If you watched ants scurrying about, you certainly wouldn't think that they work in any organised way, but they do. All ants are social, which means they live in organised colonies. A lot is known about how the colony behaves but not much is known about the underlying cause of the organised behaviour. A new book, Ants at Work: How an Insect Society is Organised, by Deborah M. Gordon (The Free Press, 1999) sheds light on some aspects of ant behaviour.
The body of an ant has three parts - head, thorax and abdomen. The abdomen is articulated to the thorax by an abdominal stalk. The ant family contains more than 4,500 described species that are distributed widely in temperate and tropical countries. The nests of many species consist of chambers excavated underground, or under stones or logs, in mounds of earth, or in decayed trees.
An ant colony contains sterile workers (wingless females) and one or more fertile females (queens). A fertilised female becomes the queen of the colony. Her main function is laying eggs that produce the workers and the next generation of reproductives - new queens and males. The new reproductives fly off to mate and then begin new colonies. The males die after mating and the workers gather food, care for the young, and maintain and defend the colony.
Ms Gordon studies red harvester ants (Pogonomyrmex bar batus) in the Arizona Desert. The ants are each about 1 cm long and they forage for plant seeds. She has kept track of about 300 colonies of ants over many years. New colonies are born, struggle to occupy a foraging area, grow, reproduce and then settle down among their life-long neighbours. The colonies live for 15 to 20 years. Individual ants live only for one year.
Ms Gordon has noted that the way a colony behaves changes as the colony gets older and larger. This cannot be because, as the colony ages, it is guided by older and more experienced ants. So why does colony behaviour change if the nature of individual ants remains the same?
There are many tasks to be done in the ant colony: collection and distribution of food, nest-building, caring for eggs, larvae and pupae, maintaining the nest, defending the nest, feeding and caring for the queen, etc.
The nest must behave in a dynamic and flexible manner, otherwise it would be unable to survive in a changing environment. Thus, if the food-foraging ants find a new rich source of food, the colony swells the ranks of the forager ants in order to exploit Nature's temporary largesse. Again, if the nest suffers some damage, the ranks of the nest-maintenance ants are augmented in order to effect quick repairs. The queen exercises no authority, so how is task allocation controlled in an ant colony? Ms Gordon investigated this by carrying out perturbation experiments, in which she deliberately impeded or accelerated the rates of certain ant tasks.
In one experiment, she impeded the movement of forager ants by placing plastic barriers on the foraging trails. The barriers were not impermeable - the ants could go around or through them via a few holes in the bottoms of the barriers - but they did slow up progress. It might be thought that the colony would respond by sending out more foraging ants to compensate for the slower progress, but the opposite happened - fewer forager ants left the nest. A decline in the rate at which food is gathered signals a scarcity of food and the colony responds by "considering" it not worth the effort to forage. Foraging is a costly activity in which ants use up water and energy.
Another experiment perturbed the activity of the nest-maintenance ants. Ms Gordon placed piles of toothpicks next to the nest entrance. These were quickly carried away by the nest-maintenance ants and placed around the edge of the nest mound. During this activity the numbers of maintenance ants increased.
FORAGING and nest-maintenance are strongly related. When perturbations decreased the numbers of foragers, the number of nest-maintenance workers increased, and vice-versa. Since the foragers were not directly affected by the presence of toothpicks, and the nest-maintenance ants were not directly affected by the plastic barriers, it seems that changes in numbers of either party is a response to changes in numbers in the other party.
In other clever experiments, Ms Gordon showed that when her experiments created a need for extra workers, ants switch tasks, i.e., for example an ant that normally forages for food will switch to nest maintenance. She suggests that all of this is regulated by a mixture of external cues and the frequency with which ants meet other ants in the upper chambers of the nest directly inside the entrance. Colonies are active outside the nest for about seven hours a day and all external workers frequently enter and leave the nest. A foraging trip typically takes about half an hour. Nest-maintenance workers enter the nest even more frequently.
Ms Gordon suggests that the probability that an ant performs a task depends on the history of its recent encounters. Ants engaged on different tasks have different odours. Supposing a rule operated such as: "If I meet five successful foragers, go out and forage", this would explain the increase in forager ant numbers when a new source of food is located, and a decrease in forager numbers when a food scarcity occurs.
Her proposal has powerful explanatory capacity. External cues and individual rule-based behaviour leads to predictable patterns of group behaviour. However, she points out that the process of task allocation is not rigidly deterministic even at the individual ant level. An ant does not respond every time in the same way to the same stimulus - nor do colonies. Certain events influence the probability that certain ants will carry out certain tasks and these regularities lead to predictable tendencies.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC.