Complexity_ A Guided Tour - Melanie Mitchell [83]
Task allocation is another way in which an ant colony regulates its own behavior in a decentralized way. The ecologist Deborah Gordon has studied task allocation in colonies of Red Harvester ants. Workers in these colonies divide themselves among four types of tasks: foraging, nest-maintenance, patrolling, and refuse-sorting work. The number of workers pursuing each type of task adapts to changes in the environment. Gordon found, for example, that if the nest is disturbed in some small way, the number of nest maintenance workers will increase. Likewise, if the food supply in the neighborhood is large and high quality, the number of foragers will increase. How does an individual ant decide which task to adopt in response to nestwide environmental conditions, even though no ant directs the decision of any other ant and each ant interacts only with a small number of other ants?
FIGURE 12.4. An ant trail. (Photograph copyright © by Flagstaffotos. Reproduced by permission.)
The answer seems to be that ants decide to switch tasks both as a function of what they encounter in the environment and as a function of the rate at which they encounter other ants performing different tasks. For example, an inactive ant—one not currently performing a task—that encounters a foreign object near the nest has increased probability of taking up nest-maintenance work. In addition, an inactive ant that encounters a high rate of nest-maintenance workers entering and leaving the nest will also have an increased probability of adopting the nest-maintenance task; the increased activity in some way signals that there are important nest maintenance tasks to be done. In a similar way, a nest-maintenance worker who encounters a high rate of foragers returning to the nest carrying seeds will have an increased probability of switching to foraging; the increased seed delivery signals in some way that a high-quality food source has been found and needs to be exploited. Ants are apparently able to sense, through direct contact of their antennae with other ants, what task the other ants have been engaged in, by perceiving specific chemical residues associated with each task.
Similar types of mechanisms—based on pheromone signals and direct interaction among individuals—seem to be responsible for other types of collective behavior in ants and other social insects, such as the construction of bridges or shelters formed of ants’ bodies described in chapter 1, although many aspects of such behavior are still not very well understood.
Biological Metabolism
Metabolism is the group of chemical processes by which living organisms use the energy they take in from food, air, or sunlight to maintain all the functions needed for life. These chemical processes occur largely inside of cells, via chains of chemical reactions called metabolic pathways. In every cell of an organism’s body, nutrient molecules are processed to yield energy, and cellular components are built up via parallel metabolic pathways. These components are needed for internal maintenance and repair and for external functions and intercellular communication. At any given time, millions of molecules in the cell drift around randomly in the cytoplasm. The molecules continually encounter one another. Occasionally (on a scale of microseconds), enzymes encounter molecules of matching shape, speeding