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defense, nutrition, and reproduction. Close study of any social insect species would reveal examples of individual traits that have evolved for colony function, but the honey bee is the best-studied case. For example, honey bees use floral food sources, and they need to track this ever-changing resource. Their famous dance language allows workers to communicate the quality and location of a resource to nestmate workers. But they also have mechanisms to effectively allocate work among multiple food sources, even when each individual has quite limited information. Fidelity to a good food source is not absolute; a fraction of workers always seeks to discover new food sources. Returning foragers adjust the intensity of their waggle dancing (the number of waggle runs) according to the profitability of their trip. Foragers from better sources therefore recruit more followers, so the colony concentrates on the better food source (Seeley, 1997).

Foragers can also tune the intensity of their recruitment dances according to how much the colony needs food, but this requires coordination with the workers that specialize in nectar processing. If a colony needs more nectar, the nectar-processing bees that have this information crowd closer to the hive entrance. This means that a returning forager unloads her nectar quickly, which cues her to intensify her dancing to recruit more foragers (Seeley, 1997) and allay the shortage of nectar. On the other hand, there may sometimes be more nectar coming into the colony than the nectar processors can handle, resulting in inefficiently long unloading times. In this event, foragers perform the tremble dance, which is different from the waggle dance in that it stimulates other workers to become nectar processors (Seeley, 1997).

Such integrated behaviors of many workers in a honey bee colony allow the colony to find and exploit food efficiently, to alter group foraging based on individual information, and to adjust the number of foragers and nectar processors to meet changing needs. No individual is doing anything that by itself would be very useful; instead, each is performing a role in a process that only makes sense in terms of increasing colony function. Such a smooth coordination among workers in finding, harvesting, and processing food makes the argument for the colony as an organism compelling. Similar kinds of coordination are found wherever they are looked for in social insects, for example, in nest construction in Polybia wasps (Jeanne, 1986), in nest-finding in honey bees (Seeley and Visscher, 2004) and Leptothorax ants (Mallon et al., 2001), and in the establishment of foraging trails by army ants (Franks et al., 1991).

THE SUCCESS OF SOCIAL INSECTS

If colonies have found ways to be more efficient than separate individuals, one might expect social insects to be particularly successful. In



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