Colony-level Response to Perturbation

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Dr. Gordon will investigate the social organization of harvester ants, so called because they collect and eat seeds. These ants do various tasks outside the nest. Foragers travel along cleared trails to collect seeds; nest-maintenance workers clear the surface of the nest mound and maintain tunnels inside the nest; patrollers inspect the nest area and respond to disturbances or new sources of food, and midden workers sort and arrange the colony refuse pile, or midden. Each task is done each day, in a predictable sequence that Dr. Gordon calls the "daily round" of the colony. The daily round changes in response to changes in the colony's environment. Dr. Gordon has found experimentally that groups of ants doing different tasks are interdependent. For example, if she increases the amount of nest maintenance by giving the colony extra work to do, the colony will also do less foraging. These interactions between different worker groups depend on the age of the colony. When disturbed experimentally, 5-year-old colonies emphasize foraging more than do 2- year-old colonies. The older colonies also respond in the same way if an experiment is repeated many times, while younger colonies respond in many different ways. As individual ants live only about a year, this cannot be because 5-year-old colonies contain older, "wiser" ants than do 2-year-old colonies. Somehow, the organization of the colony must change as it gets older. Dr. Gordon will carry out experiments designed to help answer the following questions: (1) When an increase in the numbers doing one task causes a decrease in the numbers doing another task, is this because ants are switching tasks, or do increases result from recruitment of ants from reserves inside the nest? Dr. Gordon will mark ants distinctively to identify individuals. (2) Do older colonies respond to experiments differently from younger colonies because they are older, or because they contain more ants? Dr. Gordon will remove ants from older colonies to see if they then act like younger, smaller colonies. (3) Do experiments that change the behavior of one colony cause changes in the behavior of neighboring colonies? If so, this may help to explain how colonies interact in an environment in which they are competing for food. Dr. Gordon's research will help us to understand how a complex society functions at the group level. A forager's behavior depends on the conditions affecting foraging: where food is, how accessible it is, how much food the colony has. But Dr. Gordon's experiments show that a forager's behavior also depends on events affecting the ants doing other tasks--for example, on how much clean-up work there is to be done on the nest mound. The dynamics of relationships between different worker groups suggest a complex model for how an ant society functions in an ever-changing environment. Such models may apply to other societies as well.