contributes to the reproductive success of a colony by enabling it to continue to grow, develop, and ultimately produce a new generation of reproductive males and females during changing colony conditions.

Plasticity in division of labor in advanced eusocial species is achieved by a variety of mechanisms of behavioral integration. These mechanisms enable workers to respond to fragmentary information with actions that are appropriate to the state of the whole colony. This makes sense because it is unlikely that any individual workers have the cognitive abilities to monitor the state of their whole colony and then perform the tasks that are needed most or direct others to do so.

Mechanisms of worker behavioral integration often involve social interactions. For example, in many species, including Polybia wasps and honey bees, nest workers routinely relieve the foragers of their newly acquired loads, whether nest material or food. Foragers that are unloaded immediately upon their return to the nest are likely to continue foraging for the same resource, apparently because the quick unloading signals to them that they have brought something of high value back to the colony. In contrast, foragers that experience a significant time delay before being unloaded respond by changing their behavior, perhaps shifting to the collection of another resource. The nutritional status of a fire ant colony strongly influences the behavior of its foragers, with the relevant information transferred during social feeding. In colonies of the desert-dwelling red harvester ant, Pogonomyrmex barbatus, workers obtain information on the needs of the colony by changes in their encounter patterns with members of various task specialist groups. For example, red harvester ant foragers are more likely to leave the nest to forage when they encounter greater numbers of successful returning foragers.

Social inhibition is a potent mechanism of integration in insect colonies. In colonies of honey bees, social inhibition acts to keep the division of labor synchronized with changes in colony age demography. Older workers inhibit the rate of maturation of younger workers. Some young workers in a colony deficient in older workers, for example, exposed to lower levels of social inhibition, respond by becoming precocious foragers. The specific honey bee worker factor that causes this inhibition has not yet been identified, but other sources of social inhibition have, emanating from the queen and the brood. The regulation of the size of the soldier force in Pheidole colonies also is based on a process of social inhibition. In this case, the presence of adult soldiers inhibits the production of new soldiers. Involvement of a pheromone is suspected, but no specific soldier inhibition pheromone has been identified yet.

The integration of activity in primitively social insect societies appears to be more centralized than in advanced eusocial societies. Primitively eusocial colonies often contain only a few dozen individuals, making centralized control more feasible. Queens act as central pacemakers and modulate worker activity via behavioral interactions in sweat bees and polistine wasps. Queens do not appear to be able to get workers to shift to different tasks, but they do cause them to work harder at the tasks they are already doing.

We are far from understanding how the behavior of individual workers is integrated into a well-functioning colony. Studies of behavioral integration are aided by various kinds of theoretical models. In some models, an insect colony is likened to a developing organism, i.e., the "superorganism" metaphor. In other models, an insect colony is analyzed with perspectives from neural network theory, with individual workers serving as analogs of individual neurons. Still other models view an insect colony as a self-organizing entity and use complex systems theory to develop ideas on colony function.

See Also the Following Articles

Ants • Caste • Colonies • Hymenoptera • Isoptera • Recruitment Communication • Sociality

Further Reading

Beshers, S. N., Robinson, G. E., and Mittenthal, J. (1999). The response threshold concept and division of labor. In "Information Processing in Social Insects" (C. Detrain et at., eds.), pp. 115—141. Birkhauser, Basel.

Bloch, G., Wheeler, D. L., and Robinson, G. E. (2002). Endocrine influences on the organization of insect societies. In "Hormones, Brain and Behavior" (D. Pfaff et at., eds.), pp. 195-237. Academic Press, San Diego.

Detrain, C., Deneubourg, J. L., and Pasteels, J. M. (eds.) "Information Processing in Social Insects." Birkhauser, Basel.

Page, R. E., and Robinson, G. E. (1991). The genetics of division of labour in honey bee colonies. Adv. Insect Physiol. 23, 117-171.

Robinson, G. E. (1992). The regulation of division of labor in insect societies. Annu. Rev. Entomot. 37, 637-665.

Robinson, G. E. (1998). From society to genes with the honey bee. Am. Sci. 86, 456-462.

Wilson, E. O. (1971). "The Insect Societies." Harvard University Press, Cambridge, MA.

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